The Journal of
Physical Chemistry
0 Copyrighr, 1985, by the American Chemical Society
VOLUME 89, NUMBER 24 NOVEMBER 21, 1985
LETTERS Femtosecond Photofragment Spectroscopy: The Reaction ICN
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CN
+I
N. F. Scberer, J. L. Knee, D. D. Smith: and A. H. Zewail** Arthur Amos Noyes Laboratory of Chemical Physics,s California Institute of Technology, Pasadena, California 91 125 (Received: September 28, 1985)
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In this Letter we report our first results on femtosecond photofragment spectroscopy. The reaction studied is I-CN [I-CN] I CN. A photolysis-and-probe scheme is used to measure the rate of the primary photodissociation of ICN under collisionless conditions. The CN buildup time is measured from these preliminary results to be 600 100 fs. These direct measurements for the time of bond breakage in ICN are discussed in relation to the dynamics of the recoil process of the reaction.
Introduction Studies of the dynamics of primary photofragmentation processes offer new opportunities for the direct viewing of the process of bond breakage, its dependence on the nature of the transition state, and the resultant internal states of the products. Information about (average) photodissociation lifetimes, t,has in the past been obtained from knowledge of the product angular distribution(s) and from the calculated average rotational period of the parent, Q, as is illustrated, for the case of ICN, in the pioneering work of Ling and Wilson.' To obtain, t,however, one must assume a model for the sharpness of the angular distribution and rely on the classical or quantum model for relating t and Q to the spatial anisotropy, /3, of the reaction. Picosecond and femtosecond photolysis-and-probe techniques allow one to obtain 7 directly and test the validity of the models involved in the description of the rotation of the parent and the recoil process. Comparison of the direct time-resolved results with steady-state angular distribution measurements can then be made. In recent work from this laboratory we have reported on the picosecond photolysis-and-probe monitoring of chemical reactions Permanent address: Department of Chemistry, Purdue University, West Lafayette, IN 47907. *Camille and Henry Dreyfus Foundation Teacher-Scholar. *Contribution No. 7308.
in molecular beams.24 The primary process of alkyl iodide, RI, bond fragmentation to R and I was not, however, resolved because the fragmentation, in this case, is on a repulsive surface and the time scale for dissociation is shorter than the picosecond resolution of our experiment. In this Letter we wish to report our first results on the femtosecond time resolution of the primary photofragmentation (on the repulsive surface) of the reaction I-CN [I-CN] I CN -+
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In these experiments, as outlined in Figure 1, a femtosecond pulse (photolysis pulse) initiates the dissociation by exciting ICN (with a well-defined E-field polarization direction in the-laboratory frame) to the continuum absorption of the repulsive A state with 306-nm light. The second pulse (probe pulse) monitors the recoiled C N at 388 nm (with its E field orientated parallel or perpendicular to that of the photolysis pulse polarization). By observing the laser-induced fluorescence of the rotationally excited C N fragment (1) (2) 4715. (3) 1996. (4) 4659.
Ling, J. H.; Wilson, K. R. J . Chem. Phys. 1975, 63, 101. Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1985,82, Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Chem. Phys. 1985,83, Knee, J. L.; Khundkar, L. R.; Zewail, A. H. J. Phys. Chem. 1985.89,
0022-3654/ 8 5/ 2089-5 141$01.50/0 0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 24, 1985
Letters
c
:,/ time ( p s )
Figure 2. The leftmost transient is the system response function. The right-shifted transient is the ICN photofragmentation taken under identical conditions of overlap and time delay. The shift and change of slope between the transients are quite evident. The solid line which passes through the points of the ICN transient is a convolution of the measured response function (REMPI transient) with a 600-fs exponential buildup, which agrees with a nonlinear least-squares fit of the same.
