J. Phys. Chem. 1996, 100, 4729-4733
4729
A Direct Determination of the Direction for the Transition Dipole Moment by the Polarized Laser Photolysis of the Oriented CH3I Beam Hiroshi Ohoyama, Tetsuya Ogawa, Hiroshi Makita, Toshio Kasai,* and Keiji Kuwata Department of Chemistry, Faculty of Science, Osaka UniVersity, Toyonaka, Osaka 560, Japan ReceiVed: March 8, 1995; In Final Form: January 4, 1996X
A beam-intensity-depletion (BID) method to determine the direction of the transition dipole moment in the molecular frame was demonstrated in the polarized laser photolysis of the oriented CH3I beam. The depletion of the primary beam intensity of oriented CH3I via the photodissociation with linearly polarized laser light was measured as a function of molecular orientation and the polarization angle of the polarized laser light. The (X-A) transition of CH3I in the |111〉 rotational state was probed with linearly polarized laser light at 248 nm. The orientational dependence of the BID indicates that this transition is predominantly (96 ( 4%) a parallel-type transition at this wavelength. This result is in good agreement with the previous result deduced by the angular distribution of the photofragment, showing the usefulness of this direct method.
Introduction Studies on photodissociation provide information about structures and potential energy surfaces of excited states of molecules. The angular distribution of the photofragment indicates the correlation between the polarization and the dissociation vector, and it is governed by the direction of the transition moment in the molecular frame and dynamics on the excited state potential surface. Information about the direction of the transition moment allows us to identify excited states. The dissociation dynamics are of interest as a so-called halfcollision in the excited states of molecules. In general, the direction of the transition dipole moment was deduced by the angular distribution of the photofragment from the randomly oriented molecule. In most cases, the axial-recoil approximation has been used in analysis, where the angular distribution was expressed in terms of the “anisotropy parameter”, β.1 Though this approximation is appropriate for the transition to a shortlived excited state, in the case where β has a different value from the ideal case (e.g., β ) 2 for a parallel transition and β ) -1 for a perpendicular transition), it is difficult to distinguish whether the difference in β implies the existence of a mixed parallel-perpendicular transition or the lifetime of the excited state, because the anisotropy parameter originates from both the direction of the transition dipole and the dynamical factor, e.g., rotation of the molecule on the time scale of the dissociation.2 For the transition to a long-lived excited state (or the predissociation case), especially, the angular distribution becomes isotropic and provides no information. The use of a molecule oriented in space offers an opportunity to perform photodissociation experiments on an anisotropic ensemble of molecules without the loss of information about dissociation by averaging over an isotropic ensemble. For example, the orientation of the molecules allows the direction of the transition moment to be separated from the dynamical recoil distribution in the molecular frame. Therefore, even in the case where the angular distribution becomes isotropic, the direction of the transition moment with respect to the molecular axis can be probed without influence from any dynamical factor. In the present study, we demonstrate a new method to determine the direction of the transition dipole moment in the molecular frame by measuring the orientational dependence of X
Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-4729$12.00/0
the beam-intensity depletion (BID) of an oriented molecular beam by irradiation using linearly polarized laser light. This new method was applied for the (X-A) transition of the benchmark system of CH3I. CH3I is one of the few molecules whose photodissociation dynamics has been extensively investigated both by experiment3,4 and theory.5-7 Photofragment anisotropy measurements have indicated that the photodissociation originates from the dominant parallel transition to 3Q0 that experiences a succeeding surface crossing to the 1Q branch for both I(2P3/2) and I*(2P1/2) states.3,4 The anisotropy parameters have been proposed to be β(I) ) 1.44 ( 0.04 and β(I*) ) 1.52 ( 0.043 from van Veen et al., and the more ideal value β ) 1.95 was proposed by Barry and Garry4 via a forward convolution method. The orientational dependence of the BID indicates that this transition is predominantly a parallel-type transition (96 ( 4%) at this wavelength and gives the anisotropy parameter β ) 1.9 ( 0.1 on the basis of the axial-recoil approximation. This result is in good agreement with the photofragmentation result3,4 deduced by the angular distribution of the photofragment under the axial-recoil approximation, showing the usefulness of this direct method. Method The experimental geometry is schematically shown in Figure 1. The orientational angle θ is defined as the angle of the molecular axis R against the electric field direction E. The linearly polarized laser light propagates in the direction at a right angle to the electric field. The polarization angle φ is defined as the angle between the laser electric vector and the orienting electric field E. The absorption probability of the light is proportional to the square of the scalar product of with the transition dipole µ, i.e., |‚µ|2. Since the transition dipole is tilted by an angle R against the molecular axis, and is tilted by the angle φ against the space axis (the same direction of the orientation field), the use of a molecule oriented in space gives a definite relation between the transition dipole µ and the electric field vector through the electric field. Therefore the excitation yield by the laser light depends on the polarization angle φ. After averaging |‚µ|2 over all azimuthal angles and orientational angles θ, weighted by the orientational distribution of the molecule in space, the excitation yield by the linearly polarized laser light, or BID, is determined by eq 1.8 © 1996 American Chemical Society
4730 J. Phys. Chem., Vol. 100, No. 12, 1996
Ohoyama et al.
