Multiphoton ionization of ammonia: Mass analysis and photoelectron spectra

Abstract

Mass and photoelectron spectra following the resonantly enhanced four-photon ionization of ammonia have been recorded. The mass spectral data show that extensive fragmentation occurs at a distinct wavelength threshold. This threshold energetically corresponds to the minimum number of photons required to reach the first excited state of the ion. The branching ratio for the various fragments (H+, H+2, N+, NH+, NH+2, NH+3) is independent of the neutral resonantly excited vibronic state, and the ratios differ from those recorded by conventional ionization techniques. Photoelectron energy measurements show that low (∼0 eV) kinetic energy electrons are produced following ionization of all but one of the resonantly excited Rydberg states investigated. The origin of these zero energy electrons is thought to be due to vibrational autoionization. Internal conversion may give rise to the neutral vibronic levels responsible for the autoionization. For one excited state, higher electron energies are recorded corresponding to direct ionization. This mechanism leaves the positive ion in a single vibrational quantum state which is the same vibrational state as that of the initially excited neutral-ammonia with cross sections governed by the Franck–Condon principle. The results suggest that the fragmentation occurs entirely within the ionic manifold.