rI-cp.l
Figure 1. Schematic representation of the ICN state structure relevant to the photodissociation processes being considered. The femtosecond photolysis pulse excites the ICN to the A state continuum. Product CN
radicals are detected with the probe pulse by laser-induced fluorescence. as a function of the time delay between the photolysis and probe pulses, we measure the buildup (rise) time for the formation of the C N photoproduct. These experiments provide a direct view of the process of bond breakage and illustrate some of the difficulties inherent in the earlier indirect methods. Experimental Section Apparatus. The two femtosecond pulses (306 and 388 nm) were generated by using the following arrangement: A modelocked argon ion laser synchronously pumps a cavity dumped dye laser (pulse width 5 ps, 612 nm), the output of which is compressed in a fiber optic/grating double-pass arrangement. The compressed pulse autocorrelation is typically 350-400 fs in duration. These pulses are amplified in a three-stage dye amplifier which is pumped by the second harmonic of a Nd:YAG laser. The first two gain stages are isolated from each other by a spatial filter, while the second two are optically separated by a dye jet of the saturable absorber DQOCI. The resultant amplified pulses have typical energies of 150 pJ and are usually 400 fs in duration. The pulse compressor is adjusted to precompensate for the dispersion encountered in the amplifier which, along with the saturable absorber jet, serves to prevent any significant pulse broadening. Mixing the amplified pulse in a nonlinear crystal with the 1.06-pm YAG fundamental produces the 388-nm probe light, while the 306-nm light is the second harmonic of the amplified fundamental. This method of light generation produces pumpand-probe pulses with an insignificant amount of relative timing jitter. We have also taken care to ensure that we are able to phase match the entire frequency bandwidth to minimize the extent of pulse broadening in the two mixing processes. The 306- and 388-nm pulses traverse separate arms of a Michelson interferometer, one beam path containing a fine-resolution variable delay. The relative polarization is adjusted with a Soleil-Babinet compensator/thin-film polarizer combination in the 388-nm arm. The beams are recombined and adjusted to travel collinearly through the experimental (ICN) and response function cells. The ICN cell is extensively baffled to reject scattered laser light and allow straightforward detection of the resonant laserinduced fluorescence from the C N fragment. The response function of the system is obtained by replacing the ICN-LIF cell with an ionization cell of NJ-diethylaniline (DEA) and generating a resonance-enhanced 1 1 ionization transient. The cell repositioning is done without changing any of the overlap adjust-
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ments or adding neutral density filters; therefore, a calibrated set of data (ICN transient, DEA REMPI response) is obtained. Calibration of the ICN transient against the REMPI response allows us to measure the rise and the “ t = 0 shift” for the 306/388-nm pulse excitation instead of relying on the autocorrelation of the dye laser pulses. Finally, the handling of ICN and the signal processing are straightforward and have been described el~ewhere.~ Treatment of the Data. Figure 2 displays the apparent rise and shift observed in these femtosecond photolysis-and-probe experiments. This form of the delayed rise is similar to the observation made by Smith et alS5on the picosecond time scale. To obtain T from our measurements, we have treated the data in the following way. We used the REMPI transient as the response function of the system for fitting the ICN transient using a nonlinear least-squares single-exponential fitting routine. This gave 7 = 600 f 100 fs. The REMPI transient rises and becomes flat with no indication of any decay components. This indicates that the resonant intermediate is long-lived (nanoseconds) and that the observed REMPI is the response function for the 306/388-nm experiment. We have also considered the possibility that the intermediate state of DEA has an additional fast decay component due to off-resonant ionization and/or IVR processes.6 In this case a biexponential decay is the expected behavior, and the fast component can result in further sharpening of the response function, leading to an apparent shift from the actual system response. We have modeled the observed rise by convoluting a Gaussian pulse (the integration of which gives the ICN transient) with a decaying biexponential function, fixing the long component at 8-ns decay and the fast component as pulse width limited (to be of the order of the pulse). It is found that the experimentally observed shift cannot be obtained while simultaneously maintaining the shape of the REMPI transient. When the experimental shift is matched, the modeled REMPI response shows a pronounced decay component, which is inconsistent with the experimental observation. We found that by making the fast component much shorter ( < l o fs) we could, in principle, reproduce the shift but with the ratio of the fast to slow component being unphysically large (>50). The above treatment of the data confirms that the REMPI transient is the system response and that the observed shift is due to the finite 7 of 600 f 100 fs. This is consistent with several experimental facts: (1) We observe similar temporal behavior when the polarization of the probe is perpendicular to that of the (5) Smith, D. D.; Lorincz, A ; Siemion, J.; Rice, S . A. J . Chem. Phys. 1984, 81, 2295. ( 6 ) (a) Scherer, N. F.; Perry, J. W.; Doany, F. E.; Zewail, A. H. J . Phys. Chem. 1985,89, 894. (b) Perry, J. W.; Scherer, N. F.; Zewail, A. H. Chem. Phgs. Lett. 1983, 103, 1. (c) Scherer, N. F.; Shepanski, J. F.; Zewail, A. H. J . Chem. Phys. 1984, 81. 2181.
