Diffuse vibrational structures in photoabsorption spectra: A comparison of CH3ONO and CH3SNO using two-dimensional a b i n i t i o potential energy surfaces

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Abstract
We investigated the photodissociation of methyl nitrite (CH3 ONO) and methyl thionitrite (CH3 SNO) within the first absorption band (S 1 ←S 0 ). The calculations were based on a two‐dimensional model including the O–NO/S–NO and N=O bond distances as active coordinates. The S 1 ‐potential energy surfaces were calculated with quantum chemical methods and the dynamical calculations were performed exactly within the time‐independent approach. The main emphasis is on the origin of diffuse vibrational structure in the photoabsorptionspectrum of both molecules. A low potential barrier of 0.086 eV along the O–NO dissociation coordinate in CH3 ONO prevents immediate dissociation and leads to an initial state dependent lifetime for the excited complex of 100–250 fs corresponding to 3–8 NO vibrational periods. CH3 ONO decays nonadiabatically via vibrational predissociation. The absorptionspectrum of CH3 ONO is dominated by narrow Feshbach‐like scattering resonances which can be characterized by two quantum numbers, m and n*: m=0 and 1 specifies the quanta of excitation in the O–NO bond and n*=0,1,2,... specifies the excited vibrational level of the N=O bond. The potential barrier is absent in CH3 SNO 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 absorptionspectrum exhibits diffuse vibrational structures. The shape of the individual absorption peaks is determined by the classical Franck–Condon reflection principle. The dissociation of CH3 SNO 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 time‐dependent theory we calculated the autocorrelation function of the time‐dependent wave function by inverse Fourier transformation of the energy‐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 absorptionspectrum, and the lifetime of the excited complex.

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