J. Phys. Chem. 1995,99, 4360-4363
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Two-Dimensional Imaging of the Photolysis of Oriented Molecules J. W. G. Mastenbroek, C. A. TaatjesJ K. Nauta, M. H. M. Janssen, and S. Stolte" Laser Centrum en Faculteit der Scheikunde, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands Received: October 31, I994@
The angular distribution of fragments produced from the photolysis of oriented methyl iodide molecules, prepared in a single IJKM) state by hexapole selection, is state selectively detected by two-dimensional ion imaging. For the simple, direct, photolysis of methyl iodide at 266 nm, the angular distribution reflects the shape of the prepared oriented symmetric top wave function. Analysis of the CD3 rotational product state distribution from photolysis of CD31(JK= 11) indicates a propensity for conservation of the initial K quantum number and a minor excitation of rotational angular momentum around the figure axis resulting from the excited state dynamics.
The investigation of the simplest kind of chemical reaction, photodissociation, has been of great importance in the elucidation of intramolecular forces and determination of potential surfaces. However, study of initial quantum state effects in photolysis is still underdeveloped. Using the technique of electric hexapote selection, it is possible to create essentially single-state beams of symmetric top molecules which can be oriented in Oriented-molecule photolysis contains inherently more information about the dissociation dynamics than photolysis of an initially isotropic ensemble of molecule^.^*^ The reduction in averaging over the initial rotational states and the initial spatial orientation allows a detailed examination of the molecular frame dissociation dynamics when using stateselected and oriented parent molecules. By combining hexapole state selection with ion imaging detection we can detect state-specific angular distributionsof photofragments from oriented molecules. It is thereby possible to image the photolysis of a single quantum state. For the simple photodissociation of oriented methyl iodide (CH31 or CD31) at 266 nm, the fragment angular distribution reflects the shape of the initially prepared wave function. The great progress which has been seen in reaction dynamics over the past decades has been due largely to the increased state specificity that has become possible in the study of chemical reactions. True initial state selection in photodissociation studies had previously been limited to predissociation,8where the initial quantum state can be spectroscopically selected, and sequential multiple-resonance ~ c h e m e swhich ,~ allow selection of energy and angular momentum states via the first absorption step. Investigation of the temperature dependence of photolysis products has also implied an effect of initial product motion on final state distributions.I0 The experiments described here use a new apparatus which allows final state-, velocity-, and angleresolved detection of fragments from oriented molecules in a single selected quantum state. The measurements are made possible by the combination of two powerful techniques: rotational state selection by hexapole fields'-3 and angular resolution of photofragmentation by ion Hexapole state selection and orientation have a long history in scattering studies, and the actual state selection and degree of orientation
* To whom correspondence
should be addressed. Combustion Research Facility, M / S 9055, Sandia National Laboratories, P.O. Box 969, Livermore, CA 9455 1-0969. Abstract published in Advance ACS Abstracts, March 15, 1995.
' Present address: @
of the molecules have been established theoretically and experimentally."-14 Imaging of half and full collision processes is rapidly developing as an extremely powerful technique to elucidate the detailed dynamics of inelastic and reactive collisions. 5,16 The mechanism of state selection by a hexapole has been previously The transmitted intensity of certain symmetric-top molecules (e.g., methyl iodide) through the hexapole changes as a function of the voltage applied to the hexapole rods, as different rotational quantum states are focused into the detection region. The dissociation of methyl iodide, CH31 or CD31, at 266 nm is an obvious choice to demonstrate the capabilities of the oriented molecule imaging method, since the photolysis has been extensively studied by other methods and has become one of the benchmark systems for the study of dissociation dynamics both theoretically and experimentally. In the present experiments we concentrate on state-selected methyl iodide with one quantum of angular momentum directed along the C-I axis ( K = 1). In Figure 1 we compare the rotational product state distribution of the vibrationless CD3 fragment from photolysis at 266 nm of a state-selected beam of CD31 parent molecules, panel a, with the results of ref 17 from photolysis of a non-state-selected cold parent beam, panel b. In the rotational distribution from focused CD31molecules (principally in rotational state I J K ) = 111)) CD3 product states with K = 1 dominate. This directly demonstrates, as was assumed in the non-state-selected experiments," a propensity for preservation of the K quantum number (angular momentum about the figure axis) from parent to fragment. However, the population distribution (Figure 1) contains more population in rotational levels with K = 2 and 3 than can be explained from residual higher rotational states in the focused beam. Classical trajectory calculation^'^,'^ have shown that rotational excitation from an initially nonrotating parent J = 0 is dominated by excitation perpendicular to the C3 figure axis, Le., AK = 0 excitation. Our experimental results indicate that this propensity prevails for a rotationally excited parent in IJK) = 111), but with minor rotational excitation around the figure axis. This provides the first direct indication of a small but significant K excitation in CD31 dissociation dynamics. A key advantage of hexapole state selection is that the molecules can be oriented in space. The angular distribution from an initially oriented photodissociation has been shown
0022-365419512099-4360$09.0010 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 13, 1995 4361
