Dynamics of photofragmentation of dimethylnitrosamine from its first two excited singlet states

The photofragmentation of dimethylintrosamine (DMN) from its first two excited singlet states was studied by monitoring the scalar and vector properties of the nascent NO via one-photon laser induced fluorescence, combined with polarization and sub-Doppler spectroscopy. The DMN was fragmented following irradiation at 363.5 nm [S₁←S₀(π*←n)] and 250 nm [S₂←S₀(π*←π)]. The photofragmentation is characteristic of a direct dissociation mechanism on a repulsive potential surface for both dissociation wavelengths. The NO fragment ejects with its velocity along the bond that breaks, and its angular momentum vector tends to be perpendicular to the plane of the C₂NNO frame of the parent molecule. The experiments corroborate that the transition dipole moment is perpendicular to the plane of the parent molecule for the S₁←S₀ transition and lies parallel to this plane, along the bond which breaks, for the S ₂←S₀ transition. The Λ-doublet population ratio obtained for the two dissociation wavelengths is consistent with an A″ symmetry for the S₁ and an A′ symmetry for the S ₂ state. Finally, a comparison between the photodissociation of tert-butyl nitrite (TBN) and DMN is presented. In particular, it is shown that in both molecules, for both states, the fragmentation is largely planar with the main forces acting approximately along the bond which breaks. On the other hand, retainment of vibrational energy in the NO fragment is observed only for TBN S₁. Also, a preference of the antisymmetric Λ component in NO from TBN S₂ and of the symmetric component in DMN S₂ is found.