Dynamics of molecular stereochemistry via oriented molecule

Facile alignment of molecular rotation in supersonic beams. David P. Pullman , Břetislav Friedrich , Dudley R. Herschbach. The Journal of Chemical Ph...
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J. Phys. Chem. 1987, 91, 5427-5437

5427

Dynamics of Molecular Stereochemistry via Oriented Molecule Scattering David H. Parker,* Chemistry Department, University of California, Santa Cruz, Santa Cruz, California 95064

Henk Jalink, and Steven Stolte Physics Department, Catholic University, Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands (Received: April 8, 1987)

Crossed-beam reactive scattering experiments employing electrostatic hexapole fields to control the initial collision geometry of chemical reactions are described. New results are presented on the reactions of oriented NO with ozone and oriented N 2 0 with Ba. Preliminary results are also given for the oriented CH3F + Ca* CaF* CH3 reaction. Recent technical advances in state selection and product detection are detailed. We discuss the effects of rotational coupling and nonzero impact parameters in changing the molecular precollision orientation selected by the hexapole fields to a different in-collision orientation at the moment of impact with the reaction partner. Uncoupling of 1 doubling in N20at strong orientation fields is demonstrated via the observed reactive anisotropy. Steric effects are found to govern many aspects of the reactions investigated thus far. Strong correlations are observed of the reactivity, product recoil, and rotational angular momentum distributions with the collisional orientation. These correlations ultimately provide information on the anisotropic part of the reaction potential energy surface. We conclude with a brief outline of possible future directions in oriented molecule scattering.

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I. Introduction Investigation of the steric effect can be likened to an archaeologist working in an ancient chamber (near Jerusalem!) with primitive tools, trying to unearth a delicate mosaic. This steric mosaic is uncovered by experiments that avoid averaging in an atom-molecule collision over all angles of attack by the atom on the molecular framework. Two tools are available for preparing reactant molecules with controlled directions in space: electrostatic hexapole fields and laser photoselection. At present the only two schemes successful in detecting the outcome of oriented molecule scattering are surface ionization and chemiluminescence. In this text we describe experiments using hexapole fields and chemiluminescence detection for study of the steric properties of chemical reactions. Although hexapole orientation techniques were introduced many years ago by Kramer and Bernstein,’ until recently progress has been slow in their full utilization. These are difficult experiments to perform, and full interpretation of their results surpasses the present resources of reaction theory. Faced with these challenges one may wonder if the effort has merit. The answer, as we will try to convince the reader, is obviously yes. The rough outlines of the steric picture have been known for many years and are incorporated into almost all modern chemical concepts. As an example, molecular mechanic models,2 which promise to greatly enhance our understanding of many fields, especially molecular biophysics, embody steric factors as a basic premise. We concentrate on symmetric top molecules, with rotational quantum numbers J , K , and M where J is the total angular momentum, K the projection of J on the molecular symmetry axis, and M the magnetic sublevel, the projection of J on an applied field direction. In scattering experiments the applied field direction is set parallel (or antiparallel) to the collision approach direction so that M transfers to the projection of J on k, the relative velocity vector. Hexapole electrostatic fields focus via the first-order Stark effect molecules whose product pelKM is negative where pel is the permanent electric dipole moment of the molecule. If the reactive or “heads” end of the molecule is defined parallel to r, the molecular axis, in order to convene with the collision setup (at least for the examples considered in this article), r lies opposite to the dipole moment [ p e l t i r ]and thus pelis negative. The +K,+M or the -K,-M states, which have the same orientational preference, are deflected to the scattering center. Hexapole focusing by (1) Kramer, K. H.; Bernstein, R. B. J . Chem. Phys. 1965, 42, 767.

(2) Burket, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982.

0022-3654/87/209 1-5427$01.50/0

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selecting J,KM orients the molecular axis. Laser photoselection can prepare single J,IKW states; at fixed K the population may differ between individual 111.11 levels, a situation labeled alignment. Hexapole focusing provides both oriented and aligned reactants (Le., even and odd moments of polarization) while photoselection prepares only the even moments. In all reactions studied so far except those of oriented CHC13 the observed orientation effects are appreciable, especially compared to the effects seen for reactions of laser-aligned atoms or molecules. Bernstein and Kramer’ introduced hexapole focusing in 1965, and Brooks and Jones3 and Bernstein and co-workers4 reported on alkali metal reactions with oriented methyl iodide in 1966. A number of revealing but somewhat qualitative experiments followed as reviewed by Brooks5 in 1976. More quantitative measurements began in the early 1980s and are discussed in this text. There is lately a strong sense of excitement in orientation studies,6 brought out in a November 1986 workshop in Jerusalem on the Dynamics of Molecular Stereochemistry from which this report is generated. We discuss here only experiments using the hexapole technique; the many successes of laser photoselection methods have been reviewed by Leone7 in 1985 and in several articles in this issue. A review of oriented molecule scattering was given in 1982 by Stoltes and extended more recently in a technical treatise on the use of state selectors in scattering experiment^.^ Keeping pace with experiment are models that directly incorporate steric data to obtain orientation-dependent opacity functions, and conversely, studies which generate steric opacity functions from realistic potential energy surfaces for reactions. Detailed examples by Janssen and Stolte’O on modeling experimental systems including those discussed in this paper appears elsewhere in this issue along with papers by other authors including Levine and co-workers” on modern orientation models. We refer the reader to these other texts for a more complete discussion of steric models, and also to the related article by SimonsI2 on polarization of reaction products. Section I1 of this paper defines the basic goals of steric studies and describes some of the experimental complexities, especially impact parameter averaging. (3) Brooks, P. R.; Jones, E. M. J . Chem. Phys. 1966, 45, 3449. (4) Beuhler, R. J.; Bernstein, R. B.; Kramer, K. H. J . A m . Chem. SOC. 1966,88, 5332. (5) Brooks, P. R. Science 1976, 193, 11. (6) Leone, S.R. Phys. Today 1987, January, S-12. (7) Leone, S . R. Science 1985, 227, 889. (8) Stolte, S. Ber. Bunsenges. Phys. Chem. 1982, 86, 413. (9) Stolte, S . In Azomic and Molecular Beam Methods; Scoles, G . , Ed.; Oxford University Press: New York, 1987; Vol. 1, Chapter 25. (10) Janssen, M. H. M.; Stolte, S., this issue. ( 1 1 ) Schechter, I.; Levine,

R. D.; Prisant, M. G., this issue.

(12) Simons, J. P., this issue.

0 1987 American Chemical Society

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The Journal of Physical Chemistry, Vol, 91, No. 21, 1987

A

Parker et al.

P(b=O)

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Figure 1. Steric aspects of the CHpI Rb reaction, showing the angle of attack, yr, of the Rb atom upon ICH3 (represented by a space-filling

steric model within an ellipse). The outer dashed figure is a "best-fit" shell from an ellipsoidal line-of-centers model for this reaction. Adapted from ref 14. In section 111 we describe the apparatus and its present technical capabilities. Results of current steric experiments appear in section IV and a brief forecast of future directions concludes the paper.

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11. Steric Measurements: Simple Theory and Methods

The collision geometry is one of many variables that determine the probability of reaction.13 By varying the relative collision geometry the reaction orientation dependence is obtained; this reflects the configurations that are able to pass the (steric-dependent) barrier of the potential energy surface toward products. Measurement of the directional properties of reaction products, especially from oriented molecule reactants, provides even more detail on the concerted motion reactants follow in traversing the surface saddle point into the product channel. Section IV describes such a directional measurement, in oriented N 2 0 reactions with Ba. Figure 1 illustrates a quantitative steric measurement, derived14 from data on the reaction of oriented C H J Rb.15 A spacefilling model of CH31is shown superimposed on a polar diagram of reactivity (a polar steric opacity function) with Rb. Heads-on attack at the I atom yields full reactivity while steric blocking of the I atom by C H 3 appears as a 53 f 2' cone of nonreaction. Remarkably, this subtends the same angle as the projection of the CH3 van der Waals radius from the CH31 center of mass. Chemical intuition is confirmed (as in Figure 1) by the steric measurements reported so far. The predicted nonreactive end of the molecule is nonreactive to some extent; the idealized experiment maps out the size or steric factor of the blocking group. Indeed, oriented molecule reactivity data has been used in the reverse sense to experimentally determine the direction of a molecular dipole moment.I6 Orientation is a directional property of reaction, a central theme of D. R. Herschbach, recipient of the 1986 Nobel Prize in Herschbach and Chemistry. In a far-sighted series of his co-workers pointed out the important clues the measurement of directional or vector properties offer in deciphering reaction mechanisms. Vector correlations recover dynamical information otherwise lost by the averaging over all possible azimuthal ori-

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(13) Levine, R. D.;Bermtein, R. B. Molecular Reaction Dynamics and Chemical Reactivity; Oxford University Press, New York, 1987. Bernstein, R. B. Atom-Molecule Collision Theory: A Guide for the Experimentalist; Plenum: New York, 1979. (14) Choi, S. E.; Bernstein, R. B. J . Chem. Phys. 1985, 83, 4463. (15) Parker, D. H.; Chakravorty, K. K.; Bernstein, R. B. J . Phys. Chem. 1981, 85, 466; Chem. Phys. Lett. 1982, 86, 113. (16) Jalink, H.; Parker, D. H.; Stolte, S . J. Mol. Spectrosc. 1987, 121, 236. (17) Herschbach, D. R. Discuss. Faraday SOC.1962, 33, 283. (18) Case, D. A.; McClelland, G. M.; Herschbach, D. R. Mol. Phys. 1978, 35, 541.

