J. Phys. Chem. 1993,97, 12481-12484
12481
Molecular Rotation Clocking of the Subpicosecond Energy Redistribution in Molecules Falling Apart. 2. Excess Energy Dependence of the Rates of Energy Redistribution in the Two Photodissociation Channels of Iodobenzene John E. Freitas,+ Hyun Jin Hwang,t and Mostafa A. El-Sayed' Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024 Received: August 12, 1993"
For some dissociating molecules, the photodissociation rate becomes comparable to the energy redistribution rate and molecular rotation frequency. In this situation, a correlation could be observed between the recoil angular and speed distributions of the fragments. In a previous communication, Hwang and El-Sayedl were able to use this correlation to clock the energy redistribution process and to determine its rate for predissociating iodobenzene molecules excited to their *,a*excited surface a t 304 nm. In this work we use this technique to examine the excess energy dependence of this rate. In addition, we use the width of the observed recoil velocity distribution for the rapidly dissociating molecules excited to their repulsive, n,u* surface to examine the excess energy dependence of their rate of energy redistribution. We find that while the rate of energy redistribution increases with excess available energy (i.e., with decreasing the wavelength of the photodissociation laser) for rapidly photodissociating molecules, no definite correlation is observed for the predissociating ones. These observations are explained by the fact that for rapidly dissociating molecules the doorway optical state and the dissociating state are the same, which is not the case for the predissociating molecules.
Introduction Alkyl iodides (e.g., CH31, C2H51, etc.) are known to photodis~ociate~-~ rapidly upon excitation from the ground electronic state to the so-called A-band continuum, which extends from approximately 200 to 350 nm. This A N transition*+9 involves the promotion of a nonbonding p electron from the iodine atom valence shell to an antibonding u* orbital centered on the C-I bond. The result of this u* n excitation to the A band is a very fast dissociation in which an alkyl radical fragment and an iodine atom in either the ground I(2P3/2)state or the spinorbit excited I*(2P1/2) state are produced. The ground- and excited-state iodine photofragments will be referred to as I and I*, respectively. Mulliken9 used molecular orbital theory and predicted that the A-band absorption is due to the u* n promotion and gives rise to three repulsive excited states, the 3Ql(n,u*), 3Qo(n,u+), and 'Ql(n,u*) states in order of increasing energy. It was predicted that the 3Q0 N transition should be polarized parallel to the C-I bond axis and should correlate with the production of I*, while the 3Ql N and 'QI N transitions were predicted to be polarized perpendicular to the carbon-iodine bond axis and to correlate to the formation of I. Magneticcircular dichroism'o was used experimentally to resolve the three states of the A band, and their maxima were found to occur at approximately 240 nm (e = 70 L/(mol-cm), 260 nm (e = 300 L/(mol.cm)), and 300 nm (e = 4 L/(molcm)) for the 'Q1,3 Q ~ , and 3 Q l states, respectively, where e is the extinction coefficient for absorption. However, the results of photofragment translation spectroscopy (PTS) experiments on alkyl iodides at 248, 266, and 304 nm have consistently show$-7 that the formation of I results predominantly from absorption resulting from the parallel transition, not from the perpendicular transitions as was predicted by Mulliken. This observation has been attributed to a curve crossing between the 3 Qand ~ lQl states, resulting in ground-state iodine atoms being produced from a parallel transition. The spatial distribution and the value of the recoil velocity of the fragment produced by photolysis with a polarized laser depend
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t Present address: Department of Chemistry, University of Texas at Austin, Austin, TX 78712-1167. 8 Present address: Department ofchemistry, University, Seoul, . Kyung-Hee . Korea. Abstract published in Adounce ACS Absrrucrs, November 1, 1993.