2 IOR ) x4πσb00 c0 + c2P2(cos φ)P2(cos R) 5
[
]
(1)
where cn are the expansion coefficients of the orientational distribution in terms of Legendre polynomials Pn(cos θ). The orientational distribution of the molecular axis for the oriented molecule was expressed as follows,8,9 N
I(cos θ) ) ∑cnPn(cos θ)
(2)
n
The excitation yield with randomly oriented molecules is expressed as IRAN ) x4πσb00c0 for I(cos θ) ) 0.5 (i.e., cn ) 0(n > 0)). Therefore, the relative excitation yield, defined as R ) IOR/IRAN, is expressed in the following equation:
R(φ,R) )
2c2 IOR (φ,R) ) 1 + P (cos φ)P2(cos R) IRAN 5c0 2
Figure 1. Experimental geometry and the definition of the symbols: R, molecular axis; E, orientation field; , electric vector of polarized laser light; µ, transition dipole moment; φ, polarization angle of laser light; θ, orientational angle of the molecule in space; R, angle between R and µ.
The actual polarized light having a degree of polarization fp ) Ip/(Ip + Is), the above equation is rewritten as follows:
R(φ,R,fp) ) 1 +
2c2 [f P (cos φ) + 5c0 p 2 (1 - fp)P2(cos(φ+90°))]P2(cos R)
Thus the angle R can be determined by the φ-dependence of the relative excitation yield. For the pure |111〉 state, the orientational distribution is expressed as the square of the wave function Ψ(cos θ) and is given by the next equation (see Figure 3b).10
3 1 I(cos θ) ) 0.5 + P1 (cos θ) + P2(cos θ) 4 4 Finally R(φ,R,fp) is given by eq 3.
R(φ,R,fp) ) 1 + 0.2 [fpP2(cos φ) + (1 - fp)P2(cos(φ+90°))]P2(cos R) (3) The excitation yield can be obtained in two ways. One is the direct detection of the net excited species populated following excitation or the change of the original species. Another is the detection of the fragment species produced by photodissociation. For the purpose of the general application of the wide range of molecules, in the present study, we employ the detection of the BID of the original beam by use of a mass spectrometer. That is, the excitation probability for the photodissociation process can be determined as a BID intensity of the original beam due to the succeeding dissociation. Under oriented and randomly oriented conditions, the BID intensity was measured as a function of the polarization angle of the polarized laser light. Experimental Section Figure 2 schematically shows the experimental apparatus used in the present study. The CH3I beam was produced by a 500 µs pulsed supersonic expansion of 2% CH3I seeded in Ar at the stagnation pressure of 200 Torr. A single rotational state selection of the |111> state of CH3I was performed by using a 2 m electric hexapole field.11 To perform single state selection at the area irradiated by the laser light, the focusing curve was measured by sampling the beam intensity with time intervals of 6 µs (corresponding to the 3 mm irradiation spot diameter) in the middle of the beam profile (300 µs). The rod voltage
Figure 2. Schematic view of the experimental apparatus.