Letters photolysis pulse. (2) The temporal behavior is not sensitive over a range of pulse energies, and the ion signal is linear in the probe intensity. (3) The pump wavelength is on resonance with the absorption of DEA, which at room temperature is continuous at 306 nm.’ Finally, to reiterate, the fitting of the ICN rise (600 f 100 fs) was a convolution of the measured response function with a single-exponential buildup, reflecting the prompt photodissociation process on the repulsive surface(s).
Results and Discussion ICN is a prototypical triatomic molecule for the study of the dynamics of axial recoil.”” The photofragmentation spectroscopy of the A continuum of ICN (maximum -255 nm)I2 has been extensively studied, a summary of which is found in the recent article by Nadler et al.13 Our interest in this problem stems from a desire to directly measure the photodissociation lifetime to obtain a better understanding of dynamics on repulsive surfaces. The reaction on this repulsive surface(s) produces C N radicals and iodine in its ground(1) and excited (I*) spin-orbit states. The C N is produced in the X2E+ ground state with essentially no vibrational excitation. On the other hand, there is extensive rotational excitation of the C N which is produced, and the distribution is known. Moreover, recent studies by Wittig’s groupI3 indicate that the spatial anisotropy parameter, 0, is quite large for this reaction (1.3-1.6), which is slightly less than the maximum expected value of 2.0. Their values of 0,which were obtained for 266-nm photolysis, should be compared with the values obtained earlier by Ling and Wilson (1.02 and 1.4). Knowledge of /3 can give an average photodissociation lifetime, 7, assuming a simple classical model for relating the average rotational period of ICN, 9, and i to p. This model assumes that the “loss” of the anisotropy is solely determined by the rotation of the parent and that no other channels, such as surface crossing or transverse recoil, are involved. Assuming that the decrease in the value of p is only due to the rotation of the parent both Ling and Wilson and Nadler et al. infer that the average photodissociation time is 100-200 fs. Figure 2 shows results obtained when the C N photoproduct is monitored with the femtosecond photolysis-and-probe scheme in a parallel configuration of the laser p01arizations.l~ Similar transients were obtained for the perpendicular configuration. The (7) Kimura, K.; Tsubomura, H.; Nagakura, S. Bull. Chem. Soc. Jpn. 1964, 37, 1336.
