Resonance-enhanced multiphoton ionisation spectroscopy of the NH(ND) radical. Part 1—The d 1Σ+ state

Abstract

The d ¹Σ⁺ state, previously identified in emission, is confirmed as being the state responsible for the two-photon resonance enhancement in the MPI spectrum of a ¹Δ state imidogen radicals at excitation wavelengths around 285 nm. We present the first reliable estimate of the bond-dissociation energy of this excited state. Two-photon resonant MPI via the d state is observed to yield both parent NH⁺(ND⁺) and atomic, N⁺ and H⁺(D⁺), ions. Three possible mechanisms for fragment ion formation are considered. Four-photon ionisation doubly resonance-enhanced, first (at the two-photon energy) by levels of the d ¹Σ⁺ state and then (at the three-photon energy) by autoionising members of the Rydberg series converging to one or more of the excited ionisation limits, is considered best able to explain the substantially enhanced N⁺ ion yields observed when MPI occurs via a select subset of the d–a resonances. Parallels between the ionisation behaviour found in this present investigation and that seen in previous studies of two-photon resonant MPI of b ¹Σ⁺ state PH(PD) radicals suggest the need for a reassignment of the resonance-enhancing excited state of PH(PD).