was scanned from 0 to 8 kV in steps of 0.05 kV. The stateselected CH3I beam traveled through the 45 cm long guiding field (400 V cm-1) and oriented in the orientation field (800 V cm-1), whose direction was set perpendicular to the axis of the laser light. Random orientation of the molecules was achieved by reducing the guiding field strength to zero. KrF excimer laser light (248 nm) was polarized using a 10plate Brewster angle stack polarizer, which transmits 45% of the primary laser beam and provides 95% linear polarization. The polarization ratio was experimentally determined by measuring the reflected light intensity from a quartz plate set at the Brewster angle, as a function of the rotation angle of the polarizer. The power of polarized laser was monitored by a pin photodiode. The typical intensity of the linearly polarized light was ∼25 mJ/pulse. The linearly polarized light (25 mJ) was focused on the oriented CH3I beam in the |111> state within the 3 mm area, and it photodissociated the oriented CH3I beam. The laser irradiation in the orienting field causes the BID for a time width of 6 µs. The excitation probability for the (X-A) transition of CH3I determined as the BID due to such photodissociation. The beam intensity was measured by use of a quadrupole mass spectrometer through a 1 mmφ pinhole mounted 51 mm downstream from the light irradiated point. The typical pressure of the main chamber during the experiment is ∼1 × 10-7 Torr. The beam intensity was measured by a gated pulse-counting method. With oriented and randomly oriented CH3I molecules, BID was measured as a function of the polarization angle of the polarized laser light (i.e., φ ) 0°, 54.7°, 90°). To reduce the background signal of the mass spectrometer, the signal intensities under three experimental conditions, S1 (beam off, laser off), S2 (beam on, laser off), and S3 (beam on, laser on), were successively measured in the same molecular beam pulse. The pulse sequence is illustrated in Figure 2, where BID is defined by BID ) (S2 - S3)/(S2 S1). To switch the orientation conditions, the guiding field was switched on and off every 500 beam pulses. The data under
Direction of the Transition Dipole Moment
J. Phys. Chem., Vol. 100, No. 12, 1996 4731
Figure 4. Intensity of the reflected light from the quartz plate set at the Brewster angle as a function of the rotation angle of the polarizer. (Open circle) Experimental; (solid line) calculated reflected light intensity for the 95% polarization.
Figure 3. (a) Focusing curve of CH3I in V0 ) 1.0-4.0 kV. (Open circle) Experimental; (solid line) contribution of the |111〉 state in the focusing curve calculated by Monte Carlo simulation including secondorder stark. The peak at 2.1 kV is assigned as the pure (>98%) |111〉 rotational state. (b) Orientational distribution of the molecular axis in space for the |111〉 rotational state.
the two conditions were accumulated up to 1 × 105 times to obtain a better signal-to-noise (S/N) ratio. Results and Discussion Figure 3a shows the observed focusing curve of the CH3I beam in the V0 ) 1.0-4.0 kV region, and the calculated contribution of the |111〉 state (the solid line) is shown in the focusing curve. The detail on the focusing curve is described elsewhere.12 From the calculation, the first peak at 2.1 kV was found to be the |111〉 rotational state of CH3I with >98% purity. The orientational distribution for the pure |111〉 state is given in Figure 3b for the case of full decoupling of the nuclear spin. It has been reported that CH3I suffers from a strong hyperfine interaction, and relatively high field strengths (about 1000 V cm-1) are needed to fully decouple the nuclear spin from the rotational angular momentum.13 Furthermore, Bulthuis et al. address the saturation of 〈p2(cos θ)〉 and calculate an imperfect alignment at a field strength of 500 V cm-1.14 Although it is difficult to estimate the precise value, an empirical extrapolation of the calculated curve of 〈p2(cos θ)〉 at 800 V cm-1 can coarsely estimate an alignment of about 90% of the perfect alignment of 〈p2(cos θ)〉 ) 0.1. According to this estimation, the imperfect alignment gives the lower value of 〈p2(cos θ) 〉 ) 0.09. This means that eq 3 becomes as follows
R(φ,R,fp) ) 1 + 0.18[fpP2(cos φ) + (1 - fp)P2(cos(φ+90°))]P2(cos R) (4) In the following discussion, we tentatively assumed the above imperfect alignment and analyzed the experimental result in the present study on the basis of eq 4. The polarization ratio of the polarized light was experimentally determined in the following manner. According to Fresnel’s law, only the s-wave component of the laser light,
Figure 5. Time profile of the BID due to laser irradiation. (Dashed line) experimental; (solid line) simulated time profile of the BID for the following experimental conditions: velocity distribution of VS ) 565 m s-1 and RS ) 35 m s-1, distance from laser irradiation point to detector ) 51 mm, Gaussian shape of the laser spot focused into a 3 mm area (standard deviation of 3 mm).
