Wave packet calculations on the effect of the femtosecond pulse width in the time-resolved photodissociation of CH3I in the A-band

Abstract

The effect of changing the temporal width of the pump and probe pulses in the time-resolved photodissociation of CH₃I in the A-band has been investigated using multisurface nonadiabatic wave packet calculations. The effect is analyzed by examining properties like the photodissociation reaction times and the CH₃ fragment vibrational and rotational distributions, by using four different widths of the pump and probe pulses, namely pulses with full-width-at-half-maximum of 100, 50, 20, and 10 fs. Simulations are carried out for two different excitation wavelengths, 295 and 230 nm, located to the red and to the blue of the maximum of the absorption spectrum, in order to explore possible effects of the excitation wavelength. The reaction times are found to decrease significantly with decreasing pulse temporal width. The times associated with the CH₃ + I*(²P_1/2) dissociation channels decrease more remarkably than those of the CH₃ + I(²P_3/2) channels. The results indicate that for excitation wavelengths located to the blue of the absorption spectrum maximum the effect of changing the pulse width is less pronounced than for wavelengths to the red of the spectrum maximum. On the contrary, the CH₃ vibrational and rotational distributions show little variation upon large changes in the pulse width. The trends found are explained in terms of the changes in the spectral bandwidth of the pulses and of the shape and slope of the absorption spectrum at the different excitation wavelengths.