Resonance ionization mass spectrometry of nickel odd-parity autoionization states

Abstract

Efficient photoionization in isotope separation and laser ion source requires detailed characterization of autoionization states. This study presents a comprehensive investigation of nickel autoionization spectra via resonance ionization mass spectrometry (RIMS) to advance multi-color multi-step photoionization techniques. The three-color excitation scheme populates atoms into target high-lying excited states with the 3d⁸4s(⁴F)5s configuration in the 48,000–52,050 cm⁻¹ range, followed by systematic autoionization scanning in the 64,600–68,700 cm⁻¹ range. We identified 170 odd-parity autoionization states, including 150 previously unreported states. Spurious peaks arising from unintended transitions were eliminated through cross-validation of spectra from multiple scanning pathways and λ₂-blocked experiments. Precise characterization of energy positions, peak asymmetry and full width at half-maximum (FWHM) values was determined through Fano fitting. Remarkable consistency was observed for energy positions with literature data. Angular momenta (J) of 161 states were assigned using ΔJ = 0, ±1 selection rules, with the majority exhibiting J = 4 or 5. These states are characterized by narrow FWHM (< 1 cm⁻¹) and significant clustering in the target energy range. The strongest transitions were characterized for enhanced photoionization cross-sections in laser isotope separation. This research substantially expands the nickel autoionization spectral database.