MULTIPHOTON GAS PHASE SPECTROSCOPY OF EXPANSION-COOLED AMMONIA: NEW ELECTRONIC STATE ASSIGNMENTS

Author Institution:The two and three photon spectrum of expansion cooled ammonia has been recorded in the 380 - 490 nm region using multiphoton ionization (MPI) detection. Features due to transitions to eight electronic states are observed in the 5.7 to 9.3 energy range, where only five states 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 vacuum uv spectra, assignments can be made with much more certainty. Three previously assigned states have been reassigned, and four additional electronic states identified. It is found that nd orbitals are important in the description of several excited states, in contrast to previous experimental and theoretical work where they were excluded from consideration. Evidence is also found for direct competition between photodissociation and photoionization of $NH_{3}$ excited states