Resonance-enhanced multiphoton ionisation spectroscopy of the NH (ND) radical. Part 2.—Singlet members of the 3p Rydberg complex

Abstract

We report new, or improved, spectroscopic data for three singlet excited electronic states, the f¹Π, g¹Δ and h¹Σ states of the imidogen radical (NH and ND). Each of these states is observed via the two-photon resonance enhancements they provide in the multiphoton ionisation spectrum of a ¹Δ state NH (ND) radicals. All three states are assigned as members of the Rydberg 3p complex, with leading configuration … 3σ²1π¹3pλ¹. The available evidence points to the h ¹Σ state having + parity, but this cannot be confirmed in the present study. The separation of the f ¹Π and g ¹Δ origins is comparable to the energy of one quantum of N—D stretching vibration; as a result, transitions to vibrationally excited (v′>0) levels of the f ¹Π state of ND are seen to gain intensity from, and to be perturbed by, the near-resonant g ¹Δ levels with quantum number v′–1.