Rotational distributions of the CH 3 fragment

1. Wave packet calculations on the effect of the femtosecond pulse width in the time-resolved photodissociation of CH3I in the A-band A. García-Vela1 and L. Bañares2 1Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain 2Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Introduction: The effect of varying the temporal width of the pump and probe pulses in femtosecond experiments of the CH3I photodissociation in the A band has been studied by means of multisurface nonadiabatic wave packet simulations [1,2] using the best ab initio potential surfaces currently available [3,4]. Specifically, the effect of decreasing the temporal width of the pulses on magnitudes like the photodissociation reaction time in the different dissociation channels and the CH3 fragment vibrational and rotational distributions has been explored. Four widths (FWHM) for the pump and probe pulses have been used, namely 100, 50, 20, and 10 fs. In addition, in order to investigate a possible effect of the excitation wavelength used, the simulations are carried out for two excitation wavelengths, λ=295 and 230 nm, which are located to the red and to the blue of the maximum of the absorption spectrum, respectively. Reaction times for the different dissociation channels Reaction times are found to decrease significantly as the temporal width of the pump and probe pulses decreases, for all the dissociation channels and excitation wavelengths. The effect is more pronounced for the CH3(ν) +I* channels, and for the 295 nm wavelength. The decrease of the reaction times is larger as the excitation in the umbrella mode ν increases, which causes the relative time τ1-τ2 between the I* and I dissociation channels to diminish rapidly with decreasing temporal width. These results indicate that by changing the width of the pump and probe pulses some control can be exerted on the reaction times. Time and energy profiles of the different pulses used in the simulations Absorption spectrum of CH3I Vibrational distributions of the CH3I fragment Vibrational distributions of the CH3I fragment are affected very little by decreasing the width of the pulses. Thus, in contrast with the behavior of the reaction times, the CH3 vibrational distributions are magnitudes predicted to remain very stable upon substantial changes in the temporal width of the pump and probe pulses. The simulations show that the variations found in the vibrational distributions are almost entirely due to the changes in the pump pulse width. Rotational distributions of the CH3 fragment Similarly as the vibrational distributions, the CH3 fragment rotational distributions are also little affected by changing substantially the width of the pulses. The shape of both vibrational and rotational distributions is originated in the CH3-I interaction region where the wave packet is created, and that shape is retained as the CH3 and I*(I) fragments separate. Thus the shape of these distributions is essentially unaffected by the increasing extent of the detection energy window produced by the probe pulse spectral width. The largest changes are found for the distributions of the CH3(ν) + I* dissociation channels, and for the 295 nm excitation wavelength. Conclusions: The reaction time of the CH3I photodissociation is found to decrease significantly with decreasing temporal width of the pump and probe pulses. An effect of the excitation wavelength is also found, being the changes in the time smaller for wavelengths located to the blue of the maximum of the absorption spectrum than the changes found for wavelengths to the red of this maximum. These results are explained in terms of the shape and slope of the absorption spectrum at the different excitation wavelengths. In contrast, the vibrational and rotational distributions of the CH3 fragment are found to remain very stable upon significant changes of the width of the pulses. The trends found here for CH3I seem to be general to other photodissociation processes of similar characteristics References [1] R. de Nalda, J. Durá, A. García-Vela, J. G. Izquierdo, J. González-Vázquez, and L. Bañares, J. Chem. Phys. 128, 244309 (2008). [2] D. Xie, H. Guo, Y. Amatatsu, and R. Kosloff, J. Phys. Chem. A 104, 1009 (2000). [3] A. B. Alekseyev, H.-P. Liebermann, R. J. Buenker, and S. N. Yurchenko, J. Chem. Phys. 126, 234102 (2007). Acknowledgements: Ministerio de Educación y Ciencia, Spain, Grants No. CTQ 2005-08493-C02-01 and FIS-2007-62002, Consolider program SAUUL, No. CSD2007-00013, and the Centro de Supercomputación de Galicia (CESGA).