A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

Abstract

The real time photodissociation dynamics of CH 3 I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times ͑clocking͒ of the different ͑nonadiabatic͒ channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited ͑symmetric stretch and umbrella modes͒ CH 3 fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH 3 ͑ =0͒ and I͑ 2 P 3/2 ͒ and I * ͑ 2 P 1/2 ͒ that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I * channels when the CH 3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.

The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

“…2 Similarly, the constant and equilibrium values of the corresponding magnitudes used in the present calculations are also the same as those specified in Ref. 2.…”

“…Let us denote by a and b ͑which are complex quantities͒ the sum of the amplitudes projected out asymptotically of the 2,v and 2,v packets, respectively, onto the CH 3 ͑ =0͒ + I product fragment states. Thus, the total amplitude projected out of the 2 If the b amplitude is very small compared to a, the asymmetry ͑ab ‫ء‬ + a ‫ء‬ b͒cos R sin R term becomes negligible. However, if b ͑which is expected to be smaller than a, as discussed above͒ has a non-negligible value compared to a, the ͑ab ‫ء‬ + a ‫ء‬ b͒cos R sin R term will cause an appreciable manifestation of asymmetry in the angular distribution.…”

“…The projected out distributions are obtained using the method of Balint-Kurti et al 56,57 The procedure followed in our specific case has been described elsewhere. 2 The final distributions of interest obtained from each simulation started by exciting a given v 00 ͑R , r , ͒ initial state are weighted by a Maxwell-Boltzmann distribution f͑E v , T͒ ͓being E v the eigenenergy of the v 00 ͑R , r , ͒ state and T the vibrational temperature͔ and then added together in order to obtain distributions comparable with the experimental ones. Unfortunately it is hard to estimate the vibrational temperature in the experiment, so actually T becomes a parameter of the theoretical model.…”

“…A detailed description of the X 1 A 1 potential surface used has been given elsewhere. 2 For the 3 Q 0 and 1 Q 1 excited electronic states and the nonadiabatic coupling between them, we used the ab initio surfaces calculated by Xie et al, 45 which are an improved version of the previous nine-dimensional surfaces of Amatatsu et al 43 The surfaces and nonadiabatic coupling of Refs. 43 and 45 have the same functional form, only differing in the value of some of the parameters used to fit the ab initio data, as a result of the larger basis set used in the ab initio calculations of Ref.…”

“…45͒ and, more recently, focusing on time-resolved properties. 2,46 Recently, Alekseyev et al 1,47 employed larger atomic basis sets and a larger configuration interaction treatment to recalculate the X 1 A 1 ground state and the low-lying excited states, 3 Q 1 , 3 Q 0 , and 1 Q 1 , together with the transition moments relevant to the photodissociation of CH 3 I in the A band, with the aim of improving the agreement between the experimental and calculated absorption spectra.…”