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Figure 1. Comparison of the rotational distribution in CD3 after photolyzing a state-selected IJK) = I I I ) parent (a) and the distribution from ref 17 where a cold (Taround 15 K) non-state-selected CD3I beam was used (b). Note the dominance of the (N, K = I ) levels in (a) whereas the strong modulation with even/odd N of the population in (N,K = 0) levels in (b) reflects the nuclear spin statistics in CD3. The state-selected populations are measured via the (2 + 1) multiphoton ionization magic angle spectrum of the CDj fragment (in the vibrational ground state) from photolysis at 266 nm of a IJK) = 11 I ) state-selected beam of CD3I parent molecules. Instead of collecting the spatial angular distribution of the recoiling fragments by imaging the output of the two-dimensional microchannel plate/phosphor screen ion detector with a CCD camera, the total CD? fragment signal was collected with a photomultiplier tube.
theoretically to contain more information about the molecule data, is compared with the theoretical distribution expected for frame dissociation dynamics than that from an initially isotropic a direct, parallel dissociation (/? = 2). The asymmetry of the di~tribution.~For the limiting case of a direct, prompt dissociaexperimental recoil distribution for the oriented parent is not tion such as methyl iodide at 266 nm, the shape of the fragment as pronounced as the theoretical distribution for a completely angular distribution reflects the shape of the initially prepared oriented IJKM) = 1111) parent molecule. This deviation can quantum state, convoluted with a cos2 8 term which reflects be attributed to an incomplete orientation of the initial parent the dependence of the absorption probability on the angle 8 beam because of the relatively low orienting field strength between the electric field of the photolysis laser and the (approximately 200 V/cm) used in our present setup. Methyl transition dipole of the molecule. This fact has been previously iodide suffers from strong hyperfine interaction, and relatively applied to the determination of the up-down asymmetry of high field strengths (about lo00 V/cm) are needed to fully hexapole-oriented molecules.'2.'3 Because of the angular decouple the nuclear spin from the rotational angular momenresolution of the imaging technique, we are able to probe not tum.I3-I4 Theoretical results for disorientation of CH3I agree only the up-down asymmetry but also higher-order terms of qualitatively with the experimentally measured CD3I angular the angular distribution. distributions. Experimental improvements currently underway In Figure 2 we show two images of raw data, representing will allow considerably stronger orienting fields. the angular recoil distribution projected onto a plane parallel to the polarization of the photolysis laser (along the Y direction), As discussed recently, control of the spatial orientation of which was chosen parallel to the direction of the dc electric molecules is very important for understanding the dynamics of field used to orient the CD3I molecule in IJKM) = 11 11). The reactive collisions.20 The experiments reported here represent data clearly show the asymmetric recoil distribution of CD3 a further step toward full experimental control of the initial fragments from oriented parent molecules, compared to the quantum state and spatial orientation of collision processes with image obtained from a state-selected but nonoriented beam. The complete final state-selected, angular-resolved detection. ApCD3I molecule was oriented with the methyl group pointing plication of hexapole methods to photodissociation dynamics predominantly in the positive Y direction. Because of the offers new opportunities for investigation of the effects of extremely fast dissociation (less than 100 fs), the methyl vibrational and rotational excitation on photodissociation. fragments are preferentially ejected in this direction producing Linear triatomic molecules with bending excitations can also the asymmetric angular recoil distribution seen experimentally. be hexapole-selected, and these offer extremely attractive targets The nonoriented image was obtained by grounding the guiding for study. Initial rotational state selection offers uniquely fields in-between the hexapole state selector and the laser detailed insight into angular momentum disposal in photodisinteraction region. This effectively scrambles the M quantum sociation. In addition, just as the angular distribution from a number of the parent due to the strong hyperfine interaction in simple dissociation, such as CH3I at 266 nm, reflects the shape CD31, producing an isotropically oriented parent in rotational level IJK) = 11 1) before the dissociation laser c r o ~ s e s . ~ ~ . ~ ~ of the initial quantum state, distributions from more complex photolysis processes will reflect the molecular frame dissociation In Figure 3 the three-dimensional recoil distribution, obtained by Abel-inverting the cylindrically symmetric two-dimensional dynamicss
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4362 J. Phys. Chem., Vol. 99, No. 13, 1995
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Figure 2. Images of CD3 fragments produced from the dissociation of hexapole-focused CDJ molecules, both unoriented and oriented. The IJKM) = 1111) state is selected by the hexapole. As the molecules exit the hexapole and travel to the interaction region, where they will be photolyzed, a small electric field (guiding field) must be applied to define a lab frame axis and maintain M selection. In the interaction region itself, a stronger electric field (approximately 200 Vlcm) is applied to orient the molecule in space, with the CD3 pointing preferentially in the positive Y direction. This orientation field is switched off after the photolysis pulse, and an ion extraction field is switched on in the few tens of nanoseconds between the photolysis and probe laser pulses. The switching of the fields is carried out for both the unoriented (top) and oriented (bottom) dissociations in order to ensure identical ion collection conditions. The sole difference between the two cases is that the unoriented image is taken with the guiding fields grounded, so that the M distribution is scrambled.
