Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using twohyphen;dimensionalabinitiopotential energy surfaces

摘要

We investigated the photodissociation of methyl nitrite (CH3ONO) and methyl thionitrite (CH3SNO) within the first absorption band (S1larr;S0). The calculations were based on a twohyphen;dimensional model including the Ondash;NO/Sndash;NO and N=O bond distances as active coordinates. TheS1hyphen;potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the timehyphen;independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorption spectrum of both molecules. A low potential barrier of 0.086 eV along the Ondash;NO dissociation coordinate in CH3ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100ndash;250 fs corresponding to 3ndash;8 NO vibrational periods. CH3ONO decays nonadiabatically via vibrational predissociation. The absorption spectrum of CH3ONO is dominated by narrow Feshbachhyphen;like scattering resonances which can be characterized by two quantum numbers,mandnast;:m=0 and 1 specifies the quanta of excitation in the Ondash;NO bond andnast;=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3SNO and the dissociation is direct on the time scale of about 10 fs corresponding to only one third of a NO vibrational period. Nevertheless, the absorption spectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franckndash;Condon reflection principle. The dissociation of CH3SNO is primarily adiabatic which leads to a pronounced energy dependence of the final NO vibrational state distribution. The diffuse structures originate in both cases from excitation of the NO stretching vibration. In order to make contact with timehyphen;dependent theory we calculated the autocorrelation function of the timehyphen;dependent wave function by inverse Fourier transformation of the energyhyphen;dependent spectra. The agreement with available experimental data for both molecules is quite satisfactory. This includes the energy spacing of the vibrational structure, the overall shape of the absorption spectrum, and thelifetime of the excited complex.