(8) (a) Zare, R. N.; Bernstein, R. B. Phys. Today 1980, 33, 43. (b) Levine, R. D.; Bernstein, R. B. ‘Molecular Reaction Dynamics”; Oxford Press: New York, 1979. (9) (a) Bersohn, R. J . Phys. Chem. 1984.88, 5145. (b) Siinons, J. P. J . Phys. Chem. 1984, 88, 1287. (c) Leone, S. In “Dynamics of the Excited State”; Lawley, K. P. Ed.; Wiley: New York, 1982; p 255. (10) (a) Shapiro, M.; Bersohn, R. J . Chem. Phys. 1980, 23, 3810. (b) Freed, K. F.; Band, Y. B. “Excited States 3”; Lim, E., Ed.; Academic Press: New York,1978; p 104. (c) Gelbart, W. M. Annu. Rev. Phys. Chem. 1977, 28, 323. (1 1) (a) Heller, E. J. Acc. Chem. Res. 1981, 14, 368. Lee, S. Y.; Heller, E. J. J . Chem. Phys. 1982, 76, 3035. (b) Imre, D.; Kinsey, J. L.; Sinha, A.; Krenos, J. J . Phys. Chem. 1984, 88, 3956. (12) (a) King, G. W.; Richardson, A. W. J . Mol. Spectrosc. 1966, 21,339. (b) Rabalais, J. W.; McDonald, J. R.; Scherr, V.; McGlynn, S. P. Chem. Rev. 1971. 71. 73. (13) Nadler, I.; Mahgerefteh, D.; Reisler, H.; Wittig, C. J . Chem. Phys. 1985, 82, 3 8 8 5 . (14) Parallel (11) and perpendicular ( I )refer to the orientation of the polarization of the photoiysid pulse to the probe pulse. ~
The Journal of Physical Chemistry, Vol. 89, No. 24, 1985 5143 measured rise is the time required for the recoiling C N fragment to separate from ICN following the photolysis pulse excitation. With this information we would like to address the point (vide supra) about relating this directly measured T to the dynamics of the recoil process. In a simple classical model for this axial recoil, our measured 7 can be related to p by using the relationship for the spatial distribution of the reaction products advanced by Zare, Herschbach, Bersohn, Wilson, Jonah, and others.I5 For a parallel-type transition and for recoil along the internuclear axis in a center of mass coordinate system p/2 = [1
+ (UT)’] [ 1 + 4(w~)’]-’
The values of iir at 300 and 337 K have been computed to be 1 X 10” l 3 and 1.89 X 10l2rad s-I,l respectively. Using our T and these values of 0, we obtain 0 = 1.1 and 0.7 for the two cases. Furthermore, our results indicate that the angle of rotation of the parent is 34 and 65O, respectively. In other words, the parent ICN molecule rotates, on the average, 30-60’ prior to dissociation if /3 is determined by this rotational process. It is interesting to compare the range of /3 values of 0.7-1.1 (depending on the value of iir chosen) obtained here to the measured range of p’s (1.2-1.613 and 1-1.4’) for 266-nm photolysis. There are a number of considerations that must be taken into account in comparing these time-resolved measurements to previous “CW” photofragmentation results. First, the photolysis wavelength may have an effect on T , as these experiments were performed by pumping at 306 nm, considerably to the red of the 266-nm pumping used in the other studies.’~’~ Second, the 0’s in ref 1 and 13 were obtained from the classical formula given above which neglects level crossing, transverse recoil, and orbital angular momentum. It also assumes that the optical transition is purely parallel in this case. Inclusion of these dynamical effects is essential to the understanding of the recoil process, and we hope to be able to isolate their separate influences on the dynamics by these femtosecond time resolution experiments. We also plan experiments with shorter time pulses and at lower temperatures (beams) to separate these effects. In conclusion, this work shows that the femtosecond photofragment spectroscopy of reactions is experimentally feasible and can directly measure the rate of formation of products in given states. It is our hope that these experiments, when completed, will provide the details of the transition state involved in the recoil process. Since the recoil velocity of this reaction is - 2 X lo5 cm/s, the fragment separation is 10 A on the time scale of the experiment (-500 fs). With this time resolution we must therefore consider the proximity of the fragments at the time of probing, i.e. the evolution of the transition state to final products. There is a wealth of experiments to be done on this (ICN) and other systems to directly elucidate the dynamics of photofragmentation under collisionless conditions.
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Acknowledgment. This research was supported by the National Science Foundation. We thank Prof. C. Wittig for his interest in this work and useful discussions on ICN. (15) (a) Zare, R. N.; Herschbach, D. R. Proc. IEEE 1963, 51, 173. (b) Kinsey, J. L. J . Chem. Phys. 1984, 80,2589. (c) Yang,S.; Bersohn, R. J . Chem. Phys. 1974,61, 4400. (d) Jonah, C. J . Chem. Phys. 1971, 55, 1915. (e) Greene, C. H.; Zare, R. N . Annu. Rev. Phys. Chem. 1982, 33, 119.
