Resonance-enhanced multiphoton-ionization photoelectron spectroscopy ofnpandnfRydberg states of atomic nitrogen

Abstract

(2+1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy studies are reported for the metastable $^{2}$${\mathit{D}}_{\mathit{J}}^{\mathit{o}}$ and $^{2}$${\mathit{P}}_{\mathit{J}}^{\mathit{o}}$ nitrogen atomic species presumably produced by photodissociation of the ${\mathrm{N}}_{3}$(X $^{2}\mathrm{\ensuremath{\Pi}}_{\mathit{g}}$) radical. Two-photon transitions to a multitude of doublet Rydberg states of odd parity are observed in the wavelength range 225.5--294.5 nm. This comprehensive coverage follows all np Rydberg states from n=3 to the ground-state ionization limit ${(}^{3}$${\mathit{P}}_{0,1,2}^{\mathit{e}}$) with n=25 being the highest identifiable. Perturbations are observed around n=5 due to an interloping np Rydberg state (n=3) converging to the first excited ionic limit ${(}^{1}$${\mathit{D}}_{2}^{\mathit{e}}$). A limited multichannel-quantum-defect theory (MQDT) analysis was performed to estimate the composition of the wave functions in this bound-bound interaction. In addition, nf Rydberg states from n=4 to 10, and some other weaker quartet states, have also been identified; these, and higher np states (ng8) show a breakdown of LS coupling. Overall, a considerable number of new states have been identified. Photoelectron kinetic-energy scans give information on the ion state distributions from the branching ratios to the $^{3}$${\mathit{P}}_{0,1,2}^{\mathit{e}}$ ground and the $^{1}$${\mathit{D}}_{2}^{\mathit{e}}$ and $^{1}$${\mathit{S}}_{0}^{\mathit{e}}$ excited states of ${\mathrm{N}}^{+}$; these can be compared to calculated values obtained from the MQDT analysis. Core- and non-core-preserving transitions are observed, and in one case almost complete state selection of excited ${(}^{1}$${\mathit{D}}_{2}^{\mathit{e}}$) nitrogen atomic ions is observed. Analysis of the Doppler profiles of the atomic transitions provides evidence for the mechanism which is at play in the formation of N metastable states.