0022-3654/93/2097-12481$04.00/0
on the characteristic times of different molecular processes. For a photodissociation process with a lifetime T D , which is shorter than the energy redistribution time, ~ ~ dand i ~the , molecular rotation time in the excited state, Trot, the spatial distribution is highly anisotropic, and a large fraction of the excess energy appears in translational energy, i.e., a large average recoil velocity, vrW. This is the prompt (rapid) dissociation limit, to which the photodissociation of alkyl belong. The other extreme is the statistical limit for which T D is >> ~ ~> Trot. d i In~this case, the fragments will be isotropically distributed with most of the excess energy appearing as vibrational and rotational excitation of the fragments, which leads to small recoil velocities. An interesting limit is the one for which q,= Trdist = Trot. In this case, the spatial distribution is expected to be correlated with the recoil velocity. As T D increases, more energy is redistributed and more molecular rotation takes place prior to photodissociation. This yields fragments with lower recoil velocities that are detected with a more isotropic distribution of angles relative to the axis of the polarized photoexcitation. This would be the case for predissociation.' Aromatic iodides, such as iodobenzene, present such a case. In addition to the n,u* repulsive electronic states of the C-I bond, the presence of the aromatic ?r-electronic system provides a set of singlet and triplet T,T* bound electronic states. Mixing between these a,?r*and then n,u* states results in giving a predissociation character to the T,T* states. In their first photofragment translation spectroscopic (PTS) experiments on iodobenzene, Bersohn and co-workersll observed spatial anisotropies that are lower than those observed for alkyl iodide photodissociation e~periments,~-~J I suggesting that a predissociation plays an important role in the photodissociation of this molecule. The PTS technique was initially developed by Wilson and his group2 by interfacing lasers with molecular beams. Since then, the technique was applied and modified by a number of workers. Morerecently, Houston and his group havediscussed thedifferent vector correlations that can be observed in these types of experiments.3 Recently, Hwang and El-Sayed have used a modified7 state-selective PTS technique with improved resolution to clearly show' that at 304.67 nm photolysis of iodobenzene results in the formation of two distinct velocity distributions for I(2P3/2). The high-velocity one corresponds to an excitation to the alkyl iodide type 3Qo(n,u*) state followed by crossing to the 0 1993 American Chemical Society
Freitas et al.
12482 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
lQIstate which correlates with the ground state of the iodine atom. This results in prompt dissociation (call this channel A). The low-velocity distribution corresponds to absorption to the predissociative benzene type T,T* state(s). Energy redistribution as well as crossing to the repulsive n,u* states then occurs. Iodine atoms resulting from this channel (call this channel B) are found to have spatial anisotropy parameters that are correlated with their velocities. As the velocity decreases, so does the anisotropy parameter. Using the relation12J3 between the anisotropy parameter, @, and the time-dependent rotational correlation function, Hwang and El-Sayedl were able to convert the relation between unC and @ into a plot between translational energy and time (scaled by the rotational period of iodobenzene). From this they were able to determine the rate of energy redistribution prior to dissociation for molecules that are excited to the T,T* excited-state surface. In this method, molecular rotation was used' to clock the energy redistribution processes. In this article, we use the above method to study the effect of the excess energy on the rate of energy redistribution produced in channel B, Le., upon excitation to the T,T* states. In addition, the effect of excess energy on the rate of this process is examined for molecules excited to the repulsive n,u* state by examining the effect of excitation wavelength on the width of the translational energy distribution of the high-velocity distribution resulting from channel A. Experimental SeCtionl~c The experimental apparatus and methodology are described in detail el~ewhere.1.~Briefly, a single-stage pulsed extraction TOF mass spectrometer is used in combination with two pulsed lasers. Linearly polarized pulsed laser light of 218 or 266 nm is used to photodissociate room temperature iodobenzene vapor, while pulsed 304.67-nm laser light is used to selectively ionize ground-stateiodine photofragments via 2 1 resonance-enhanced multiphoton ionization. Photodissication and photoionization occur within the overlapping pulses of the lasers used, each having a 6 4 s pulse width. Following photodissociation and photoionization, the iodine photoions recoil under a field-free condition with the same recoil velocities of the iodine atoms formed in the photodissociation process. After a delay time 7 d (1.5 ps), an acceleration field of ca. 1050 V/cm is pulsed for 1.0 ps, and time-of-flight (TOF) distributions of the iodine atoms are measured after a field-free drift of about 78 cm. As discussed in detail el~ewhere,'.~ the TOF measured in our method depends on the one-dimensional position of the iodine ion along the detection axis when the electric field pulse is turned on. Thus, we measure via the TOF of the iodine photofragments the component of the recoil velocity of the iodine atoms which recoil along the detection axis. A discrimination pinhole of diameter 6.0 mm placed in front of the detector reduces the detection solid angle, thus allowing detection of only those photoions whose recoil directions are closely aligned with the detection axis. As has been demonstrated elsewhere,lJ this velocity discrimination pinhole greatly enhances the velocity resolution of this PTS method. The angular distributions of the iodine photofragments are determined by performing the experiment a t two photodissociation laser polarization angles, a = Oo and a = 90°, with respect to the detection axis.
