Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of Rydberg states of NH

Abstract

Results of combined theoretical and experimental studies of photoelectron spectra resulting from (2+1) resonance enhanced multiphoton ionization (REMPI) via the f ^1Π(3pσ), g ^1Δ(3pπ), and h ^1Σ^+(3pπ) Rydberg states of NH are reported. The overall agreement between these calculated and measured spectra is encouraging. Strong ΔN=N+−N’=even peaks, particularly for ΔN=0, are observed in these spectra. Low‐energy Cooper minima are predicted to occur in the l=2 wave of the kπ(^1Σ^+), kπ(^1Σ^−), and kπ(^1Δ) photoelectron channels for the f state, the kπ(^1Δ), kδ(^1Π), and kδ(^1Φ) channels for the g state, and the kπ(^1Σ^+) and kδ(^1Π) channels for the h state of NH. Depletion of the d wave (l=2) contributions to the photoelectron matrix element in the vicinity of these Cooper minima subsequently enhances the relative importance of the odd l  waves. The observed ΔN transitions are also affected by strong l  mixing in the electronic continuum induced by the nonspherical molecular potential. Interference of continuum waves between degenerate ionization channels also determines the spectral pattern observed for photoionization of the f ^1Π state of NH. Photoelectron angular distributions and the angular momentum compositions of photoelectron matrix elements provide further insight into the origin of these Cooper minima.