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J. Phys. Chem. 1996, 100, 6376-6380
Photodissociative Pathways of C2H2 at 121.6 nm Revealed by a Doppler-Selected Time-of-Flight (a 3-D Mapping) Technique Lih-Huey Lai Department of Chemistry, National Tsing-Hua UniVersity, Hsinchu, Taiwan 30043
Dock-Chil Che† and Kopin Liu* Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10764 ReceiVed: December 11, 1995; In Final Form: January 31, 1996X
The photodissociation dynamics of C2H2 near the H-atom Lyman-R transition was investigated by a newly developed, Doppler-selected time-of-flight technique. The aim of this study is to elucidate the detailed dynamics via the directly measured fragment 3-D distribution. An alternative, preliminary analysis of a fraction of the data is presented here which already reveals a rich dynamics involved in the photodissociation. A strong ˜ 2Σ) state, a prominent C-H stretching excitation propensity against the formation of the ground electronic C2H(X ˜ ) state, and two distinct dissociation pathways being likely involved in the in the production of the C2H(A title process have been found.
Introduction The past decade has witnessed a remarkable progress in photodissociation dynamics of small molecules.1, 2 The impetus behind this widespread growth has been the development of new and innovative experimental techniques that has provided photochemists with highly sensitive tools that can be used to interrogate the dynamics of the photochemical process in an unprecedented manner. Certainly, laser spectroscopic technique is foremost among the new tools in catalyzing this progress. Techniques such as laser-induced grating,3 Rydberg H-atom TOF,4-6 Doppler-shift7-9 or ion TOF detection,10-12 2-D imaging,13-14 and their variants,15-17 just to mention a few, have all enjoyed a wide range of applications. Among these, the Rydberg H-atom TOF and the 2-D imaging technique are particularly attractive. The former can potentially yield the rotationally resolved distribution of the polyatomic coproduct; the latter, combined with multiplexed velocity detector and stateselective spectroscopic detection, has made possible the complete 3-D characterization of the photofragments from a 2-D image through proper inverse projection transformation. Reported here is the development of a multidimensional imaging variant using a 1-D detector and the preliminary analysis of the results obtained in its application to the study of the photodissociative process of C2H2. The spectroscopy and photochemistry of C2H2 in the UV and near-VUV have been extensively investigated.18 However, much less is known at shorter wavelengths. Several Rydberg series and valence states are involved in the VUV absorption spectrum. Because both C2H2 and C2H2+ are linear in the ground states, all Rydberg transitions of C2H2 exhibit a strong ν2 progression (totally symmetric CtC stretching vibration mode). Near the hydrogen Lyman-R (L-R) transition at 121.6 nm, two Rydberg states denoted as 4R (1Πu, ν2 ) 1) and 4R′′ (1Πu, ν2 ) 1) have previously been identified.19 Both bands are diffuse on account of predissociation. With 10.2 eV photon energy, at least two † IAMS visiting scholar, permanent address: Department of Chemistry, Osaka University, Toyonaka, Osaka 560, Japan. X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-6376$12.00/0
fragment channels, the formation of C2 + H2 and C2H + H, are energetically accessible. The production of C2 + H + H requires 10.3 eV (based on a recent determination20 of D0(HCC-H) ) 5.713 eV and an ab initio result21 of D0(CC-H) ) 4.6 eV) which is too close to the energetic limit to be completely ruled out a priori. This exploratory work focuses on the characterization of H-atom elimination. Experimental Approach The basic idea of our experimental approach is quite obvious: to characterize the fragment c.m. (center of mass) 3-D distribution, f(ν b), all one needs is to be able to specify the density at any given point (νx, νy, νz) in the c.m. velocity space. Operationally, this can be realized by combining three 1-D projection techniques, preferably in the c.m. frame, in an orthogonal manner. Figure 1 illustrates our method. The Doppler-shift technique is used to select a subgroup of products with the velocity component along the