having the electric vector out of the reflection plane, is reflected by a quartz plate set at the Brewster angle. Rotation of the polarizer converts the p-wave component in the polarizer frame into the s-wave component in the laboratory frame. Thus, it is possible to determine the polarization ratio by measuring the reflected light intensity from the quartz plate set at the Brewster angle as a function of the rotation angle of the polarizer. The experimental result is given in Figure 4. The ratio fp ) Ip/(Ip + Is) was determined to be 0.95 by fitting the experimental results using the following equation:
I ) Ip sin2 φ + Is cos2 φ Figure 5 is a representative time profile of the CH3I beam depleted by the laser light accumulated over 10 000 pulses by a storage oscilloscope. To avoid the broadening of the BID peak due to time constant, each ion pulse from the mass spectrometer was amplified with a pulse width of 200 ns. Since the pulse count rate is small due to the inefficiency of ionization by electron impact in the mass spectrometer, ion pulses rarely appear in each sampling channel and this results in a reduction of the dc-averaged beam intensity at each channel. Consequently, the S/N ratio of the time profile in Figure 5 is relatively low owing to the electric noise originating from both a preamplifier and a radio frequency (rf) modulator of the quadrupole mass filter. To improve the S/N ratio, the actual BID measurement was carried out by the grated-pulse counting method. In this method, a discriminator can completely cut
4732 J. Phys. Chem., Vol. 100, No. 12, 1996
Figure 6. Relative BID intensity as functions of φ and R. (Closed circle) experimental; (lines) relative BID intensity calculated from eq 4 under the imperfect alignment effect due to the saturation of 〈p2(cos θ)〉, for four angles of R: R ) 0°, 30°, 60°, and 90°.
off such noise from the ion pulses, and the discriminated ion pulses are summed over the sampling gate to obtain a good S/N ratio. For a sampling gate width of 12 µs, the beam intensity was typically 470 counts for each set of 500 beam pulses with the standard deviation less than 4%. In Figure 5, the BID appeared 90 µs later than the laser irradiation time (corresponding to a 51 mm flight length). The intensity of the BID is ∼15% of the primary beam intensity. The laser irradiation of a 3-mm crossed beam area produces a BID in time width of 6 µs. The BID spreads due to a velocity distribution of the beam. The solid line is a simulated time profile of the BID, assuming a Gaussian shape space profile for the laser light focused into the 3 mm area. As mentioned above, the BID measurement was carried out by the gated-pulse counting method. The gate was set to a width of 12 µs at the position indicated in Figure 5. Figure 6 shows the observed relative BID intensity, R, at three polarization angles of the linearly polarized light (φ ) 0°, 54.7°, and 90°). R was found to be enhanced at parallel polarization (φ ) 0°) and decreased at the perpendicular polarization (φ ) 90°). This experimental result was compared with the one calculated by use of eq 4. The four lines in Figure 6 indicate the expected enhancement behavior of R ) R ) 0°, 30°, 60°, and 90°. The experimental result was found to be in good agreement when the transition dipole moment lies along the molecular axis with an angle of R ) 0°, indicating that the transition of (X-A) is dominantly a parallel-type transition. Absorption Ratio of 3Q0/1Q The A ˜ states of CH3I are due to 5pn f σ* excitation, and the transitions to the three component states are the allowed ones from the ground electronic state, which consist of three components 3Q1,3Q0 and 1Q in order of increasing energy. The transitions to the 3Q1 and the 1Q states are known to be perpendicular transitions and correlate to the CH3 + I(2P3/2) dissociative channel. On the other hand, the 3Q0 state is produced by a parallel transition, and it correlates to the CH3 + I*(2P1/2) channel.15 Zewail and co-workers have directly measured the lifetime of A ˜ states to be 175 fs.16 The fragment 2,3,17 and emission18 also show that the dissociation anisotropy of the A ˜ state is a direct one. For the (X-A) transition of CH3I, it has been reported that the 3Q0 state contributes 78% of the absorption strength at 248 nm, whereas the 1Q accounts for the remaining 22% by means of the MCD (magnetic circular dichroism) technique19 and theoretical calculation.5 On the other hand, the photofragment anisotropy measurements have indicated that the I atoms, formed in both the 2P3/2 and 2P1/2 states, originate from the transition involving the parallel transition moment, even though the I(2P3/2)