(19) McClelland, G. M.; Herschbach, D. R. J . Phys. Chem. 1979, 83, 1445. (20) Barnwell, J. D.; Loeser, J. G.; Herschbach, D. R. J . Phys. Chem. 1983, 87, 2781.

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-1.0

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COSYr Figure 2. Steric opacity functions, P(b,cos yo), for a barrier located at a sphere of radius R, showing limiting behavior for (a) zero impact parameter and (b) heads-on collisions. Panel (c) shows an angle-dependent barrier to reaction, V(cos yr), for CH31 Rb,'O where yr is the angle of attack at reaction. With the parametrization V(cos yI)= Vo + V , cos y,,it can be shown that, from E,, cos2 yr 1 Vo V , cos yr, P(0,cos 7 , ) = 1 for cos y,2 -0.644, and from b = R ( l - cos2 y,)lI2that P(b,l) = 1 for b 5 3.64 A (Etr= 0.13 eV, Vo = 0.092 eV, VI = -0.059 eV, and R = 4.16 A).

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entations (about k) of b, the collision impact parameter. Section IV surveys the vector correlations measured so far in oriented molecule experiments. Impact parameter averaging also smears the selected collision orientation at the moment of impact. We address this more experimental aspect in this section. A . Controlling the Angle of Attack. Averaging, as in crossed-beam experiments, over b allows a change of yo,the initial angle of attack selected in the precollision region to yr, the incollision actual angle of attack. With the applied orientation field set parallel to k, yo is the angle between k and the reactant molecular axis, r, prior to reaction; yo = 0 describes a heads-on collision. At the moment of impact the trajectory is displaced by b from the line of centers, leading to a different angle of attack on r. Deconvolution of Y~ requires introduction of a steric model, cf ref 10. Dynamically, b affects both 1, the reactants' orbital angular momentum, and 6, the product recoil angle. Under certain conditions a direct one-to-one correlation of b and 0 is predicted by extended optical models2' for reactive scattering. At a given collision energy the total reaction cross section, uR, relates the opacity function P(b) and the product angular distribution Z(6) as (21) Kwei, G. H.; Herschbach, D. R. J . Phys. Chem. 1979, 83, 1550.

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987 5429

Dynamics of Molecular Stereochemistry bR

=

Jx

27r sin 8 I(8) d8 =

1-

27rbP(b) db

(1)

P(c0s 0)

I

I

1

In some direct backscattering reactions such as CH31 + Rb, b and 8 are approximately related by b R cos (8/2) where R is an adjustable hard-sphere radius.14 The results illustrated in Figure 1 are for RbI product detected at 8c.m.= 180, corresponding in this approximation to collisions taking place with b = 0. While under this constraintthe data may not fully reflect bR, yrand yo are equivalent. Reactant orientation can be incorporated in the opactiy function COS e as P(b,cos yo). Figure 2 shows the simplest limiting behavior of Figure 3. The probability of finding the molecular axis, r, a t the prea steric opacity function using a barrier V(cos yr) (Figure 2c) cession angle, 8, with the electric field, E, for the (J,K,M) = ( l , l , l ) and deduced1° for the CH31 Rb reaction. Reactivity is evaluated (2,1,2) states. For the (1,-1,-1) and (2,-1,-2) states similar distributions at this barrier which surrounds the CH31 molecule as a shaded are found (see text). The average angle is given by cos 8 = ( P E ) = sphere of radius R. Only collisions with line-of-centers energy K M / ( J ( J + 1)) and is and for ( l , l , l ) and (2,1,2), respectively. above the barrier react ( P = 1). The simple cases of heads-on and zero impact parameter collisions (at 0.13 eV collision energy) are isolated. With b = 0 the line-of-centers energy is always 0.13 eV; thus in Figure 2a reaction stops at the cutoff angle corresponding to the cone of nonreaction of Figure 1. For perfect heads-on collisions the line-of-centers constraint stops reaction before R is reached, as shown in Figure 2b. Less favorable orientations at large impact parameters approach closer to R, diluting the observed U,(COSyo) steric dependence, by as much as a factor of 2 for some systems.1° Bernstein, L e ~ i n e , and ~ ~ .other ~ ~ ~ ~ r k e rhave ~ developed ~ ~ - ~ ~ - ~ ~ methods for deducing V(cos yr)from steric data or from accurate Figure 4. The focusing machine of Nijmegen: as, absorption cell; ch, potential energy surfaces, extending the simple modified linechopper; m, mirror; so, source; sk, skimmer; st, Stark plates; de, laser power detector; ss, hexapole state selector; cs, collimator of scattering of-centers model of Smith.26 Measuring the steric effect as a region; bf, barium oven; hf, harp field; 11,12, lens system; pmt, bialkali function of the collision energy provides the most direct means photomultiplier; cd, collimator of mass detector; io, ionizer; qmf, quadof obtaining V(cos yr),our first (one-dimensional) probe of the rupole mass filter. Parallel plate guiding fields follow the hexapole fields. potential energy surface anisotropicity. Section IV describes a ) the Ba + N 2 0 reaction. measurement of U,(COSY ~ , E , , for precession angles are plotted in Figure 3. The precollision oriFactors besides b averaging may also alter yo. The collision entation is widespread, and velocity and angular spreads of the energy and reactant rotational angular momentum, j, together crossed beams further smear the range of attack angles. control the “disruption of favored ~ r i e n t a t i o n ”i.e., , ~ ~ the rotation Molecular axis polarization for a selected J,K,M rotational state of the target molecule before collision. Fast collisions and low is described by a Legendre polynomial expansion*of the precession rotational energy lessen this effect. With oriented symmetric tops angle distribution as rotational precession about the top axis occurs rather than endover-end tumbling, and thus the basic steric effect survives but control of the azimuthal angle about the top axis is lost. Finally, forces such as dipole-induced dipole attraction operating over distances of a few angstroms could serve to “reorient” the molecule, where PJKM is the quantum probability distribution function and enhancing the observed reactivity especially of the initially favored Pn(cos yo) are the Legendre polynomials. Choi and Bernstein30 orientations.28 A quantitative treatment of the actual torques have tabulated Cn coefficients for J,K,M values up to J = 4. The induced during the collision will require a critical knowledge of J,K,M value determines how many moments are present in the the electron density distributions of the reaction system. Fast summation; the (1, l ,1) state for example contains only a P, and collisions again lessen the importance of “reorientation”. a Pzmoment, while many moments (of decreasing reliability) are B. Selection and Description of the Initial Angle of Attack. available from a “hot” impurely state-selected source. Supersonic beams are usually employed in hexapole focusing The following experimental section describes the hexapole foexperiments. As pointed out by Stolte et al.29the low quantum cusing apparatus, the only experimental method which controls states focused behave nonclassically. They precess about a large the molecular orientation (odd and even moments) over the full average angle, due to quantum effects about a large range of range of angles of attack. Hexapole focusing complements the angles. Choi and Bernstein30 have recently described in detail potentially very powerful intramolecular photodissociation method the classical and quantal characteristics of state-selected symmetric pioneered by Wittig,31 discussed elsewhere in this issue. This top molecules. Two single N 2 0 rovibrational states are selected photodissociation technique does not vary the collisional orientation in the N 2 0 Ba reaction; the ( l , l , l ) and (2,1,2) J,l,M states but does select the in-collision impact parameter and one average (1 is the v2 bending vibrational quantum number); their range of direction of attack.