laser propagation axis, Z, lying between νz and νz + dνz through a REMPI process. The Doppler-selected 2-D velocity distribution is then projected onto an orthogonal axis, Y, through the ion TOF measurement. The information on the third degree of freedom, νX, can be obtained if a position-sensitive detector is employed. In this exploratory study, a nonimaging detector with a slit lying along the Z direction in front of it was used; thus, νX was restricted to νX ≈ 0 through spatial dispersion. By slicing the 3-D distribution via Doppler selection at different wavelengths, it can then be reconstructed directly through the successive TOF spectra. The experiments were conducted in a pulsed, crossed-beam apparatus detailed previously.22, 23 In brief, only a single beam was employed here. The 121.6 nm photon which served as both the photolysis and probe lasers was produced by the third harmonic generation scheme in a Kr gas cell.24 It has ca. 0.25 cm-1 bandwidth in VUV. To avoid space-charge effects and other multiphoton complications, the VUV laser beam was kept ca. 2 mm (X) × 4 mm (Y) at the interaction region and the more intense UV (365 nm) beam diverged. The polarization change of the VUV photon was accomplished by inserting a λ/2 waveplate in the UV beam path. As indicated in Figure 1, the laser was always directed along the Z axis, the polarizations, © 1996 American Chemical Society
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Figure 1. Schematics illustrating the idea of Doppler-selected TOF technique for mapping the product 3-D velocity distribution.
|| or ⊥, were referenced to the ion-TOF detection axis Y. Due to the relatively large size of laser employed, a single-stage TOF arrangement was adapted for the kinetic energy release measurement rather than the widely accepted Wiley-McLaren twostage arrangement.25 The main advantage of the present setup lies in the fact that the first-order space-focusing condition can readily be fulfilled over a wide range of extraction fields which allows for the use of an unfocused laser without energy resolution tradeoff. In addition, it gives the flexibility of performing 1-D (Doppler profile), 2-D, and 3-D (Dopplerselected TOF) measurements all in a single setup. The details of these multidimensional comparisons and the overall performance of the apparatus will be reported in the future. It should be noted that the molecular beam is directed orthogonally to both the laser propagation and ion-TOF axes (Figure 1); hence, the selections of νZ (Doppler) and νY (TOF) are essentially in the c.m. frame. Since the speed of the parent molecule is much smaller than the energetically allowed H-fragment speed, 8.8 × 104 vs 2.9 × 106 cm/s, the small correction in νX due to the parent molecule speed is neglected in the preliminary analysis to be presented below. Results and Discussion Figure 2 displays two raw TOF spectra obtained with 5.5 V/cm ion extraction field at ω0, i.e., nominally νz ≈ 0, for two different polarization configurations. Typically, several runs of 2000 laser shots were accumulated with the laser power kept constant, alternating between two polarizations in order to minimize any effects of long-term drifts. Thus, the two spectra are normalized to each other. Clearly, the ⊥ configuration yields a higher intensity than the || one. More strikingly, though the two spectra display nearly identical shape around the center, they differ substantially for the fast-moving H fragment, and in particular, some structures are clearly seen for the ⊥ polarization. These prominent structures are not only reproducible but also show up on both sides of the TOF spectra as they should. Similar structures were also revealed at wavelengths other than ω0. Though the dynamics can be more clearly shown from the full 3-D presentation of the distributions, the main dynamical features can also be gleaned using a simpler analysis based on the results shown in Figure 2. To extract the fragment c.m. speed distribution g(ν) and anisotropy parameter β from Figure 2, two problems have to be addressed. First, the instrument function needs to be accounted for which includes
Figure 2. Raw TOF spectra for two polarization configurations. The laser frequency was set at the center of the H-fragment Doppler profile, i.e., ω0.
