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

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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.