Ohoyama et al. state correlates to the perpendicular transition. These results have been explained as the dominant parallel transition to 3Q0 that experiences a succeeding surface crossing to the 1Q branch for both the I(2P3/2) and I*(2P1/2) states.3,4 Recently, Hammerich et al. have proposed that the formation of both I* and I products is attributed to the excitation of the vibronically active asymmetric mode which is connected to any nonadiabatic transition on the conically intersecting surfaces of Jahn-Teller systems.6 van Veen et al. have proposed that the anisotropy parameters for the CH3 fragments are β(I) ) 1.44 ( 0.04 and β(I*) ) 1.52 ( 0.04.3 Combining these values with the value of the I/I* branching ratio of 0.41 will yield an overall value for β of 1.5. Barry and Gorry have measured the angular distribution of the I fragment and reported the more ideal value of β ) 1.95 via the forward convolution method of the distribution,4 although the vibrational distribution of CH3 is extremely different from the recent experiment.20 The full state resolved experiments indicate that this picture is too simple, because the early mass spectrometric studies considered only the umbrella mode.21 The gross disagreement between the MCD data and the photofragmentation result has been recognized,7 because the MCD result forbids the large value of β (i.e., β < 1.34). The basic difference between the MCD and the other techniques is that the former probes only the first event in the photodissociation process, namely, the absorption, whereas the other methods also probe the product fragment formed after the dissociation process. The key advantage of the present experiment is to probe solely the absorption event precluding the dissociation. Therefore, one can directly probe the contribution of the perpendicular transition within the parallel transition without any influence of dynamics (e.g., the lifetime of the A ˜ state and surface crossing events) and any convolution procedure (e.g., the data transformation from the laboratory frame to the center-of-mass one) and might reveal that the decrease of β implies either the mixing of the perpendicular transition or the reflection of the dissociation dynamics. As mentioned, we can determine the ratio of parallel and perpendicular transition cross sections, σr ) σ⊥/σ|, without any influence of surface crossing events. In the present study, the dissociation processes producing the I(2P3/2) and I*(2P1/2) states were observed at the same time. In this case, the relative BID intensity is expressed by
R(φ) )
σ|IOR(φ,0°,fp) + σ⊥IOR(φ,90°,fp) σ|IRAN + σ⊥IRAN
)
1 [R(φ,0°,fp) + σrR(φ,90°,fp)] 1 + σr
Although the relative BID, R(φ), experimentally obtained seems to be consistent with R(φ,0°,fp) (i.e., σr ∼ 0, R ) 0°, and β ) 2), the present S/N ratio prevents us from determining the precise contribution of the 3Q0 state in the absorption at 248 nm. There is a possibility that a mixed parallel-perpendicular transition caused by the vibronic coupling induced by motion in an asymmetric mode6 may decrease the value of the anisotropy parameter, β, from the ideal case (β ) 2). Further improvement of the S/N ratio is required to make this point clear. On the basis of R(0°,0°,0.95) ) 1.167 and R(0°,90°,0.95) ) 0.924, our result (R(0) > 1.15) corresponds to σr < 0.08. This result indicates that the transition is characterized by a dominant parallel transition and σr is estimated to be 0.04 with the experimental error of 0.04. Since the dissociation of the A ˜ state is direct,16 the axialrecoil approximation might be suitable. Therefore, the present
Direction of the Transition Dipole Moment results can be directly compared with those of the existing studies. In this approximation, the anisotropy parameter is expresses as follows:
β)
2 - σr 1 + τσr
On the basis of this relation, our result (σr ) 0.04 ( 0.04) indicates that β ) 1.9 ( 0.1. The lower limit (β ) 1.8) is lower than the β value of 1.95 obtained by the angular distribution from Barry and co-workers but larger than β ) 1.6 reported by Veen et al. The lower limit (σr ) 0.08) is somewhat smaller than the MCD result (σr ) 0.29). This tendency may correspond to the discrepancy between the MCD data and the photofragmentation result.7 The basic difference between the MCD and the BID is that the former probes the overall absorption, whereas the BID methods probe the absorption followed by the dissociation process. Another difference is the initial distribution of the rovibrational states in the prepared sample gas. Although it is difficult to understand this disagreement, it may be partly concerned with the emission process of the dissociating molecule to the ground electric state of the parent molecules18 and/or the effect of the initial rovibrational motion of the parent molecule on the vibronic coupling. Our results indicate that the (X-A) transition is predominantly a parallel-type transition at this wavelength. In summary, the principal result here demonstrates a new method to determine the direction of the dipole moment by using an oriented beam, suggesting the usefulness of the present technique to investigate the photoexcitation process in longlived states and predissociation states. Acknowledgment. This work was supported by a Grantin-Aid on Priority-Area-Research “Photoreaction Dynamics” from the Ministry of Education, Science, and Culture, Japan (06239239). References and Notes (1) Zare, R. N. Mol. Photochem. 1972, 4, 1.