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111. Experimental Section (22) Levine, R. D.; Bemstein, R. B. Chem. Phys. Lett. 1984, 105, 467. (23) Blais, N. C.; Bernstein, R. B.; Levine, R. D. J. Phys. Chem. 1985, 89, 10. (24) Evans, G. T.; She, R. S. C.; Bernstein, R. B. J. Chem. Phys. 1985, 82,2258. She, R. S. C.; Evans, G. T.; Bernstein, R. B. J. Chem. Phys. 1986, 84, 2204. Evans, G. T. J. Chem. Phys. 1987,88, 3852. (25) Agmon, N. Chem. Phys. 1981,61, 189. (26) Smith, I. W. M. J. Chem. Educ. 1982, 59, 9. (27) Loesch, H. Chem. Phys. 1986, 104, 213. (28) Luo, Y.;Benson, S. W. J. Phys. Chem., to be submitted for publication. (29) Stolte, S.; Chakravorty, K. K.; Bemstein, R. B.; Parker, D. H. Chem. Phys. 1982, 71, 353. (30) Choi, S . E.; Bernstein, R. B. J. Chem. Phys. 1986, 85, 150.

Figure 4 shows the oriented molecule reactive scattering apparatus in operation at Nijmegen. Note the length of the machine, a total of -2.8 m from nozzle source to scattering center! Such distances are required for pure state selection, and practical only due to the -20-fold gain in beam flux by focusing. Three components are distinguished: state selection and orientation of a molecular beam and the subsequent detection of reaction with a crossed atomic beam. (31) Radhakrishnan, G.; Beulow, S.; Wittig, C. J. Chem. Phys. 1986,84, 747.

5430

Parker et al.

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987

f

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I%CF3H/Ar Po:SoO torr V, ;555 m/s

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beam hexapole focusing apparatus capable of focusing lOI3 molecules/cm3 of pure state-selected CH3Cl. Bernstein and cow o r k e r ~present ~ ~ focusing curves in this issue generated by a pulsed-beam hexapole apparatus at UCLA. Figure 5 shows focusing curves from the continuous-beam Nijmegen machine for CH3F, CF3Br, and CF3H under similar nozzle conditions 10 K). The prolate top C H 3 F (translational temperature (rotational constants A = 5.28 cm-I, B = 0.085 cm-I) shows nearly complete rotational cooling; only a few J,KM states are populated. On the other hand, CF3Br, a prolate top with small rotational spacings ( A = 0.191 cm-I, B = 0.070 cm-l) even at -10 K has such a wide range of states populated that resolved state focusing is not possible since many states focus at the same rod voltage. Note that CF3H, an oblate top also with small rotational constants ( A = 0.919 cm-I, C = 0.188 cm-I), does show a resolved focusing spectrum. Imperfect alignment of the field axis and beam axis, velocity spreads, and finite collimator size account for most of the individual peak width in the focusing curves. State-dependent second-order Stark effects may also perturb the focusing resolution. A powerful enhancement of hexapole state selection is also illustrated in Figure 5 where the laser pumping apparatus in Figure 4 is used to vibrationally excite substantial fractions of a CH3F beam to single rotational levels of the u3 C-F stretching mode.34 The laser is tuned into resonance with CH3F by Stark fields and pumping of unfocusable M = 0 states into M = 1 levels is detected by an enhancement of the focused beam. The ( l , l , l ) u3 = 1 state selected in Figure 5 focuses at the same rod voltage as the ( l , l , l ) u3 = 0 state. A 66% increase in focusing is seen which should lead to easily detected contributions of vibrational energy in reactive scattering. B. Orientation. Hexapole fields focus and also orient polar symmetric tops or polar molecules which exhibit symmetric toplike behavior, including diatomics with electronic angular momentum such as NO, linear polyatomics excited in a bending mode such as N 2 0 (u2 = l ) , and even polar asymmetric tops, as described by Jones and Brooks.35 With N,O, J,K,M is replaced by J,I,M where 1 is the bending mode quantum number, and in NO, K is replaced by 9, the electronic angular momentum. In scattering experiments the electric field direction defining M is eventually set along k. At the end of the hexapole field an adiabatic transform is made from the six-pole field to parallel plate fields situated along the beam direction, overlapping the hexapole and scattering center assembly. The voltage gradients from one field to t h e next must be small compared to the spacing between Stark levels to avoid “flips” in the selected quantum numbers.36 In the scattering center the applied field is set along k by tilting the electric fields appropriately. These “orientation” fields must be uniform and of minimal bulk t o avoid scattering disoriented

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Figure 6. Artists view of the focusing machine (Figure 4) scattering region showing the harp fields, a polarization calibration device, and the polarizer rotator stage.

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1/5 I

1/6 1/7 1/8 I

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Figure 5. Focusing curves for CF3H, CF3Br, and CH3F. The beam intensity enhancement a t the mass detector, AI, by focusing with the hexapole divided by the intensity for the non-state-selected direct beam, Io, is plotted as a function of the hexapole voltage, V . The upper horizontal axis indicates the corresponding average cosine of the angle between the molecular axis and the electric field inside the hexapole. For the lower focusing curve a computer-simulated spectrum (solid line) is plotted along with the experimental points (closed dots). The open dots represent the beam intensity after populating the u3 = I , (J,K,M) = ( l , l , l ) state by pumping the u3, (l,l,O) state in Stark resonance with a COz laser.

A. State Selection. Both pulsed and continuous source hexapole state selectors can produce oriented beams of molecules in single rotational states. Gandhi et al.32have recently described a pulsed

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(32) Gandhi, S. R.; Curtiss, T. J . ; Xu, Q. X.; Choi, S. E.; Bernstein, R. B. Chem. Phys. Lett. 1986, 132, 6 . (33) Bernstein, R. B.; Gandhi, S. R.; Xu, Q. X.; Curtiss, T. J., this issue. (34) Jalink, H.; Janssen, M.; Harren, F.; Van Den Ende, D.; MeiwesBroer, K. H.; Parker, D. H.; Stoke, S. In Recent Advances i n Molecular Reaction Dynamics; Vetter, R., Vigue, J. Eds.; CNRS: Paris, 1986; p 41. (35) Jones, E. M.; Brooks, P. R. J . Chem. Phys. 1970, 53, 5 5 . (36) Maltz, C.; Weinstein, N. D.; Herschbach, D. R. Mol. Phys. 1972, 24, 133.

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987 5431

Dynamics of Molecular Stereochemistry molecules back into the reaction zone. Detecting chemiluminescence by viewing through a series of thin parallel wires ("harp fields"), shown in Figure 6, placed symmetrically about the scattering zone proved most functional in studies of the Ba + oriented N 2 0 reaction. The series of wires are tilted 33O relative to the N 2 0beam and connected by a resistor chain to give a linear stepwise voltage gradient parallel to k. Three orientations are specified: (i) Etfk, (cos yo) = 0.5, favorable orientation, Ba approaches the 0 end of N20;(ii) Etik, (cos yo) = -0.5, unfavorable orientation, Ba approaches the N end of NzO; and (iii) Elk, (cos yo) = 0.0, side-on orientation, Ba approaches the N,O molecule broadside. From the measured signals Z(fav), I(unfav), and I(side-on), I,,, the signal corresponding to unoriented (but state-selected) molecules is obtained as

Io = Y6[I(fav) + Z(unfav)]

+ Y3Z(side-on)

i

+

-

+

(cos YO)(E,,,,,,)= )/2[1/ ( l + (~//EStark)2)1'21

'

I

'

'

'

1

'

'

'

I

(4)

where qr is the 1-doubled splitting and &[ark = peffEactual with F,ff = peIlM/J(Jt l), the effective dipole moment. Substituting ql = 23.75 M H z and ~ ~ EStark (in MHz) = 0.0436Ea,tu,l(in V/cm) yields the excellent fit (solid line) to the data. Figure 7 is used to correct the Ba N 2 0 reaction data for lower orientation field voltages. A similar calculation is mentioned by Jones and Brooks35 in their treatment of oriented slightly asymmetric tops.

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(37) Townes, C. H.; Schawlow, A . L. Microwave Spectroscopy; McGraw-Hill: New York, 1955. ( 3 8 ) Meerts, W. L.; Dymanus, A. J . Mol. Spectrosc. 1972, 44, 320. (39) Reinartz, J. M. L. J.; Meerts, W. L.; Dymanus, A. Chem. Phys. 1978, 31, 19.

