UV photodissociation of N-methylpyrrole: The role of 1πσ∗ states in non-hydride heteroaromatic systems

The photophysics of jet-cooled N-methylpyrrole molecules following excitation to their first excited singlet state (the 1A2 state, arising from a 3s/σ∗←π electron promotion) has been investigated by resonance enhanced multiphoton ionisation spectroscopy, by measurements of wavelength resolved 'action' spectra for forming CH3 photoproducts, and by velocity map imaging studies of these CH3 products (in their v = 0 and v2 = 1 vibrational levels). CH3 products are observed at all excitation wavelengths within the NMP (1A2←X∼1A1) absorption band. Direct dissociation on the 1A2 potential energy surface (PES) yields 'fast' CH3 fragments, with an average total kinetic energy release (TKER) of ∼6500 cm−1, but this product channel is only observed in a narrow wavelength range near the absorption band origin. All of the measured CH3 images also show a broad component, peaking at lower TKER (∼1700 cm−1); this component extends beneath the 'fast' feature in images recorded at wavelengths near the 1A2←X∼1A1 origin, and accounts for all of the CH3 products observed at shorter photolysis wavelengths. These products are attributed to decay of highly vibrationally excited ground state molecules formed by radiationless transfer from the 1A2 state. Similarities and differences with the results of previous studies of the H + pyrrolyl products arising in the UV photodissociation of pyrrole are discussed in terms of the likely nuclear motions on the relevant ground and excited PESs (along RN-CH3/RN-H), and the possible couplings between these surfaces. The present study confirms that the proposed model of 1πσ∗ state induced bond fission in heteroaromatic molecules [A.L. Sobolewski, W. Domcke, Chem. Phys. 259 (2000) 181] is also applicable to non-hydride substituted heteroaromatics, but that mass effects can have an important influence on the subsequent nuclear dynamics.