Ultrafast dissociation of nitromethane from the 3p Rydberg state

We report the time-resolved observation of the ultrafast dissociation of the 3p Rydberg state manifold of nitromethane, which decays with a time constant of 53 +/- 5 fs. We access this manifold using a femtosecond high-harmonic-generation source, allowing us to track the dynamics of high-lying Rydberg states. Combining ultrafast VUV/UV pump-probe ultrafast photoelectron spectroscopy with ab initio non-adiabatic surface-hopping calculations, we find that a major channel of this ultrafast relaxation is NO bond cleavage. In contrast to previous literature, we find that intersystem crossing is not necessary to explain the dynamics, as the multiplicity of the system is retained along the reaction pathway. The photoinduced homolytic fission results in the formation of a nitrosomethane radical, with possible implications for atmospheric chemistry.