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
The Journal of Physical Chemistry, Vol. 91, No. 21, 1987
5438
Gandhi et al. 1
1
1
1
1
1
1
1
1
1
1
,
1
1
1
I-
t II
1 l
c 1
0
I
1
2
l
1
4
1
1
1
6
1
1
8
10
1
1
1
12
1
1
14
0
2
1
1
4
1
1
6
V,
V, / K V
l
I
8
I
I
10
I
I
12
I
l
I
14
/KV
Figure 1. Experimental "focusing curves" ("raw" data points vs. V,)
Figure 3. Focusing curves (similar presentation to that of Figure 1) for
obtained with the EI-QMS detector for pulsed, seeded beams of CH3F. Upper: CH3F/C0 beam with CH3Fvelocity u = 0.72 km sW1. Lower: CH,F/Kr, L' = 0.66 km s-l.
CF3H/Kr (upper) and CCI,H/Ar (lower). Velocities 0.39 and 0.43 km s-' for CF3H and CC13H,respectively.
,-. 0)
4 .3
K 3
< -?
]CH3F/Kr
q
Experimental Section A brief description of the molecular beam machine has been presented in ref 13. The present experiments employed the electron-impact ionization quadrupole mass spectrometer (EIQMS) detection scheme with gated integrator, synchronized with the beam pulse from the PSV (R.M. Jordan Co.) via a variable time delay (in the range 4-10 ms). In a few cases, the 40-ws chopper was employed for more accurate velocity selection, but for most experiments the PSV pulse was used without further chopping. There were a few minor changes in the experimental conditions (vs. those of ref 13), including the size of the PSV orifice, skimmer, collimators, etc., but these are only of secondary concern here.
4
K
. ... ... ..* I
I
Recently, Stolte and co-workers showed that it was possible to take advantage of the first-order Stark effect associated with the /-doubling in the excited bending vibrational state of the linear molecule NNO to obtain rotational state selection and orientation of ~ ~ 0 . 9 In what follows, we present structured "focusing curves" for pulsed, supersonic, seeded (as well as neat) beams of a number of polar polyatomic molecules (which are considered possible oriented molecule reagents in reactive asymmetry experiments), showing a range of focusing characteristics. In some cases, essentially pure JKM rotational states are obtained; in others, only focusing and orientation with less well-defined ensembles of states. The results are indicative of the power and limitations of the electrostatic hexapole technique.
I
VOp/mv2
1
I
I
(Arb.
I
I
I
I
I
Units)
Figure 2. Semilog, composite presentation of focusing curves; abscissa is the "reduced" voltage V o / ( m u 2 / p ) The . upper and middle curves are replots of the data of Figure 1 for CH,F; the lower one is for CH3CN/Ar; L; = 0.71 km s-]. The dipole moments taken for CH3Fand CH3CN are 1.86 and 3.91 D, respectiveIy.*laRotational state assignments are as in ref 13.
(16,) Butkovskaya, N. I.; Larichev, M. N.; Leipunskii, I. 0.;Morozov, I . I.; Tal Rose, V. L. Chem. Phys. 1976, 12, 267. Chem. Phys. Lett. 1979, 63, 375. (17) Butkovskaya, N. I.; Morozov, I. I.; Tal'Rose, V . L.; Vasiliev, E. S. Chem. Phys. 1983, 79, 21. (18) Kaesdorf, S.; Schonhense, G.; Heinzmann, U. Phys. Reu. Letr. 1985, 54, 8 8 5 . (19) Kasai, T.; Ohashi, K.; Ohoyama, H.; Kuwata, K. Chem. Phys. Lett. 1986, 127, 5 8 1 . (20) Novakoski, L. V.; McClelland, G. M. Phys. Reu. Lett., in press.
The Journal of Physical Chemistry, Vol. 91, No. 21, 1987 5439
Oriented Molecule Beams I
I
I
l
l
1
I
I
1q
CF, H / K r
I
I
1
1
1
1
I
1
12212
: -." 1111, . .-.- .'. .. lZz1> . " .... .13222 . ..,?;. ..-.......:. .... ;'..." 1222> . -" -. .... 1333> .: . ..'*:.1331>: .. --. ....".. ........ . . . . . . . .-.... .- . ...." .. . .-. 1222> l l l l > 1211> I
1332,
.
-e
',
3 8
*.
*
9 .
.
"
I.
I
I-.
.*
*"
I
,
\-
.................."---9 .
"
. . . . . . . ..
t1 tt
5-
I I
I
I
l
l
I
1
1
I
I
I
(Arb.
V0p/mv2
I
I
l
l
1
1
1
.:.
1.........f.
c-
.
1'*: .."..**-',....* 1
Units)
t
-
*.
1
1
1
1
1
1
1
1
1
l
l
I
I
Figure 4. Semilog, composite presentation (as in Figure 2) of data of Figure 3. Dipole moments taken for CF3H and CC13H are 1.65*la and 1.04 D,21brespectively. (See text regarding state assignments.)
3 K
r"
alE
t
m
I
D -
.