the time-to-speed transformation and the restricted field-of-view of the detector. Second, because the L-R is a doublet split by 0.365 cm-1 which corresponds to ∆νz ) 1.34 × 105 cm/s in speed, at a given laser wavelength, Doppler selection actually samples two subgroups of H fragments from these two optical transitions. Hence, the TOF spectra displayed in Figure 2 consist of two contributions separated by ∆νz in the 3-D velocity distribution. A scheme was then devised to remove the doublet complication experimentally and to obtain the TOF spectra corresponding to a single L-R optical transition.26 After these problems have been dealt with, one can proceed with the inversion of g(ν) and β(ν) from the results. In general, the photofragment c.m. velocity distribution27 in terms of the differential cross section can be expressed as
f(ν b) )
d3σ 1 ) g(ν)[1 + β(ν)P2(cos θ)] dν dΩ 4π
(1)
where θ is the angle between the fragment recoil velocity and the polarization axis of the dissociating light and P2(cos θ) ) 1/ (3 cos2 θ - 1). It can be shown26 that the inverted ν 2 Y
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Figure 3. (a) Photofragment c.m. translational energy distribution. The arrows mark the energetic thresholds for the corresponding electronic states of the cofragment C2H. The structures are roughly equally spaced by 3100 cm-1 as indicated by the caps on top and are tentatively assigned to the C-H stretching excitations (ν1 ) 0-4) of the C2H(A ˜) state. (b) The anisotropy distribution, β(Et). Note the mild oscillation is exactly out-of-phase with the prominent structure seen in (a).
distribution from Figure 2 for a single νZ-valued selection represents
for ||-polarization: f |(νY;νZ)0,νX≈0) ≈ g(ν)[1 + β(ν)]
(2)
for ⊥-polarization:
thresholds of various electronic states of the cofragment C2H, there is very little intensity beyond the threshold for the C2H(A ˜ ) state indicating a propensity against the formation of C2H(X ˜ ) from photodissociation. In view of the strong vibronic perturbations among X ˜ and A ˜ states, this observation alone is quite intriguing. Second, the prominent structures between C2H˜ ) arrows are roughly equally spaced by 3100 (A ˜ ) and C2H(B cm-1, which is tentatively assigned mainly to the excitation of ˜ ) state. (As far as we the C-H stretching mode of the C2H(A know, the vibrational frequency of the C-H mode of A ˜ has not yet been reported.) Since the photoexcitation is into the CtC stretching of the Rydberg states of C2H2,19 the energy redistribution must proceed before dissociation. The observa˜ ) and the strong preference for C-H tions of the lack of C2H(X ˜ ) suggest that the predissociation has stretching mode in C2H(A to be relatively fast rendering the coupling between X ˜ and A ˜ states and energy randomization. Third, the β values shown in Figure 3b are all negative, indicating a perpendicular-type transition in accord with the spectroscopic assignments of the ˜ 1Σg+fRydbergs(1Πu)).19 In addiinitial excitation of C2H2(X tion, a clear dependence of β on fragment translational energy is seen, varying from about -0.05 at low energies to about ˜ ). At least two -0.75 near the energetic limit for C2H(A possibilities can result in such an energy dependence of β values. It can be attributed to a range of dissociation times on the parent molecular rotation time scale, resulting in a more effective energy redistribution and more rotational depolarization of the fragment spatial distribution.27, 28 In other words, the dissociation occurs entirely via a single pathway. Alternatively, there are in fact two distinct dissociation pathways and the energy dependence of β arises from the energy dependence of the branching ratio of these two pathways. In light of the discussion just given and the observation that a mild oscillation over the higher energy range was also seen in Figure 3b which upon closer inspection turned out to be exactly out-of-phase to the structures appearing in Figure 3a, it seems that the latter possibility is more likely to be the origin of the energy dependence of β. With that assumption and the absence of any coherent effects, eq 1 can be rewritten as
f(ν b) ) (1/4π)[g1[1 + β1P2(cos θ)] + g2[1 + β2P2(cos θ)]] (6) Accordingly, (4) and (5) become