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Results and Discussion Figure 1 presents thelabvelocity distributions in thez (detection axis) direction for the ground-state iodine photofragments produced upon the photodissociation of iodobenzene at (a) 266 and (b) 218 nm. Negative recoil velocities correspond to iodine photofragments which recoil away from thedetector, while positive recoil velocities correspond to photofragments which recoil toward thedetector. Ineachfigure, theupper andlowertracescorrespond to photolysis laser polarizations parallel (a = Oo) and perpen-
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u z (m/s) Figure 1. Lab velocity (u,) distributions of ground-state iodine atoms resonantly ionized at 304.67 nm followingphotodissociationof iodobenzene at (a) 266 nm and (b) 218 nm. At each wavelength studied, polarization angles (a)parallel and perpendicular to the detection axis were used. Two typesofvelocitydistributionsareclearlyobservedat each wavelength. The outside peaks exhibit high average velocities and high spatial anisotropies,which is reminiscent of what is observed for alkyl iodides. The broader inner velocity distributions have lower average velocities and lower spatial anisotropies which depend on the photofragmentrecoil velocity.
dicular (a = 90°) to the detection axis, respectively. At each wavelength, the recoil velocity distributions exhibit both a sharp high-velocity component and a broad low-velocity component. This is reminiscent of the earlier obesrvations made on the photodissociation of iodobenzene a t 304.67 nm in our 1aboratory.l Thus, it appears that the ground-state iodine formation channel observed in iodobenzene a t both 21 8 and 266 nm is also composed of a prompt alkyl iodide type dissociation process (channel A) and a predissociation type process resulting from absorption to a benzene type state (channel B). Figure 2 presents the distribution of the total translational energy release G(E,) (Et is the sum of the translational energies of the iodine atom and the phenyl radical) and the dependence on Et of the anisotropy parameter @(Et),which is determined from the lab velocity distributions shown in Figure 1. At 266 nm (Figure 2a) and 218 nm (Figure 2b), @ is found to be 1.2 and 1.3, respectively, for the high recoil energy distribution. The positive value indicates that photodissociation occurs following an absorption that has a dominant parallel polarization component. Furthermore, the rs for the respective high translational energy distributions are independent of Et, suggesting that the iodine atoms produced are a result of a prompt dissociation whose time scale is much shorter than the molecular rotation time in the excited state. These observations are reminiscent of what is observed in the photodissociation of the alkyl i o d i d e ~ ~ -and ~ J ~of the one giving rise to high recoil energy distribution via channel A in iodobenzene' at 304.67 nm. Therefore, we assign the high translational energy distributions observed at 266 and 218 nm which result in the formation of ground-state iodine atom to a prompt, direct dissociation which results from the parallel electronic transition to the alkyl iodide type 3Qo(n,u*) repulsive state followed by curve crossing to the lQl(n,u*) repulsive alkyl iodide state (channel A). The slight decrease in the values of @ a t 266 and 218 nm as compared to that observed a t 304.67 nm (6 = 1.6) suggests small contributions from a perpendicular transition at 266 and 218 nm, presumably to the IQl state of the alkyl iodide type. Since the limiting value of @ for the prompt
The Journal of Physical Chemistry, Vol. 97, No. 48, 1993 12483
Photodissociation Channels of Iodobenzene
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E t(kcal/mol) Figure 2. Total translational energy release distributions G(El) and the El dependence of the anisotropy parameter @(El)of ground-stateiodine atoms produced from the photodissociation of iodobenzene at (a) 266 and (b) 218 nm. The distributions shown are each decomposed into a high (h) El component (Gh(Et) and @I,(&)) and a low (1) Et component (q(Et)and &(Et)). The G(Et)distributions are obtained from the lab velocity distributions in Figure 1 by using an instrumental detection function described in ref 1. The anisotropy parameter @(Et)is defined