L
L -
-
c
............. . . I
I
I
I
I
l
l
l
l
(21) (a) Gordy, W.; Cook, R. L. Microwave Molecular Spectra; Wiley: New York, 1984. (b) Reinhart, P. B.; Williams, Q.;Weatherly, T. L. J . Chem. Phys. 1970,53, 1418. (c) Tanaka, K.; Ito, H.; Harada, K.; Tanaka, T. J . Chem. Phys. 1984, 80, 5893.
Gandhi et al.
The Journal of Physical Chemistry, Vol. 91, No. 21, 1987
5440
I
1
I
I
I
l
l
1
NH, / N e a t
. r t
; I
.
‘i
A
I
(0
3
r.
3
f
I?I
ri t
OCS/Ar
*:
f
3
3
i L
6
Ik
...
J.
.
x
4
c
.. -..-. .:..: ..
NH,/Ar
.3
v)
c
U 3 ’? f QI
I..
a.
*
3
c
._:..- :.
1 .
*
.*
Y
i“-‘*
‘. ...f,. c.
N
I
.. .. ...
Z
t
I
I
2
I
I
I
5
i
..
+
I
I
I
I
I
I
10
I
l
l
1
I
I
I
I
(Arb.
V,/v2
V, / K V Figure 7. Semilog plot of focusing data for neat NH3 beam (upper), u = 1.2 km d;and for a seeded NH3/Ar beam (lower), u = 0.82 km s-l. c
l
l1
Units)
Figure 9. Semilog, “reduced”plots of focusing curves for OCS, with the
reduced abscissa variable Vo/u2(see text). Upper: OCS/Ar, u = 0.45 km s-I; lower: OCS/Ar, u = 0.66 km s-I. (See text for discussion of peak assignments).
k
I
I
I
I
I
i
l
l
1
.. ..-. .-..
BrCN/Kr
r
i
15
t L
c
V,/KV
Figure 8. Chart recorder plot (analog “raw” data acquisition mode) of focusing of a beam of OCS/Ar, u = 0.45 km s-l. (See text for discussion
x
of state assignments for the three peaks.)
cusing curves show structure but are not well-resolved. Figure 6 shows structured focusing curves for the asymmetric-top molecules CH2C12, CH3NO2,and CD,OD. Figure 7 is a semilog plot of the focusing of N H 3 at two beam conditions. Some structure is observed. Figure 8 is a plot of raw experimental data on the focusing and state selection for OCS. The curve resembles the focusing curves of the prolate symmetric tops. This behavior is quite analogous to that observed b e f ~ r e for , ~ the linear molecule N N O . State selection was achieved via the first-order Stark effect, due to the I-doubling in the first excited bending vibrational state, designated (0 1‘ 0 ) .
cn-
c
m
3 -
K
l-4
+
Z 0 -
L
m
1 1
:.;.
I
*
: * , .
a
:
a...:.
,
1
1
,
1
;
,
*
.
2
5
10
14
Figure 9 is a composite, semilog “reduced”p1ot of two other focusing curves for OCS, using the reduced abcissa variable Vo/v2 appropriate for focusing via the first-order Stark effect, in accord with ref 9.
Figure 10. Semilog plot of focusing data for BrCN/Kr (upper), ~i = 0.57 km s-’, and BrCN/Ar (lower), u = 0.45 km s-I.
(22) Focusing, orientation, and reactive scattering of a similar compound, t-C4H91.has been reported by Marcelin, G.; Brooks, P. M. J. Am. Chem. SOC. 1975, 97, 1710.
Based on the known dipole moment of the (010) vibrational state of OCS, namely 0.70 D,21cand the observed values of Vo/v2 for the first two peaks, they are readily assigned to the rotational
V, / K V
J . Phys. Chem. 1987, 91, 5441-5445 states IJIM) = 1111) and 1212), as for N20.9 However, there is a problem in assigning the relatively intense third peak in the focusing curves (consistently appearing with greater intensity than that of the 1111) and 1212) peaks), although a number of speculative explanations come to mind. Further work on OCS is in progress. Figure IO is a semilog plot of two focusing curves for BrCN. Because of the complexity of the Stark effect and the hyperfine problem for this molecule, no analysis of the structure in the focusing curve has yet been carried out. Further work on the cyanogen halides is in progress.
5441
BrCN suggests that beams of oriented cyanogen halide molecules can be used for crossed-beam reactive asymmetry experiments, Such experiments are now in the preliminary planning stage.23
Acknowledgment. This research has been supported by N S F Grant No. C H E 83-16205, hereby gratefully acknowledged. The authors express their appreciation to Dr. S. Stolte of the Katholieke Universiteit, Nijmegen, for many valuable suggestions and informative discussions. R.B.B. acknowledges additional financial support, in eonnection with participation in the workshop (sponsored by the Institute for Advanced Studies of the Hebrew University of Jerusalem) from the US-Israel Binational Science Foundation. Thanks are also due to Prof. R. D. Levine and the Fritz Haber Center of the Hebrew University for organizing this exciting Workshop on Dynamical Stereochemistry.
