Low-lying electronic states of CH3NO2 via photoelectron imaging of the nitromethane anion

Negative-ion photoelectron imaging at 532, 392, 355, and 266 nm is used to assign several low-lying electronic states of neutral nitromethane CH(3)NO(2) at the geometry corresponding to the anion equilibrium. The observed neutral states include (in the order of increasing binding energy) the X (1)A(') ground state, two triplet excited states, a (3)A(") and b (3)A("), and the first excited singlet state, A (1)A("). The state assignments are aided by the analysis of the photoelectron angular distributions resulting from electron detachment from the a(') and a(") symmetry molecular orbitals and the results of theoretical calculations. The singlet-triplet (X (1)A(')-a (3)A(")) splitting in nitromethane is determined as 2.90(+0.02)/(-0.07) eV, while the vibrational structure of the band corresponding to the formation of the a (3)A(") state of CH(3)NO(2) is attributed to the ONO bending and NO(2) wagging motions excited in the photodetachment of the anion.