as (111- 11)/(0.5111+Zl), where 111and Il are iodine ion signals obtained at photolysis laser polarizations of a = Oo and 90°, respectively, after correcting for El dependence of the detection solid angle. Note that @I,(&) at each wavelength is large and nearly independent of Et, while @I(&)at each wavelength is lower and dependent on Et. This indicates that, for the dissociation process which yields GI&), there is a range of dissociation times that are comparableto energy redistribution times and molecular rotation times. dissociation resulting from the parallel transition is in the range 1.6-1.8 under our experimental conditions, the contributions of the perpendicular transition should be about 15%20% at 218 and 266 nm. The presence of a low translational energy distribution for the iodine formed at 266 and 218 nm is also reminiscent of what is observed for the ground-state iodine photodissociation channel Bobserved in iodobenzenel at 304.67 nm. As has been observed' at 304.67 nm, the ps which correspond to the low translational energy distributions observed at 266 and 218 nm are lower than those values of the anisotropy parameter which correspond to the high translational energy peaks, and also decrease as Et decreases (see Figure 2). This suggests that these iodine atoms result from molecules whose photodissociation time is comparable to the rotation and energy redistribution times. In this case, it has been shownl2J3 that the correlation between /3 and Et (and therefore the fragment recoil velocity) can be related to the dissociation time of the molecule through the molecular rotation correlation function, which is derived elsewhere.12 Thus, the range of ps observed represents a range of dissociation times which are on the time scale of molecular rotation ( T ~= 1.7 ps for iodobenzene at room temperature). This corresponds to the low translational energy photodissociation channel (channel B) for ground-state iodine production. The overall low E,'s of these distributions observed at 266 and 218 nm indicate that, following photoexcitation, internal energy redistribution into the phenyl ring modes competes with the dissociation of the carbon-iodine bond. These observations are consistent with a picture in which the initial excitation is to a benzene type T , A * bound state which is predissociative due to mixing with the n,b* repulsive state. We
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Figure 3. Total translational energy release as a function of time for the
low translational energy distributions obtained at (a) 266 and (b) 218 nm. These time dependences are determined from the observed energy dependences of @l(Et)in Figure 2 and the theoretical time dependence of @ obtained from eq 1 of ref 1II as well as the rotational correlation function of iodobenzene at 298 K described in ref 12. The slope of each plot (-15 kcal/(mol.ps) at 266 nm, -21 kcal/(mol.ps) at 218 nm, as compared with -23 kcal/(mol-p) at 304.67 nm (ref 1)) represents the rate of internal energy redistribution (IER)into the phenyl ring modes at different excitation (excess) energy (see text). therefore assign the low translational energy distributions observed at 266 and 21 8 nm to result from excitation to the predissociative singlet u,a* states: the states corresponding to the lBzUand lBlu states in benzene, respectively. This is followed by crossing to the repulsive (n,u*) surface, which produces ground-state iodine atoms. This constitutes channel B. As stated above, the fact that /3 has values that decrease with Et for the low translational energy distribution means that El can be correlated with the dissociation time. Figure 3 presents plots of El versus the dissociation time for the ground-state iodine formation via the slow channel observed at a photolysis wavelength of (a) 266 and (b) 218 nm. The slopes (dEt/dt) represent the rates at which energy is redistributed into the internal energy modes of the phenyl ring in competition with dissociation at each wavelength. These are found to be (dE,/dt) = -1 5 kcal/(mol.ps) for photolysis at 266 nm and -21 kcal/(mol.ps) for photolysis at 218 nm. Thevaluedetermined