1

I

1000

500

+

+

'

(3)

which can be used to normalize the oriented signals at different collision velocities. Equation 3 is valid in chemiluminescence detection only if radiation is collected over all directions in the absence of strong polarization and correlation effects. C. Rotational Coupling Effects. Most polyatomics contain a t least one atom with nuclear spin, I , which couples with J to give a total angular momentum F = J I , J I - 1, ..., IJ- I1 rather than J . M becomes M F ,the projection of angular momentum on a chosen direction. Other electronic and vibrational motions can also couple with r ~ t a t i o n , ~strongly ' affecting the purity of orientation. For example, 1 doubling in N 2 0 and A doubling in NO, serve to mix wave functions corresponding to and - values of 1 or A, respectively. In NO the A-doubling effect is small compared to nuclear hyperfine coupling,38 but in N 2 0 1 doubling dominates hyperfine interaction^^^ ( I = 1 for I4N,0 for I6O). Considering N 2 0 first, an applied electric field acts via the first-order Stark effect37to decouple the 1 doublets, producing in the strong field limit a pure 1 = f l state. The nuclear spins are also fully decoupled from the molecular framework at this voltage. Because of the high hexapole field strengths, rotational decoupling is immediate, although J is assumed to remain a good quantum number. On leaving the hexapole fields the guiding (orientation) field voltage should maintain (saturate) nuclear spin and other rotational decoupling. Experimentally, low orientation field voltages are desired to avoid discharges and to allow rapid noise-free switching from parallel to antiparallel field directions for efficient signal averaging. The rotational properties of N 2 0are well-known from molecular beam electric resonance studies by Reinartz et al.,39allowing a prediction of the orientation field voltage dependence of (cos yo). Figure 7 plots the experimental dependence on orientation field strength (V/cm) of the steric effect, (Iheads - Ztails)/Z0, for the Ba oriented N,O BaO N 2 reaction. A lowering of N 2 0 orientation due to recoupling of rotational motions at lower voltages appears as a smaller steric effect. Models of the field strength inside the harp region yield E(actual) in V/cm = 0.91 V (applied) in volts/3 for a 3 cm long harp field. Following ref. 37 the resulting orientation (cos yo) for Stark uncoupling of I doubled levels is given for the ( l , l , l ) J,I,M level as

+

(If4")/10

1

Figure 7. The decoupling of the I doubled level ( l , l , l ) as a function of the Stark energy. The BaO* chemiluminescent intensity of a favorable orientation minus an unfavorable orientation divided by nonoriented molecules is plotted versus the calculated electric field on the harp plates. The solid curve is fitted through the experimental points (eq 4) and has an asymptotic value at E,, = 0.10 eV given by the dotted line.

Nuclear hyperfine effects are dominant in NO. Since the 'II3/2 electronic state is focused (j= 3/2), I = 1 yields four F (=J+I) states, F = 3/2,1/2,-1/z,-3/2, further split by A doubling which is 100 times smaller than nuclear spin coupling in NO.40 Fortunately, a simple transformation from the high field (uncoupled) limit ( 5 kV) to zero applied fields is possible and a plot similar to Figure 7 yields orientation as a function of the applied orientation field voltage. Altkorn, Zare, and Greene4' treat nuclear spin coupling effects on molecules aligned via laser techniques. Studies of vibrationally excited laser-aligned hydrogen halide reactions with metal atoms42 suffer from hyperfine coupling, which is mitigated by the Stark effect in hexapole orientation studies. Maintenance of hyperfine d e ~ o u p l i n gas~ ~ the focused molecules enter the orientation field region depends on the spin structure of selected species and is a topic of continued interest. D. Detection of Scattering. A number of steric studies of chemiluminescence reactions have appeared in the past few years, Ba N20,34,46,47 and Ca* CH3F using e.g., N O 03,40,44,45 hexapole orientation, or Ca* HC14*and Xe* IBr49using laser alignment methods. In most cases the signal strength permits at best rough analysis by narrow bandpass filters, although in the N

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+

+

Van Den Ende, D.; Stoke, S . Chem. Phys. 1984, 45, 121. Altkorn, R.; Zare, R. N.; Greene, C. H. Mol. Phys. 1985, 55, 1. Karney, 2.; Estler, R. C.; Zare, R. N. J. Chem. Phys. 1978,69, 5199. Gordy, W.; Cook,R. Microwave Molecular Spectra; Wiley: New York, 1984. (44) Van Den Ende, D.; Stoke, S . Chem. Phys. Lett. 1980, 76, 121. (45) Van Den Ende, D.; Stoke, S . ; Cross, J. B.; Kwei, G. H.; Valentini, J. J. J . Chem. Phys. 1982, 77, 2206. (46) Jalink, H.; Harren, F.; Van Den Ende, D.; Stolte, S . Chem. Phys. 1986, 108, 391. (47) Jalink, H.; Parker, D. H.; Meiwes-Broer, K. H.; Stolte, S . J Phys. Chem. 1986, 90, 552. (48) Rettner, C . T.; Zare, R. N. J . Chem. Phys. 1981, 75, 3636. (49) De Vries, M. S.; Srdanov, V. I.; Hanrahan, C. P.; Martin, R. M. J . Chem. Phys. 1983, 78, 5582.

Parker et al.

5432 The Journal of Physical Chemistry, Vol. 91, No. 21. 1987

+

laser excited Ca(’P) HCl (and Cl,, CC14) s t ~ d y spectral ~ ~ v ~ ~ resolution of the chemiluminescence was possible. One drawback to chemiluminescence is that several overlapping electronically excited product states may be produced. An advantage over surface ionization detected is that the full rather than the differential reaction cross section is sampled. Chemiluminescence can provide internal state information and the alignment of excited product molecules via polarization analysis. Polarized emission from BaO* has been measured in the reaction of oriented N,O plus Ba by using the apparatus shown schematically in Figure 6. The configuration of lenses and diaphragm allows light to be collected only from the scattering volume in this crossed beam reaction, resulting in elimination by spatial filtering of chemiluminescence from long-lived (>10 ps) excited states. Only the short-lived A(’Z+) state is detected, which avoids some of many ambiguities in the emission of this difficult molecule. Figure 6 also shows a device for rotating a Polaroid film polarizer by 60’ steps for repetitive measurement of the emission anistropy, directly providing the angle and degree of polarization. This three-point scheme circumvents assuming the angle of k, which was found to coincide with the angle of polarization by calibrating the reference position of the Polaroid sheet with the known Ba bulk NzO polarization angle. An unpolarized light source produced by luminescent paint (“timex”) shaped to the size of the scattering area was lowered into the reaction zone as shown in Figure 6 to confirm the lack of any polarization selectivity in the optics train. Results for this correlated reactant orientation-product alignment measurement are given in section IV. Laser-induced fluorescence (LIF) has been used to probe the OH internal state distributions produced in intramolecular oriented C02.31 Reactions producing alkaline earth reactions of H halides are amenable to LIF using stable, high-resolution C W dye lasers. LIF of nascent CaBr from the Ca + CF3Br reaction using a C W laser has recently been ~ b s e r v e d . ~ CF3Br ’ can also be oriented by hexapole focusing. Both LIF and laser ionization52 provide information on the alignment of product species. With pulsed lasers, however, it is difficult to avoid saturation effects which can complicate the extraction of alignment information. Zare and c o - ~ o r k e r shave ~ ~ ,addressed ~~ alignment determinations using LIF including the effects of saturation for several representative cases of product polarization. A more general treatment of saturation effects on alignment information from LIF has recently been p r e ~ e n t e d . ~Zare ~ and co-workers have also described a general experimental scheme for extraction of population and alignment data using (1 1)56 and (2 1)57REMPI. Electrostatic fields can probe the orientation or alignment of products from unoriented reactants, with the classic example by Herschbach and co-workers on CsI from the reaction of Cs with CH31.58 Hexapole fields have recently been used to roughly determine the orientation of CHF3 leaving a metal surface.59 Laser ionization has been used in a similar study to determine the rotational alignment of N2 molecules scattered from Ag surfaces.60

+

+

+

+

IV. Results A . Vector Properties. The oriented molecule reactions studied so far are “simple” atom transfers: A BC AB + C. For such systems four vector directions, the initial and final relative velocity vectors k and k’, and the reactant and product molecules’

+

-

(50) Rettner, C. T.; Zare, R. N. J . Chem. Phys. 1982, 77, 2416. (51) Janssen, M. H. M., private communication.

(52) Parker, D. H. In Ultrasensitive Laser Techniques: Kliger, D. S., Ed.; Academic: New York, 1983, p 233. (53) Greene, C. H.; Zare, R. N. J . Chem. Phys. 1983, 78, 6741. (54) Altkorn, R.; Zare, R. N. Annu. Rev. Phys. Chem. 1982, 33, 119. (55) Janssen, M. H. M.; Parker, D. H.; Stolte, S. Chem. Phys. 1987, 113, 357. ( 5 6 ) Jacobs, D. C.; Zare, R. N. J . Chem. Phys. 1986, 85, 5457. (57) Kummel, A. C.; Sitz, G. 0.;Zare, R. N. J . Chem. Phys. 1986, 85, 6874. (58) Hsu, D. S. Y.; McClelland, G. M.; Herschbach, D. R. J . Chem. Phys. 1974,61,4927. Case, D. A.; Herschbach, D. R. Mol. Pkys. 1975, 30, 1537.

