Imaging the photodissociation of CH3SH in the first and second absorption bands: The CH3(X 2A1)+SH(X 2Π) channel.

The CH₃(X ²A₁)+SH(X ²Π) channel of the photodissociation of CH₃SH has been investigated at several wavelengths in the first 1 ¹A^″←X ¹A^′ and second 2 ¹A^″←X ¹A^′ absorption bands by means of velocity map imaging of the CH₃ fragment. A fast highly anisotropic (β=-1±0.1) CH₃(X ²A₁) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C–S bond in the parent molecule. From the analysis of the CH₃ images, vibrational populations of the SH(X ²Π) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 ¹A^″ second excited state to the unbound 1 ¹A^″ first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH₃ leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 ¹A^″ state, a slower anisotropic CH₃ channel has been observed (β=-0.8±0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH₃S(X ²E) radicals via cleavage of the S–H bond in CH₃SH, followed by photodissociation of the nascent CH₃S radicals yielding CH₃(X ²A₁)+S(X ³P_0,1,2). [ABSTRACT FROM AUTHOR]