previously1 at 304.67 is -23 kcal/ (mol-ps) . It is clear that there is no real correlation between the amount of excess available energy and the rate of intramolecular energy redistributionfor the electronic excitation of the T.T* benzene typesurfaces. How about if excitation takes placedirectly to the repulsive surface, Le., in channel A? Since the rate of energy redistribution is faster than our "clock", Le., than molecular rotation, we may only estimate the rate of energy redistribution as follows. Assuming similar shapes for the translational energy release distributions produced at different excitation wavelengths and assuming independence of the dissociation time from excitation energy,14 the full width at half-maximum (fwhm) (or that divided by the average dissociation time) could give a number that is proportional to the rate of energy redistribution for the rapid dissociation in channel A.15 We do observe that fwhm's of the high-energy distributions increase as the photolysis photon energy increases (Figure 4). This result suggests that as
12484 The Journal of Physical Chemistry, Vol. 97, No. 48, 1993
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E a v i (kcal/mol) Figure 4. Full widths at half-maximum (fwhm) of the Gh(Et)'s of Figure 2 versus the excess energy available after photodissociation, (&I). The value of the fwhm at 304.67 nm is obtained from ref 1. The fact that the fwhm (which is determined by the rate of the energy redistribution and the average dissociation time) for the alkyl iodide-type dissociation channel increases with excess energy indicates that the rate of IER for this channel increases with the density of states of iodobenzene that are coupled with the 3Q&o*) 'optical doorway" state giving rise to the iodine atom observed in this channel.
iodobenzene is excited to higher and higher energies on the alkyl iodide 3Q0(n,u*) repulsive surface in channel A, the rate of internal energy redistribution increases as the excess energy increases. The difference in the behavior of the excess energy dependence of the rate of energy redistribution in the two channels can be explained as follows. In the photoexcitation to the repulsive n,u* state corresponding to channel A, the optical doorway states prepared at the three photoexcitation wavelengths are the same electronic manifold (Le., the3Q0repulsivesurface),and the energy redistribution is in direct competition with thedissociation process in the same electronic state. It is therefore clear that, as one excites to higher energies of the 3Q0 (n,u*) repulsive surface (i.e., as the excess energy increases), the density of the nearby dark vibronic states, with which the "optical doorway" state can mix to give rise to energy redistribution, increases. The result is an increase in the rate of energy redistribution as manifested by the increase in the observed full width at half-maximum of the high translational energy distribution. The situation is different and more complex in the case of channel B. In this case, the optical doorway states prepared at the three photoexcitation wavelengths are different manifolds of the T,T* electronic system (the SZ,SI, and TI or Tz excitationI4 at 21 8,266, and 304.7 nm, respectively); thus, one cannot expect a simple correlation between the photoexcitation energy and the density of the dark vibronic states as was the case in channel A. In this case, the optical doorway T,T* state is mixed with some
Freitas et al. of the dark states. Some of these states, but not all, are then mixed with the dissociative n,u* repulsive state(s) to give rise to the iodine atoms observed in our experiments. Therefore, the observed rate of energy redistribution in this channel involves two different types of matrix elements. One type couples the doorway states with the dark states of the phenyl group, and the other couples some of the latter states with the repulsive n,u+ states. These matrix elements will depend on the nature of the optical electronic state excited and thus would change in a complex manner as the excitation wavelength changes. This is why one cannot expect a simple correlation between the excess energy and the rate of energy redistribution observed in channel B.