f ⊥(νY;νZ)0,νX≈0) ≈ g(ν)[1 - 1/2β(ν)]
(3)
respectively. Hence, one has
(7)
β(ν) ) 2(f | - f ⊥)/(f | + 2f ⊥) ) χ(ν)β1 + (1 - χ(ν))β2 (8)
g(ν) ) (f | + 2f ⊥)/3 ⊥
(4) ⊥
β(ν) ) 2(f - f )/(f + 2f ) |
g(ν) ) (f | + 2f ⊥)/3 ) g1(ν) + g2(ν)
|
(5)
Figure 3 displays the results obtained from such a preliminary analysis. Note that what is measured in this experiment corresponds to the density of the distribution in the c.m. velocity space, D(νX,νY,νZ). In terms of the differential cross section D(νX,νY,νZ) ) d3σ/dνX dνY dνZ ) d3σ/ν2 dν dΩ ) f(ν)/ν2; thus, the Jacobian transformation for a distribution from per unit velocity volume element to per solid angle, as well as the transformation from H-fragment speed to fragment c.m. translational energy have been accounted for in presenting P(Et). Several interesting features in Figure 3 are worth pointing out. First, as marked in Figure 3a as arrows for the energetic
respectively, where χ(ν) ) g1(ν)/(g1(ν) + g2(ν)) denotes the branching fraction of two dissociation pathways. Since the β values range from about -0.05 to -0.75 which are close but not identical to the limiting values of 0 and -1, there is ambiguity in determining the branching ratio. Figure 4 shows the results based on that consideration. Once χ(ν) is obtained, with the aid of Figure 3a (eq 7), the individual g1 and g2 can then be inverted, as shown in Figure 4, for the two cases of β’s values. The corresponding energy-integrated branching ratios, g1/g2, are 2.3 and 1.8 in favor of the structureless, slower energy component for the cases β1 ) 0, β2 ) -1, and β1 ) -0.05, β2 ) -0.75, respectively. In either case, the two pathways are distinguished from each other by the different dissociation time scales with the faster one (compared to the parent rotational
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Figure 4. Two possible ways to partition the distributions of P(Et) and β(Et) shown in Figure 3. The numbers in the parenthesis of β value denote (β1, β2). The upper panels show the resulting branching fraction, while the lower panels display the fragment translational energy distributions of the two corresponding pathways.
motion) yielding the structured component (solid line) in Figure 4. The full analysis of this work is currently in progress. It is expected that with the availability of the directly measured 3-D distribution, more precise results can be obtained. In summary, an exceedingly simple yet powerful and versatile approach to map out the product 3-D velocity distribution has been developed. Just like many other “new” techniquessthis approach is not entirely new29sone can always trace its roots to several other precedent and elegant approaches. For example, both Doppler-shift and ion-TOF are well-established and widely used 1-D projection techniques. However, as demonstrated in this work, with proper combination and execution, the net performance can be more than additively enhanced. The quality of the data is self-evident, and the required setup is also readily affordable in almost any ordinary laboratory nowadays. In this exploratory work, we have not yet pushed the resolution of this approach to its limit. With some refinements on the apparatus, an order-of-magnitude improvement is anticipated. More specifically, it has been shown here that rich dynamical information such as the propensity against the fragmentation into the ground electronic C2H(X ˜ 2Σ) state, the prominent vibrational structures, and the identification of at least two distinct dissociation pathways, etc., are clearly elucidated even with a rather straightforward, preliminary analysis. Certainly, this approach is not restricted to the photodissociative process. In fact, vibrationally resolved angular distributions of a polyatomics product from a radical reaction have also been achieved under the crossed-beam conditions recently in this laboratory using exactly the same setup.30 Note added in proof: After this letter was submitted for publication, a communication by Wittig and co-workers appeared (J. Chem. Phys. 1995, 103, 6815) in which, using Rydberg H-atom time-of-flight technique, a propensity toward C2H(A ˜ ) in acetylene photodissociation at 121.6 nm was also found, as reported here.