Concluding Remarks As reported earlier,I3 intense pulsed beams with essentially pure (>95%) rotational state selection have been obtained for prolate symmetric-top molecules. Partial state resolution [with some inevitable overlapping of peaks due to their K M / ( p + J) degeneracies] is achieved in the case of oblate tops. Structured focusing curves are obtained for a variety of other polar polyatomic molecules, indicative of the potential for orientation of such molecules (even in the absence of knowledge of the state distribution in the oriented molecule beam). The strong focusing of
Registry No. CH3F, 593-53-3; CH3CN, 75-05-8; CF3H, 75-46-7; CC13H, 67-66-3; (CH3)3CCI, 507-20-0; H3CCC13, 71-55-6; CH2C12, 75-09-2; CH3N02, 75-52-5; CD,OD, 81 1-98-3; NH3, 7664-41-7; OCS, 463-58-1; BrCN, 506-68-3. (23) Xu, Q.-X.;Bernstein, R. B., work in progress.
Molecular Beam Study of Steric Effects in the Reaction K -t- HF ( v = I , j =2) - K F H
+
Manfred Hoffmeister, Rudiger Schleysing, and Hansjurgen Loesch* Fakultat fur Physik, Universitat Bielefeld, 4800 Bielefeld 1 , West Germany (Received: January 26, 1987)
-
H F molecules were optically aligned by use of linearly polarized infrared radiation generated by a color center laser tuned KF + H was to the R l ( l ) line of HF. The state-specific integral cross section of the reaction K H F (v = 1, j = 2) measured for the two different preparations of the approach geometry which result when the plane of polarization is positioned either parallel (uII)or perpendicular (uI) to the most probable relative velocity of the reagents. At a translational energy of E,, = 0.46 eV the resulting relative difference of the cross sections (uIl- u,)/a ( 5 = 1/2(u11 + uI)) amounts to 17 5%. This steric effect decreases with rising E,, and reaches zero near 1.2 eV. The data strongly suggest that the reaction occurs via a collinear transition state.
+
*
+
-
+
-
I
+
Experimental Details Molecular Beam Apparatus. The experiments were performed with a crossed molecular beam machine. Figure 1 shows a schematic diagram of the experimental arrangement. The K and the HF beams are generated by nozzle sources and intersect perpendicularly. The beams are collimated by conical skimmers which lead to an angular spread of 1.6O and 1.2O for H F and K, respectively. The velocity distributions of both beams are precisely
( I ) (a) Choi, S. E.; Bernstein, R. B. J. Chem. Phys. 1986, 85, 150. (b) van den Ende, D.; Stolte, S. Chem. Phys. 1984,89, 121. (c) Brooks, P. R. Science 1976, 193, 11, and papers cited therein. (2) (a) Zare, R. N. Mol. Phozochem. 1972, 4, 1. (b) Bersohn, R.; Lin, S. H. Adu. Chem. Phys. 1969, 16,67. (c) Ling, J. H.; Wilson, K. R. J . Chem. Phys. 1976,65, 881. (3) Hefter, U.; Ziegler, G.; Mattheus, A,; Fischer, A,; Bergmann, K. J . Chem. Phys. 1986, 85, 286. (4) Zare, R. N. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 422. (5) de Vries, M. S.;Srdanov, V. I.; Hanrahan, C. P.; Martin, R. M. J. Chem. Phys. 1982, 77, 2688. 1983, 78, 5582.
0022-3654 ,I87 ,12091-5441$01.50/0
-
In this paper we present the first results of an experimental KF investigation on steric effects in the reaction K H F H. The polarization (alignment) of reagent molecules was prepared optically by excitation of the v = 1, j = 2 state of HF ( u , j denote the vibrational and rotational quantum numbers) by using linearly polarized infrared radiation. We have measured the relative state-specific integral reaction cross section for the two different preparations of the approach geometry which result when the plane of polarization is positioned either parallel l u l l )or perpendicular (ui) to the most probable relative velocity Vof the reagents. The relative cross section difference (all - .,)/a (a = 1/2(u11+ aL)) was determined as a function of the reagent translational energy E,, in the range 0.44 I E,, I 1.23 eV.
Introduction The probability that a reaction is initiated in a bimolecular collision depends not only on the energy of the reagents but also on their relative orientation. The most direct information on these geometric or steric properties of a reaction can be obtained from molecular beam experiments with polarized (oriented or aligned) molecules. The majority of such investigations exploit hexapolar fields' to generate molecular polarization but there exist for this purpose also powerful optical methods such as photodissociation,2 saturated optical pumping,3 and excitation of ro-vibrational states by infrared radiation4 In reactive scaitering these optical techniques were applied as yet only in two studies: one on Xe* IBr XeBr*(I) + I(Br)S and the other on S r + HF S r F + He6
(6) Karny, Z.; Estler, R. C.; Zare, R. N. J. Chem. Phys. 1978, 69, 5199.
0 1987 American Chemical Societv -