(59) Novakoski, L. V.; McClelland, G. M., submitted to Phys. Rea. Lett. (60) Kummel, A. C.; Sitz, G . 0.;Zare, R. N. J . VUC.Technol. A , in press.

-

TABLE I: Survey of Experimental Directional Property Measurements for AB C A BC Reactions”

+

reactants Cs

+ CH31

K + CFJ Rb + CH31 Ba + N 2 0

+

correlation

ref

(k,k‘,j‘) (jW‘) (jkk’) (j,k,j’)

58 80 15 72

k and k’ refer to the initial and final relative velocity vectors and j and j’ to the initial and final rotational angular momentum vectors,

respectively. rotational angular momentum vectors j and j’, can be specified experimentally.20 Measurement of correlations between different sets of these vectors can help to characterize the collision dynamics. A well-known example is the two-vector correlation (k,k’), Le., the product angular distribution, Z(O), of eq 1, which helps distinguish direct and long-lived collisions and forward versus backward scattering.’ Reactive atom-molecule scattering implies an azimuthally unsymmetric “birefringent lens” potential energy surface which induces anisotropies and correlations among k, k’, j, and j’. With any scattering geometry k’ and j’ measured separately are azimuthally symmetric about k, but measurement of a j, j’, k’ pair simultaneously with respect to k can regain information on the azimuthal asymmetry of the reaction. Herschbach and co-workers have measured the three-vector correlation (k,k’,j’) in the reaction of Cs and CH31 and found a strong correlation of the k’ and j’ azimuthal orientations about k when k’ and j’ were determined j’ was found to lie preferentially s i m ~ l t a n e o u s l y . ~Basically, ~ perpendicular to the plane containing k and k’, a behavior as~ r i b e dto~strong ~ repulsion between the CsI and CH, reaction products. Table I surveys three-vector correlation measurements, all of which have employed inhomogeneous fields. This includes the above (k,k’,j’) experiment which used a quadrupole field (second-order Stark effect) to determine the alignment of the (CsI) product at one scattering angle in a (k,k’),(k’,j’) simultaneous measurement. The three-vector correlation corresponding to the reverse reaction, (j,k,k’), has been measured for several oriented molecule reaction systems, with hexapole (first-order Stark effect) orientation, of the reactant j with respect to k while product angular distributions, (k,k’), were simultaneously measured. In addition, the three-vector correlation (j,k,j’) has recently been measured by using hexapole orientation of j with respect to k while product rotational angular momentum alignment of j’ with respect to k was measured via the product emission polarization. In some cases such as K CHJ no correlation was observed; in others such as NO O3the orientation dependence reflects branching between two reaction geometries (known from the (k,k’) measurement). The Ba + N,O reaction shows, however, a very distinctive (j,k,j’) correlation. B. Representation of Reactivity. To deconvolute steric data COS yo) is usually represented as a Legendre moment expansion about cos y o as

+

+

m

COS yo) =

C((2J

n=O

+ 1)/(2n + l))o,C,,Pn(cos

yo)

(5)

where C, is the nth expansion coefficient for the selected rotational state (eq 2 ) , and P, is nth Legendre polynomial. Usually only ol/oo and u z / u o , the dependence on reactant orientation and alignment, can be extracted from the data. For a single (J,K,M) state the favorable, unfavorable, and side-on signals and Io, the unoriented signal, yield (e.g., for the l , l , l state) ol/uo = (Z(0.5) - Z(-0.5))/Zo and 0 2 / u o = 5(Z(0.5) + I(-O.5) - 2.0Z0)/Z0. If polarized chemiluminescence is measured, eq 5 expressed a(cos and a(cos yo)* for polarization detected parallel (11) and perpendicular (I) to k, individually. The two are normalized as a total collision cross section for chemiluminescence by d c o s Yo) = ~ll(COSYo) + 2o,(cos Yo) owing to the cylindrical symmetry about k.

(6)

Dynamics of Molecular Stereochemistry

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987 5433

TABLE 11: Survey of Quantitative Steric Measurements reactants

+ + NO + 0, Ba + N,O C a * + CH3F Rb CHJ K CFJ

quantity aR(Bc,,, gR(Be.m. .. ....

(ec.,,,

= 180) = 150) = 30)

u,,(0.71 eV) 0:;(0.14 e v j (0.08 eV) ~ ~ ~ ( 0 eV) .30

ul/uo

q/u0

1.53

0.3

2.8 -3.0 0.23 0.65 1.o 0.4

-0.06

ref

29 80 40 47 47 this paper

C . Recent Results of Oriented Molecule Crossed-Beam Scattering. C.1. Alkali Metals plus Alkyl Halide Reactions. The reactions of alkali metals with oriented methyl iodide were reported over 20 years ago by Brooks and Jones3 and by Beuhler, Kramer, and B e r n ~ t e i n .Large ~ differences were found in production of KI or RbI from heads versus tails collisions, a seminal observation since at the time it was thought that reorientation would equate all precollision orientations. More refined measurements in 1982 on the Rb CHJ reaction have been reviewed.8 Figure 1 is one result of a recent analysis of the scattering data.I5 These later experiments lacked single state resolution, collisional energy variation, and detailed product information, but owing to the well-understood nature of this reaction6' they are still quite informative. Classical trajectory studies sensitive to the methyl iodide orientation have been along with simple modeling of angle-dependent reaction barriers, e.g., Figure 2c, for this system.'" Reorientation during the collision has also been recently addressed for this system on a macroscopic Coarse recoil distributions were also reported for oriented CHJ RblS and interestingly, ul/uo was found to decrease from 1.5 at &,,, N 180' to -0 at.O,,; N 90". This may be due to a slight correlation of the recoil distribution with initial orientation; however, higher quality data are needed to further examine this aspect. The steric reactivity moments measured at the backscattering angle for this reaction at 0.13 eV collision energy are listed in Table 11. C.2. Alkali Metal plus Trifuoroalkyl Halides. Brooks has long emphasized measuring recoil distributions of products from oriented reactants, in studies of K plus a number of alkyl halides and trifluoroalkyl halide^.^ These experiments were sensitive to possible (j,k,k') correlations. As in the R b C H J (k,k') measurements, product velocities were not obtained. Brooks et have also investigated the azimuthal steric dependences of K CF31by orienting the molecule along the direction of the detector instead of along k. Recently, they have reported refined orientation measurements using SI detection of the reaction of K with CF31 and CFjBr64(without rotational state resolution or variation of the collision energy). These systems are more challenging than the alkyl halide reactions in that two products, K F and KBr or KI, are possible (and not distinguished by SI), and in addition, the direction of the dipole and thus the heads vs. tails end of the molecule is not independently known. A convincing argument settled both of these questions: K F is not believed to form, and the "heads" end is at the heavier halogen-the assigned recoil distributions are shown in Figure 8. The lower panel plots the KBr product recoil distributions for K reacting with heads and tails oriented CF3Br and the upper panel plots the KI product distribution from reaction with heads and tails oriented CF31.

+

b) K t

+

+

+

(61) Bernstein, R. B.; Wilcomb, B. E. J . Chem. Phys. 1977, 67, 5809. (62) Blais, N. C.; Bernstein, R. B. J . Chem. Phys. 1986, 85, 7030. (63) Brooks, P. R.;McKillop, J. S.; Pippin, H. G. Chem. Phys. Lett. 1979, 66,144. (64) Carman, H.S . ; Harland, P. W.; Brooks, P. R. J . Phys. Chem. 1986, 90,944. (65) Zaremba, S . K. Ann. Mat. 1966, 4, 73. (66) Clough, P. N.; Thrush, B. A. Trans. Faraday SOC.1967, 60, 915. Gauthier, M.; Snelling, D. R. Chem. Phys. Lett. 1973, 20, 178. (67) Donnelly, V. M.; Kqufman, F. J . Chem. Phys. 1978, 69, 1456. (68) Hsu,D. K.; Monts, D. L. Zare, R. N. Spectral Atlas of Nitrogen Dioxide 5530 to 6480 A; Academic: New York, 1978. (69) Kahler, C. C.; Kowalczyk, M. J . Chem. Phys. 1986, 84, 1386. (70) Snellen, A. Internal report, Molecular and Laser Physics Group, Nijmegen. (71) uR < 0.01 A*, private communication, 1982, Valentini Cross and Kwei.

A I

1

40

80

120

1

I

160

L A B ANGLE Figure 8. (a) Laboratory angular distributions of reactively scattered

+

KBr from K CF3Br. (b) Laboratory angular distribution of reactively scattered KI from K CFJ. (0)Heads orientation; (A)tails orientation. Inset nominal Newton diagrams show the lab angles where experimental intensity is maximum.

