Triplet Rydberg states of the imidogen radical characterized via two-photon resonance-enhanced multiphoton ionization spectroscopy

Abstract

Five new triplet excited states of the ND radical (three in the case of NH) in the wave-number range 85 000–91 000 cm−1 have been identified through analysis of the two-photon resonance enhancements they provide to the wavelength-resolved multiphoton ionization spectrum of X 3Σ− state NH(ND) radicals. The lowest energy of these, the B 3Π state, is found to be a ‘‘regular’’ Rydberg state which, on the basis of its observed quantum defect and its deduced rotational and spin–orbit coupling constant, is surmised to be the 3Π state derived from a 3pσ electron built on the 2Π ground-state ion core. Perturbations are evident in the B 3Π–X 3Σ− origin bands of both NH and ND. In the case of ND the perturbing state provides its own resonance enhancements, the analysis of which enables its definitive identification as the C 3Σ− state. The very small spin–orbit splitting found for the D 3Π state is taken to indicate that (at least in the Franck–Condon region) its wave function is dominated by the configuration involving one 3pπ Rydberg electron and a 4Σ− ion core. To still higher wave number we identify two more 3Σ− excited states, the upper of which (the F 3Σ− state) has a very small rotational constant which we take to imply that it has substantial valence character. Further indications that the F 3Σ− (and C 3Σ−) states possess significant valence character is provided by the observation that both parent and daughter (N+) ions contribute to the overall ion yield when the multiphoton ionization proceeds via these two states. Daughter-ion formation is considered to occur via an overall four-photon excitation process in which the coherent two-photon excitation to the 3Σ− state of interest is followed by a one-photon excitation to a ‘‘superexcited’’ state of the neutral. This is then presumed to absorb a further photon to yield the observed N+ ions and/or to predissociate, yielding highly excited N* atoms which then undergo a direct one-photon ionization.