J. Phys. Chem., Vol. 99, No. 13, I995 4363
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Acknowledgment. The authors thank the Netherlands Organization for Scientific Research (NWO) for finanicial support through SON and FOM. The research of M.H.M.J. has been made possible by a fellowship of the Royal Netherlands Academy of Arts and Sciences (KNAW). References and Notes (1) Brooks, P. R. Science 1976, 193, 11-16. (2) Parker, D. H.; Bemstein, R. B. Annu. Rev. Phys. Chem. 1989,40, 561-595.
(3) Harren, F.; Parker, D. H.; Stolte, S. Comments Ar. Mol. Phys. 1991,
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1993, 203, 363-370. (6) Chandler, D. W.; Houston, P. L. J. Chem. Phys. 1987, 87, 1445.
~.
1 .o
1
1
I(b) Oriented parent
.“
1
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(7) Thoman, J. W., Jr.; Chandler, D. W.; Parker, D. H.; Janssen, M. H. M. Laser Chem. 1988, 9, 27-46. (8) Kable, S. H.; Loison, J.-C.; Neyer, D. W.; Houston, P. L.: Burak, I.; Dixon, R. N. J. Phys. Chem. 1991, 95, 8013-8018. (9) Crim, F. F. Annu. Rev. Phys. Chem. 1993, 44, 397-428. (10) Person, M. D.: Kash, P. W.; Butler, L. J. J. Chem. Phys. 1991, 94, 2557-2563.
Experimental
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(11) Stolte, S. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 86. (12) Gandhi, S. R.; Bemstein, R. B. Z . Phys. D 1988, 10, 179-185.
0.4
(13) Choi, S. E.; Bemstein, R. B.; Stolte, S. J. Chem. SOC.,Faraday Trans. 2 1989, 85, 1097-1113.
1”
.* +
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(14) Bulthuis, J.; Milan, J. B.; Janssen, M. H. M.; Stolte, S. J. Chem. Phys. 1991, 94, 7181-7192.
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(15) Bontuyan, L. S.; Suits, A. G.; Houston, P. L.; Whitaker, B. J. J . Chem. Phys. 1993, 97, 6342-6350. 0.0 0
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Figure 3. Recoil distribution extracted from the raw data of Figure 2 by Abel inversion of the projected image. The experimental distribution is compared to theoretical distributions assuming a prompt parallel dissociation @ = 2). Panel a shows the result for a randomly oriented parent molecule. In panel b the experimental distribution from the stateselected and oriented CD3I beam is compared to a theoretical distribution for a fully oriented CDJ parent (strong-field limit) in rotational state IJKM) = 1111).
(16) Kitsopoulos, T. N.; Buntine, M. A,; Baldwin, D. P.; Zare, R. N.; Chandler, D. W. Science 1993, 260, 1605-1610. (17) Chandler, D. W.; Janssen, M. H. M.; Stolte, S.; Strickland, R. N.; Thoman, Jr., J. W.; Parker, D. H. J. Phys. Chem. 1990, 94, 4839-4846. (18) Amatatsu, Y.; Morokuma, K.; Yabushita, S. J . Phys. Chem. 1991,
94, 4858-4876. (19) Guo, H.; Huang, Z.-H. J. Phys. Chem. 1993, 98, 3395-3409. (20) Tannor, D. J. Nature 1994, 369, 445-446.
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