+

+

In the K CF31reaction there is essentially a zero steric factor, assuming the lab to c.m. transformation leaves the integrated distributions intact, yet the difference in recoil between heads and tails orientations is stunning. Heads reactant orientations yield backscattered KI while tails orientations result in side-forward scattered product. This propensity is lower for the CF3Br reaction, where the tails orientation yields less product, and thus a higher steric factor. The CF3Br reaction with K is intermediate in behavior between reactions with CFJ and CH31. Invoking a "harpoon" mechanism (with caution) this behavior was explained as a long-range electron jump to CF31 which is orientation independent. Following this jump the CFJ instantly dissociates and KI product forms along the initial CFJ axis direction. The electron affinity of CF3Br is substantially lower than that of CF31 (0.9 eV compared to 1.6 eV); thus an electron jump to CF3Br is expected at much shorter distances where orientation with the Br end pointing toward the incoming K atom becomes more efficient. The recoil distributions and total reactivity are consistent with this simple harpoon mechanism. Two factors may lower the observed stereochemical behavior from that suggested by the initial orientation. First, these are reasonably large cross section reactions (17 and 1 1 AZfor CFJ and CF3Br, respectively); thus impact parameter averaging could dilute the steric effect. In addition, these molecules have substantial hyperfine structure which may lower the initially selected orientations of the low-J molecules (the experiment employed a mild supersonic expansion) in the scattering zone. The strong orientation dependence seen in Figure 8 is probably even more dramatic for optimally oriented reactants. C.3. Oriented NO Ozone NO2 + O2 Reaction. With a beam of NO molecules focused by a hexapole in the C l = J = M = 3/2 state, the steric dependence of the visible chemiluminescence

+

-

5434

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987

A

90’

60’ -

Ohv

5

4

YO

120’

’

NO + 0,

Parker et al. A2 I

Ii

-.

NO: + 0, Etrz0.71 eV

-

-

3 -

I

Figure 10. Experimental geometry for the scattering box photodetection system used to calibrate the sensitivity for NO2 chemiluminescence detection in the NO + O3reaction.

0.8

0

-0 8

NO +03 +NOz+02 Etr=0.61 eV

-

Figure 9. (a) Dependence of the chemiluminescence cross section of NO + 0, NO2+ O2+ hv on the orientation of the NO molecule; (b) c.m. contour map of NOz flux at 0.71 eV collision energy.

yield, produced by NO2* molecules by scattering with O3gas at T = 120 K, has been i n ~ e s t i g a t e d .The ~ ~ results at E,,= 0.71 eV show reaction (see Figure 9a) for two types of approach geometry. The heads-on orientation, where the 0 end of the NO molecule points along k (cos yo = 1) and the broadside-tail orientation (cos yo = -0.3) appear to be the initial angles of attack yielding NO2*. In another experiment, at the Los Alamos laboratory, Valentini and c o - ~ o r k e r smeasured ~~ the anglevelocity distribution of NO2, produced by the same reaction with a universal crossed-beam machine. By use of seeded nozzle beams narrow velocity distributions of the parent beams were obtained. A c.m. contour map has been constructed by performing a one-Newton diagram transformation of the laboratory anglevelocity distribution using The resulting c.m. the most probable velocities for N O and 03. map for the recoiling NO2 product (Figure 9b) shows that the reaction is direct, with two modes for reaction. A narrow backward-scattered NO, peak and a broad sideways-scattered NO, lobe appear to be clearly separated,45 as is shown in Figure 9b, which represents the original recoil distribution, slightly symmetrized. (The azimuthal symmetry of the c.m. system around k is somewhat distorted by the limited accuracy of the one-Newton diagram transformation and has been altered by slightly shifting the recoil angle of the backward recoil peak.) The bimodal structure of the reaction differential cross section d3uR/d2wd b o 2 for the unoriented N O reactants in the c.m. recoil

map of Figure 9b has been associated with the bimodal steric opacity function of the chemiluminescence plotted in Figure 9a. The heads-on orientation, cos yo = 1, is believed61 to yield backward scattering, the broadside-tail orientation, cos yo = -0.3, sideways scattering. However, inspection of the steric opacity functions4 represented in Figure 9a shows that 30% of all chemiluminescence from unoriented N O molecules comes from the peak at the heads orientation cos yo = 1 whereas only 9.2% of all reactivity (as follows from numerical integration over the contours of Figure 9b) stems from the backward-scattered NOz peak. This discrepancy is addressed and resolved in the Appendix. C.4. Ba plus Oriented N,O Reaction. Chemiluminescence from BaO* produced in the highly exoergic reaction of Ba + N 2 0 has been intensively investigated (see ref 46 for references). We have detected BaO* emission in reactions of Ba + single rotational states of N 2 0 oriented by hexapole focusing. The dependence of this chemiluminescence on the internal and translational energy of hexapole state-selected but unoriented N 2 0 has been studied46 and indicates a small but significant contribution of rotational energy to the overall reactivity and a very large (factor of 4) contribution by the v2 bending mode. This reaction is believed to occur without an harpooning elecron jump.46 The translational energy dependence roughly confirms there is essentially no reaction threshold. Orientation studies have been reported that include the translational energy dependence of the steric effect47 and the dependence of the BaO* chemiluminescence p ~ l a r i z a t i o non ~ ~the N 2 0orientation. In these experiments an effusive Ba beam from a resistively heated oven ( T = 1000 K) crosses at 90’ a stateselected (J,I,M = 1,1,1 or 2,1,2) N 2 0 beam produced by the hexapole apparatus of Figure 4. Figure 6 shows the emission collection optics which, as discussed in section 111, are mainly sensitive to the short-lived A-state emission (lifetime = 350 ns). The “harp” orientation fields also shown in Figure 6 provide the reference field direction for the heads, tails, side-on, and unoriented collision geometries. Variation of the translational energy was carried out by heating the quartz nozzle source and using different mixtures of carrier gas. The degree of polarization was determined by the three-point rotation device of Figure 6 for each collision geometry. Figure 3 shows the range of precession angles of the two states selected for study (1,1,1 and 2,1,2). Note the favorable geometry contains a sizeable fraction of unfavorable and vice versa. In Figure 11 the intensity of the favorable, unfavorable, and side-on collisions, divided by I, (eq 3), for nonoriented N,O, are plotted as a function of the average translational energy, E,,,for the ( l , l , l ) state. Results for the (2,1,2) state will be reported later.73 More emission is produced by the favorable than the unfavorable orientation. As expected, the orientation effect decreases with increasing translational energy as the angle-dependent barrier, V(cos yr), is overtaken. A simple line-of-centers function for V(cos yr) incorporated in an ellipsoidal steric provided a tentative fit to the data of Figure 11 by assuming a cutoff angle, cos yc (angle where the barrier to reaction becomes infinite, see Figure (72) Jalink, H.; Parker, D. H.; Stolte, S. J . G e m . Phys. 1986, 85, 5 3 7 2 . ( 7 3 ) Jalink, H.; Parker, D. H.; Stolte, S. to be submitted for publication.

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987 5435

Dynamics of Molecular Stereochemistry

1

rill0

1

Yo Peg] 150

I '

'

120

90

60

I

I

I

0 (cosYo)/oo Ba + N2 0 (n2=J =1) O8

30

-'

1

P,(COSYo)

-

B a O t N2 Etr=0165eV

-+\

I

-01

\

i= f avourablo

t-

-

\

1

04

0

0.08

0.10

0.12

016-

0.14

Et,

2c) of -0.6 f 0.1. From the orientation effect we concludei6the sign of the N 2 0 dipole is "'0-, which was not previously settled. The dependence of ul/u? on the orientation field voltage was shown in Figure 7 which IS used to correct the data shown in Figure 11 for lack of saturation. These data were also corrected for misalignment of the orientation field direction with k as the collision energy was varied. The chemiluminescence polarization was measured for the three experimental orientations and incorporated via eq 6 to give COS yo), the dependence of the perpendicular and parallel polarized chemiluminescence on the reactant orientation. The two curves are plotted in Figure 12 where they are normalized to uo = uIl+ 2u,, the orientationally averaged cross section for total emission. The plot shows that for unfavorable orientations the polarization ( P = (u,,- u1)/(uI, uL)) is small but increases for a favorable orientation. These two curves and the fact that a parallel transition is observed yield the alignment, P,(cos yo), of the rotational angular momentum, j', of BaO* as

+

which is also plotted in Figure 12. Apparently the reaction tends to have a preference to take place in the collision plane more strongly for the heads orientation. This is not kinematically induced as in the H X A AX + H type reactions, even though the alignment is approaching the limit of P2 = -0.5 typically observed in the L H H systems.12 For the tails orientations there is less preference to react in the plane, perhaps because passage along the sweeping (and vibrating in the v2 mode) N-N tail is required. The dependence of this correlation on the collision energy which should shed more light on this fascinating reaction will be reported later.73 C.5. Cu* plus Methyl Fluoride Reaction. Chemiluminescence assigned to C a F A(211) and B(Q) is observed34when an atomic Ca beam in which a discharge is struck impinges on CH3F gas.