Acknowledgment. The authors thank the National Science Foundation for its support (Grant CHE 89 20052). References and Notes (1) (a) Hwang, H. J.; El-Sayed, M.A. J . Chem. Phys. 1992,96, 856. (b) Hwang, H. J. Doctoral Dissertation, Universityof California, L a Angeles, 1991. (c) Hwang, H. J.; Griffiths, J.; El-Sayed, M.A. Inr. J . MassSpectrosc. Ion Phys., accepted. (2) Riley, S.; Wilson, K. R. Faraday Discuss. Chem. Soc. 1972,53,132. (3) Loo,R. 0.;Hall, G. E.; Haerri, H.-P.; Houston, P. L. J . Phys. Chem. 1988, 92, 5 . (4) Sparks, R. K.; Shobatake, K.; Carlson, L. R.; Lee,Y. T. J . Chem. Phys. 1981, 75, 3838. ( 5 ) van Veen, G. N. A.; Baller, T.; de Vries, A. E.; van Veen, N. J. A. Chem. Phys. 1984,87,405. van Veen, G. N. A.; Baller, T.; de Vries, A. E.; Shapiro, M. Chem. Phys. 1985, 93, 277. (6) Barry, M.D.; Gorry, P. A. Mol. Phys. 1984,52,461. Peterson, C.; Godwin, F. G.;Gorry, P. A. Mol.Phys. 1987,60,729. Godwin, F. G.; Paterson, C.; Gorry, P. A. Mol. Phys. 1987, 61, 827. (7) Hwang, H. J.; El-Sayed, M.A. Chem. Phys. Lea. 1990,170, 161. Hwang, H. J.; El-Sayed, M.A. J. Chem. Phys. 1991,94,4877. Hwang, H. J.; El-Sayed, M.A. J . Phys. Chem. 1992,96, 8728. Freitas, J. E.; Hwang, H. J.; Ticknor, A. B.; El-Sayed, M.A. Chem. Phys. Lerr. 1991, 183, 165. (8) Poret, D.; Goodeve, C. F. Proc. R.Soc. London,A 1938, 165, 31. (9) Mulliken, S. J . Chem. Phys. 1940, 8, 382. (10) Gedanken, A,; Roiwe, M.D. Chem. Phys. Lerr. 1975, 34, 39. (1 1) Dzvonik, M.; Yang. S.; Bersohn, R. J . Chem. Phys. 1974,61,4408. Kawasaki, M.;Lee. S. J.; Bersohn, R. J . Chem. Phys. 1977, 66, 2647. Freedman, A.; Yang, S. C.; Kawasaki, M.;Bersohn, R. J. Chem. Phys. 1980, 72, 1028. (12) Yang, Y.S.;Bersohn, R. J . Chem. Phys. 1974, 61,4400. (13) Zare, R. N. Mol. Phorochem. 1972, 4, 1. (14) The dissociation time upon exciting molecules to repulsive surface is expected to decrease due to an increase in the relative velocities of the iodine and the phenyl radical during dissociation, but the effect is not significant due to a small change in the relative velocities. (15) In ref 1 for channel A at 304.67 nm, which implies longer channel A dissociation times for 266-m and 218-nm photoexcitations, the observed lower vlaues of fi at 266 and 218 nm than at 304 nm may be due to mixtures of parallel and perpendicular transitions to n,o* repulsive states of different polarizationsrather than due to longer dissociationlifetimes. This is supported by the fact that @ is independent of u d . Furthermore, the )Qo state is polarized parallel to the C-I bond, but the nearby 'QIstate and one of the a,# states are polarized perpendicular to the C-I bond.
