Spectroscopic and theoretical studies of ThCl and ThCl

The electronic structures of ThCl and ThCl⁺ have been examined using laser induced fluorescence and two-photon ionization techniques. Rotationally resolved spectra, combined with the predictions from relativistic electronic structure calculations, show that the ground state of the neutral molecule is Th⁺(7s²6d)Cl^-, X²Δ_3/2. Dispersed fluorescence spectra for ThCl revealed the ground state vibrational levels v = 0-10 and low energy electronic states that also originate from the atomic ion 7s²6d configuration. Pulsed field ionization-zero kinetic energy photoelectron spectroscopy established an ionization energy (IE) for ThCl of 51 344(5) cm^-1, and the ThCl⁺ vibrational term energies of the v = 1-3 levels. The zero-point level of the first electronically excited state was found at 949(2) cm^-1. Comparisons with high-level theoretical results indicate that the ground and excited states are Th²⁺(7s6d)Cl^- X³Δ₁ and Th²⁺(7s²)Cl^- Σ+1, respectively. Relativistic coupled cluster composite thermochemistry calculations yielded an IE within 1.2 kcal/mol of experiment and a bond dissociation energy (118.3 kcal/mol) in perfect agreement with previous experiments.