J. Phys. Chem., Vol. 100, No. 12, 1996 4733 (2) Yang, S.; Bersohn, R. J. Chem. Phys. 1974, 61, 4400. Jonah, C. J. Chem. Phys. 1971, 55, 1915. Busch, G. E.; Wilson, K. R. J. Chem. Phys. 1972, 56, 3638. Kable, S. H.; Loison, J.-C.; Neyer, D. W.; Houston, P. L.; Burak, I.; Dixon, R. N. J. Phys. Chem. 1991, 95, 8013. Beswick, J. A. Chem. Phys. 1979, 42, 191. Mukamel, S.; Jortner, J. Chem. Phys. Lett. 1974, 29, 169. Siebbeles, L. D. A.; Schins, J. M.; Los, J. Phys. ReV. Lett. 1990, 64, 1514. (3) van Veen, G. N. A.; Baller, T.; Vries, A. E.; van Veen, N. J. A. Chem. Phys. 1984, 87, 405. (4) Barry, M. D.; Gorry, P. G. Mol. Phys. 1984, 52, 461. (5) Guo, H.; Schatz, G. C. J. Chem. Phys. 1990, 93, 393. (6) Hammerich, A. D.; Manthe, U.; Kosloff, R.; Meyer, H.-D.; Coderbaum, L. S. J. Chem. Phys. 1994, 101, 5623. (7) Hwang, H. J.; El-Sayed, M. A. J. Phys. Chem. 1992, 96, 8728. (8) Taatjes, C. A.; Janssen, M. H. M.; Stolte, S. Chem. Phys. Lett. 1993, 203, 363. Choi, S. E.; Bernstein, R. B. J. Chem. Phys. 1986, 85, 150. Zare, R. N. Chem. Phys. Lett. 1989, 156, 1. (9) Stolte, S.; Chakravorty, K. K.; Bernstein, R. B.; Parker, D. H. Chem. Phys. 1982, 71, 353. (10) Choi, S. E.; Bernstein, R. B. J. Chem. Phys. 1985, 83, 4463. (11) Kasai, T.; Fukawa, T.; Matsunami, T.; Che, D.-C.; Ohashi, K.; Fukunishi, Y.; Ohoyama, H.; Kuwata, K. ReV. Sci. Instrum. 1993, 64, 1150. (12) Ohoyama, H.; Ogawa, T.; Kasai, T.; Kuwata, K. J. Phys. Chem. 1995, 99, 13606. (13) Gandhi, S. R.; Curtiss, T. J.; Bernstein, R. B. Phys. ReV. Lett. 1987, 59, 2951. Janssen, M. H. M.; Parker, D. H.; Stolte, S. J. Chem. Soc., Faraday Trans. 2 1989, 85, 1263. Choi, S. E.; Bernstein, R. B.; Stolte, S. J. Chem. Soc., Faraday Trans. 2 1989, 85, 1097. (14) Bulthuis, J.; Milan, J. B.; Janssen, M. H. M.; Stolte, S. J. Chem. Phys. 1991, 94, 7181. (15) Mulliken, R. S. J. Chem. Phys. 1940, 8, 382. Shapiro, M. J. Phys. Chem. 1986, 90, 3644. Tadjeddine, M.; Flament, J. P.; Teichteil, C. Chem. Phys. 1987, 118, 45. Gray, S. K.; Child, M. S. Mol. Phys. 1984, 51, 189. (16) Dantus, M.; Janssen, M. H. M.; Zewail, A. H. Chem. Phys. Lett. 1991, 181, 281. (17) Sparks, R. K.; Shobatake, K.; Carlson, L. R.; Lee, Y. T. J. Chem. Phys. 1981, 75, 3838. Black, J. F.; Powis, I. Chem. Phys. 1988, 125, 375. (18) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J. Phys. Chem. 1984, 88, 3956. Sundberg, R. L.; Imre, D.; Hale, M. O.; Kinsey, J. L.; Coalson, R. D. J. Phys. Chem. 1986, 90, 5001. (19) Gedanken, A.; Rowe, M. D. Chem. Phys. Lett. 1975, 34, 39. (20) Suzuki, T.; Kanamori, H.; Hirota, E. J. Chem. Phys. 1991, 94, 6607. (21) Chandler, D. W.; Thoman, J. W.; Janssen, M. H. M.; Parker, D. H. Chem. Phys. Lett. 1989, 156, 151. Continetti, R. E.; Balko, B. A.; Lee, Y. T. J. Chem. Phys. 1988, 89, 3383. Loo, R. O.; Haerri, H.-P.; Hall, G. E.; Houston, P. L. J. Chem. Phys. 1989, 90, 4222.
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