ONE-COLOR AND 2-COLOR 2-PHOTON RESONANCE-ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY OF THE D(1)SIGMA(+) STATE OF NH

The ionization and dissociation processes occurring after two-photon excitation of the d(1) Sigma(+) state of NH have been investigated using one- and two-color multiphoton ionization spectroscopy in combination with mass-resolved ion and kinetic energy-resolved electron detection. The photoelectron spectra obtained for ionization of the molecule via the d(1) Sigma(+) state (v(')=0,1) reveal a one-photon ionization process to all energetically accessible vibrational levels of the x (II)-I-2 ground ionic state, at variance with a one-configuration description of the two states. Moreover, electrons are observed that derive from a one-photon ionization of excited neutral nitrogen and hydrogen atoms. Two-color excitation experiments of the d(1) Sigma(+) state show that the influence of discrete states at the overall three-photon level on the dissociation dynamics is insignificant. Using the results of ab initio calculations it is concluded that these experimental observations can be interpreted consistently if two competing processes are assumed to take place from the d(1) Sigma(+) excited state: a molecular one-photon ionization which is forbidden in zeroth order, and a two-photon nonresonance enhanced excitation to neutral (pre)dissociative states.