+

-

I

I

I

-05

0

05

1

-04

cos Yo

0.18

Lev1

Figure 11. BaO* chemiluminescent intensity of a favorable orientation ((cosyo) = 0.5),a side-on orientation ((cos y o ) = 0), and an unfavorable orientation ((cos yo) = -0.5) divided by the chemiluminescent intensity for nonoriented molecules, Io, as a function of the average translational energy, E,, where cos yois the average initial angle of attack on the NzO molecular axis by Ba.

+

-I

-

Figure 12. The measured steric dependence of uIl(cosyo)and u,(cos yo) divided by uo = ull+ 2u,, the orientationally averaged cross section for total emission, is displayed by the two solid curves. At the lower and upper horizontal axes yo is the initial angle of attack between the molecular axis of N,O and the relative velocity between Ba and NzO. The average alignment P2(c0s yo) of the angular momentum of BaO* is calculated from these two curves (eq 7) and IS displayed by the dotted line. We have recently succeeded in detecting this chemiluminescence by crossing a focused, state-selected (J,K,M = l , l , l ) and oriented beam of CH3F (the focusing curve is shown in Figure 5c) with a metastable Ca beam using the apparatus shown in Figure 4 and 6. Modifications of the machine included adding a discharge and ion deflection electrode over the effusive metal oven and high rejection narrow bandpass filters centered on the Av = 0 vibrational bands of the A and B states (603 and 535 nm, respectively) in the optics train. In spite of the strong Ca* discharge lines these bandpass filters isolated the very low CaF* chemiluminescence signals sufficiently to observe an orientation effect. Preliminary analysis yields a value for uI/uo = 0.4 for this reaction at 0.30 eV collision energy, detected through A(211)-state chemiluminescence. Ca('D) is believed responsible for production of CaF* in the reaction with CH3F, in analogy with reactions of metastable Ca with CH,Cl, studied by Dagdigian and c o - ~ o r k e r s .Engelke75 ~~ has analyzed the nascent emission spectrum of CaF* formed in reactions of Ca* with F2, and Engelke and Meiwes-Broer have studied nascent CaF from reactions with HF, F2, and NF376using LIF. CaF* is a much better behaved emitter than NOz* and BaO*, and the reaction mechanism may also be less complex. The oriented C H 3 F + Ca* reaction may yield, hopefully, a straightforward analysis of the steric effect. More complete details will be reported later. V. New Directions and Summary A good deal of simplification will result from detection of ground electronic state products rather than the presently common (74) Furio, N.; Campbell, M. L.; Dagdigian, P. J. J . Chem. Phys. 1986, 84, 4332. ( 7 5 ) Engelke, F. Chem. Phys. 1979, 44, 213; 1979, 39, 279. (76) Engelke, F.; Meiwes-Broer, K. H. Chem. Phys. t e f f . 1984, 108, 132, Z . Phys. A 1985, 320, 39.

5436

The Journal of Physical Chemistry, Vol. 91, No. 21, 1987

chemiluminescent species, which almost always represent minor reaction channels. L I F or REMPI of products from oriented reactants will provide especially valuable information on product internal states and polarization along with steric cross sections. A healthy increase (toward -500 Hz) in the repetition rates of pulsed tunable dye lasers may be necessary to make up for the low signal levels of oriented molecule experiments. An exciting prospect of controlling chemical reactions via the steric effect has been suggested. The reaction of Mg with BrCN is known77to produce two products, MgBr and MgCN. BrCN when vibrationally excited in the vz = 1 mode can be oriented by using hexapole fields, and by simply switching the orientation field voltage, selection of the product channel may be possible.7s Similar effects may also be occurring in the reaction of Xe* with laser-depletion-aligned IBr.49 Rotationally state-selective hexapole focusing of the COz laser excited u3 = 1 vibrational state of CH3F (Figure 5) has been d e m ~ n s t r a t e d .Preliminary ~~ results for the reaction of Ca* with the ground vibrational state of oriented and single rotational state-selected CH3F were reported in section IV, the next step will be comparison of cross sections and orientation effects with the vibrationally excited beam. Other types of scattering processes involving oriented or aligned molecules are being investigated. REMPI studies of oriented NO scattering from surfaces are ~nderway,’~and similar measurements of the orientation and alignment of molecules leaving a surface have been described. In addition, angular distributions of photoelectrons produced by vacuum UV ionization of aligned molecules have already been reported and similar studies of multiphoton dissociation probes of aligned molecules are in progress.78 One conclusion of the few oriented molecule reactions studied so far is that the collision geometry strongly affects many properties of reaction. Correlations are already observed with orientation and both the product recoil angle and angular momentum alignment. These observations do not support the assumption of reorientation toward a unique transition state which then passes on to a unique recoil distribution. Chemistry is more complicated-the steric mosaic is not simple to uncover. Our tools are improving and with the growing number of clever hands brushing away at the cover, a better view will emerge. Acknowledgment. We thank Professor Richard Bernstein for many helpful discussions, Professor Rafi Levine for the invitation to the workshop, Mr. A. Snellen for carrying out some of the opacity calculations, Mr. M Janssen for critical remarks, Mr. M. Geijsberts for assistance in taking the focusing curves, and Mr. B. Peeters for calculations of the “harp” field strengths. We gratefully acknolwedge the NATO Collaborative Research Grants Programme for support.

Appendix That the detected fractions corresponding to the two reaction pathways in the NO O3 reaction do not have to be the same can be understood by considering that, in the Nijmegen chemiluminescence experiment, all kinetic energy was provided essentially by the NO beam (k E Y I J ~= 2695 m/s) and compared with k, the 0, gas ( T = 120 K) is practically standing still. This means that for this case the origin of the lab system is nearly coincident with the tip of the arrow of the L’, vector in Figure 9b; Le., the backscattered NOz*products recoil in the laboratory much slower than the sideways-scattered NOz*. Thus, because of the rather long radiative lifetime of the NO2* ( 7 = 100-200 ps) the backward-scattered product will yield a larger photon signal than the sideways-scattered products which escape more rapidly from the light collection zone. To investigate this explanation quantitatively an attempt has been made to calculate from the recoil map of Figure 9b the fraction of chemiluminescence originating from the backscattered

+

(77) Pasternack, L.; Dagdigian, P. J. J . Chem. Phys. 1976, 65, 1320. (78) Bernstein, R. B., and co-workers, work in progress. (79) Kuipers, E. W.; Tenner, M. G.; Kleyn, A,; Stolte, S., work in progress. (80) Brooks, P. R. Faraday Discuss. Chem. Soc. 1973, 55, 299.

Parker et al. peak, I h u ( B ) / l h v , where I),, stands for the detected signals at the photomultiplier output (counts/s). As a first step in this calculation the photon collection probability at the photomultiplier (pm) detector has been determined experimentally. The scattering box (see Figure 10) has been r e p l a d by a point light source, consisting of a green-light-emitting diode (LED). To prevent blinding of the pm, both the gain voltage (now used in the analog current mode) and current through the LED are greatly reduced, resulting in a brown emission by the LED. This calibrating source was mounted on the tip of a slender rod which could be moved in the i Q , i directions by means of translation stages. The light collection zone appeared to extend 14, 18, and 4.2 mm in the 2$,i directions (see Figure lo), respectively. The smaller range along the z axis, restricting the vertical dimension of the collection zone, originates from the rectangular photocathode of the RCA C3 1034 pm tube. Its long side was directed parallel to the horizontal x axis (see Figure 10). In order to determine the light collection efficiency quantitatively over the whole volume, the x dependence (Le., along the beam axis) of the LED-pm signal has been measured for 34 “Zaremba” integration points,65probing the range of -6 mm < z < 6 mm, -20 mm < y < 20 mm. For all pairs of sampled b,z) (Zaremba) points the collection efficiency of the LED appeared to possess a dependence on x which could be characterized as having the shape of a symmetric trapezoid. The center position, x,, full width at the top, and at half-maximum, and the maximum light signal a t x, were obtained as the four characteristic parameters. The maximum light signals were absolutely calibrated by comparing them with the signal obtained by placing the LED at the origin and adding a space angle limiting circular diaphragm (6 mm diameter) in front of the lens of our optical system (see Figure 10). In this way a calibrated collection space angle of 2.8 X lO-’Sr was obtained after interpolation between the “Zaremba” supporting functions for all points (x,y,z) within the scattering volume, allowing an estimate of the absolute light collection efficiency (equal to unity when the accepting space angle has its (unachieveable) maximum of 4a). To simplify the remainder of the calculation we make use of the cylindrical symmetry of the laboratory reactive cross-section, d3a/d2w ,)~ dWNO,, induced by the d3u/d2QdUNO, = ( U N ~ , ) / W N O beam gas configuration. Thus we average over all azimuthal angles @ in Figure 10 and define an absolute collection efficiency (P(8,r) for NO2*molecules originating uniformly from a “line” source consisting of the part of the x axis within the scattering box, scattered over the recoil laboratory angle 0, and emitting a photon after a straight-line trajectory over distance r (see Figure 10). Geometrically averaging over all possible recoil angles 8 yields a maximum geometric collection efficiency of 0.023 at r = 0, of 0.011 at r = 6.75 mm, 0.004 at r = 17 mm, and 24 mm. Returning to the reactive scattering process itself it is now possible to formulate an expression yielding the chemiluminescence signal I),,., with a indicating a region of backward, sideways, or total reactive recoil, as detected by the pm from the 03-filled scattering box:

