The MPI spectrum of expansion-cooled ammonia: Photophysics and new assignments of electronic excited states

Abstract

The two‐ and three‐photon spectrum of expansion‐cooled ammonia has been recorded in the 380–500 nm region using multiphoton ionization (MPI) detection. In addition, the vacuum ultraviolet spectrum of gas phase ammonia has been measured photoelectrically in the 125–160 nm region. Features due to transitions to eleven electronic states are assigned in the 5.7–9.3 eV energy range, where only five band systems were previously assigned. By utilizing the spectral simplification provided by expansion cooling, the different selection rules for multiphoton absorption, and the differences between the MPI and VUV spectra, assignments can be made with much more certainty. Three previously assigned band systems have been reassigned, and seven additional electronic states identified. It is found that states arising from nd orbitals are responsible for the most intense features in the VUV spectrum, in contrast to previous experimental and theoretical work where they were excluded from consideration. Evidence is found for direct competition between photodissociation and photoionization of NH₃ excited states. Earlier photochemical studies of ammonia are reinterpreted in light of the new spectroscopic assignments.