Acknowledgment. The financial support from the U.S. Department of Energy and the National Science Council of Taiwan (NSC 85-2113-M-001-033) are gratefully acknowledged. L.H.L. wishes to thank the NSC for a predoctoral fellowship. The efforts of Dr. Y.-H. Chiu in the initial phase of this work and of Mr. J.-H. Wang in helping with the data acquisition are also duly acknowledged. References and Notes (1) Molecular Photodissociation Dynamics; Ashfold, M. N. R., Baggott, J. E., Eds.; Royal Society of Chemistry: London, 1987. (2) Shinke, R. Photodissociation Dynamics, Cambridge University Press: New York, 1993. (3) Butenhoff, T. J.; Rohlfing, E. A. J. Chem. Phys. 1993, 98, 5460, 5469. (4) Schneider, L; Meier, W.; Welge, K. H.; Ashfold, M. N. R.; Weston, J. C. J. Chem. Phys. 1990, 92, 7027. (5) Ashfold, M. N. R.; Lambert, I. R.; Mordaunt, D. H.; Morley, G. P.; Western, C. M. J. Phys. Chem. 1992, 96, 2938. (6) Zhang, J.; Dulligan, M.; Wittig, C. J. Phys. Chem. 1995, 99, 7446. (7) Houston, P. Acc. Chem. Res. 1989, 22, 309. (8) Hall, G. E.; Houston, P. Annu. ReV. Phys. Chem. 1989, 40, 375. (9) Xu, Z.; Koplitz, B.; Wittig, C. J. Chem. Phys. 1989, 90, 2692. (10) Loo, R. O.; Hall, G. E.; Haerri, H. P.; Houston, P. J. Phys. Chem. 1988, 92, 5. (11) Black, J. F.; Powis, I. Laser Chem. 1988, 9, 339. (12) Hwang, H. J.; Grifiths, J.; El-Sayed, M. A. Int. J. Mass. Spectrom. Ion Process 1994, 131, 265. (13) Chander, P. W.; Houston, P. L. J. Chem. Phys. 1987, 87, 1445. (14) Heck, A. J. R.; Chandler, D. W. Annu. ReV. Phys. Chem. 1995, 46, 335. (15) Tonokura, K.; Suzuki, T. Chem. Phys. Lett. 1994, 224, 1. (16) Chen, K. M.; Kuo, C. N.; Tzeng, M. H.; Shian, M. L.; Chung, S. E. Chem. Phys. Lett. 1994, 221, 341. (17) Kinugawa, T.; Arikawa, T. J. Chem. Phys. 1992, 96, 4801. (18) Wodtke, A. M.; Lee, Y. T. J. Phys. Chem. 1985, 89, 4744 and references therein. (19) Suto, M.; Lee, L. C. J. Chem. Phys. 1984, 80, 4824. (20) Mordaunt, D. H.; Ashfold, M. N. R. J. Chem. Phys 1994, 101, 2630. (21) Wu, C. J.; Carter, E. A. J. Phys. Chem. 1991, 95, 8352. (22) Macdonald, R. G.; Liu, K. J. Chem. Phys. 1989, 82, 91. (23) Che, D.-C.; Liu, K. Chem. Phys. Lett. 1995, 243, 290.
6380 J. Phys. Chem., Vol. 100, No. 16, 1996 (24) Hilbig, R.; Wallenstein, R. IEEE J. Quantum Electron. 1981, QE17, 1506. (25) Wiley, W. C.; McLaren, I. H. ReV. Sci. Instrum. 1995, 26, 1150. (26) Lai, L. H.; Wang, J.-H.; Che, D.-C.; Liu, K., to be published. (27) Yang, S.; Bersohn, R. J. Chem. Phys. 1974, 61, 4400. (28) Hwang, H. J.; El-Sayed, M. A. J. Chem. Phys. 1992, 96, 856. (29) After this work was completed, a paper by Zare and co-workers appeared (J. Chem. Phys. 1995, 103, 5157) in which they combined Doppler
Letters selection and pulsed extraction TOF techniques to study a bimolecular reaction. Compared to the approach reported here, their method is nearly the same in concept but somewhat differing in practical operation. We also acknowledge R. Bersohn for communicating a 3-D LIF approach (J. Chem. Phys. 1991, 94, 4817) for achieving the same goal. (30) Lai, L. H.; Che, D.-C.; Wang, J.-H.; Liu, K., to be published.
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