In this expression the scattering strength of the N O molecules entering the scattering box and qo,, the number density of O3 molecules are proportionality factors to be considered later. From the contour maps in Figure 9b one extracts in arbitrary units d3aRa/dZudwNO2,and multiplying with ( u ~ ~ ~and / with w ~2 a~ ~ ) ~ to account for the integration about the laboratory azimuth a, one obtains an expression proportional to the laboratory flux of NOz molecules scattered with a velocity uNO2 and under an angle 8.The internal energy Eh, available for the NOz product molecule has been calculated by considering conservation of energy Eint

= ADO +

tEtr

- E’tr)

(‘4.2)

J. Phys. Chem. 1987, 91, 5437-5441

where, since disposal of energy into the O2product was not found," Ehtis set equal to the sum of the exoergicity, ADo = 2.08 eV, and translational collision energy, Et, = 0.61 eV, minus the disposed product recoil energy Et;

= 1/2WN02mN02(mN01+ m0,) /mO2

As the solid circle in Figure 10 shows, for most of the NO2 molecules one has E,,, > 1.6 eV; Le., highly excited NOz* is believed to be the main product. Donnely and Kaufmand' provide a simple empirical connection between the radiative lifetimes, q, induced by perturbative coupling in electronically excited states.@' For Eint> 1.6 eV we take lifetimes according (also slightly extrapolated to the blue) to ref 69 and calculate the frequency distribution of chemiluminescence (eq A l ) to be a simple 6function; that is, all internal energy of the NO2 molecule is put into a visible photon. Another more realistic red-shifting model assumes the pattern of thermal emission, f,(v) = ( 4 / ~ , ) ( v / v i ) ~ having hvi as the total available internal energy. For hvi < 1.6 eV, no visible photon can be emitted. The resulting chemiluminescence spectra assuming the v3 pattern of thermal emission agreed quite well with those observed6' at somewhat lower values of Etr. Denoting q ( u ) the quantum efficiency specified by the manufacturer of the red-near-infrared-sensitive pm RCA C31034 ( ~ ( u )= 0 for X > 900 nm) we are able to calculate the photon

5437

yield at the detector, i.e., jh(v)q ( v ) du equals the absolute detection efficiency of a photon hitting the pm and originating from a NO2 molecule produced with internal energy E;. Now one can carry out the integration of eq A1 numerically. As a result we obtained for the ratio of chemiluminescence originating from backward scattering and total yield'O of 25%, in good agreement with our experimental observation of 30% in Figure 9b. Inserting the estimated INO, the density of O3(4 X lo4 Torr), and the interaction length = 3.8 cm, and the observed photon signal yields uhu= 0.003 A*, a surprisingly small value, supported also by the value obtained by Valentini et al.71 for uR. Summarizing, cross sections for the two very distinct reaction geometries indicated by chemiluminescence from oriented N O scattering are in quantitative accord with the yield of the two product distributions detected by recoil velocity measurements for the ground-state (unoriented) reaction. Single rotational state selection with full control of rotational coupling was possible in the orientation study at one collision energy. Polarization of the chemiluminescence considering the beam-gas collision geometry is expected to be small and was not detected. The steric moments for this reaction, listed in Table 11, are reasonably large, considering the unoriented ozone collision partner. Registry No. NO, 10102-43-9; 03, 10028-15-6; N20, 10024-97-2; Ba, 7440-39-3; CH3F, 593-53-3; Ca, 7440-70-2.

Oriented Molecule Beams: Focused Beams of Rotatlonally Cold Polar Polyatomic Molecules Suketu R. Gandhi, Qi-Xun Xu, Thomas J. Curtiss, and Richard B. Bemstein* Department of Chemistry, University of California, Los Angeles. California 90024 (Received: January 12, 1987)

Pulsed, supersonic beams of polar polyatomic molecules are focused and oriented via the electrostatic hexapole technique. Essentially pure rotational state selection has been achieved for all prolate symmetric tops. Examples are shown for CH3F and CH3CN, with fully resolved states IJKM) = 11 1, 212, 313, etc. Partial resolution (with some overlapping of peaks) is obtained for oblate tops CF3H and CC13H. Rotationally structured (but not fully resolved or analyzed) focusing curves are presented for (CH3)3CC1and H3CCC13,for the asymmetric tops CH2C12,CH3N02,and CD30D, and for NH3. Pure rotational state selection has been achieved for OCS, presumably via the /-doubling effect in the first excited bending vibrational state. Sharp focusing of BrCN is also observed, implying the availability of oriented cyanogen halides for future crossed-beam reactive asymmetry experiments.

Introduction Since 1965,' there has been considerable interest in oriented molecule beams as a tool for the study of steric effects in chemical reaction dynamics.2-10 An overview of the field is presented by (1) Kramer, K. H.; Bernstein, R. B. J . Chem. Phys. 1965, 42, 767. (2) Brooks, P. R.; Jones, E. M. J. Chem. Phys. 1966,45, 3449. ( 3 ) Beuhler, R. J., Jr.; Bernstein, R. B.; Kramer, K. H. J. Am. Chem. Soc. 1966, 88, 5331. (4) Parker, D. H.; Chakravorty, K. K.; Bernstein, R. B. J . Phys. Chem. 1981, 85, 466. Chem. Phys. Lett. 1982, 86, 113. (5) Van den Ende, D.; Stolte, S. Chem. Phys. Lett. 1980, 76, 13. Chem. Phys. 1984, 89, 121. (6) Jalink, H.; Parker, D. H.; Meiwes-Broer, K. H; Stolte, S. J . Phys. Chem. 1986, 90, 552. (7) Carman, H. S.; Harland, P. W.; Brooks, P. R. J . Phys. Chem. 1986, 90, 944. (8) Jalink, H.; Janssen, M.; Harren, F.; Van den Ende, D.; Meiwes-Broer, K. H.; Parker, D.; Stolte, S. in Recent Advances in Molecular Reaction Dynamics, Vetter, R., Vigut, J., Ed.; CNRS: Paris, 1986; p 41. (9) Jalink, H.; Harren, F.; van den Ende, D.; Stolte, S.Chem. Phys. 1986, 108, 391. (10) Jalink, H.; Parker, D. H.; Stolte, S. J . Chem. Phys. 1986, 85, 5372.

0022-3654/87/2091-5437$01.50/0

Stolte and Parker elsewhere in this issue," so the present work is confined to recent experimental results of the UCLA group. A full paper with complete documentation of the new oriented molecule beam machine and detailed experimental results therefrom is in preparation.I2 A preliminary account with initial results showing pure JKM rotational state selection for methyl halides has been published e1~ewhere.l~ Following the early work on focusing and orientation of symmetric-top molecules1J4 via their first-order Stark effect, using the electrostatic hexapole technique, Jones and Brooks demonstrated the applicability of the hexapole technique to asymmetric tops.15 Others have followed, utilizing the electric hexapole as a beam focuser for a variety of polar molecules.'6-20 (11) Stolte, S.;Parker, D. H. J. Phys. Chem., this issue. (12) Gandhi, S.R.; Curtiss, T. J.; Xu, Q.-X.; Bernstein, R. B., manuscript in oreaaration. 71:) Gandhi, S.R.; Curtiss, T. J.; Xu, Q.-X.; Choi, S. E.; Bernstein, R. B. Chem. Phys. Lett. 1986, 132, 6. (14) Brooks,P. R.; Jones, E. M.;Smith, K. J. Chem. Phys. 1969,51,3073. (15) Jones, E. M.; Brooks, P. R. J . Chem. Phys. 1970, 53, 55.

0 1987 American Chemical Society