Bond Dissociation Energy, Ionization Energy, and Electronic Structure of Thorium Dimer

Diatomic thorium, Th2, has been investigated using a laser ablation, supersonic expansion source to produce the molecule and resonant two-photon ionization spectroscopy to measure its bond dissociation energy (BDE) and ionization energy (IE). The molecule has a high density of states in the vicinity of its bond dissociation energy, leading to rapid predissociation as soon as this energy is exceeded. The BDE is identified from this predissociation threshold as D0(Th2) = 2.857(7) eV, where the assigned error limit is provided in parentheses in units of the last quoted digit. Similarly, the one-photon ionization threshold has been measured, providing the ionization energy IE(Th2) = 5.042(4) eV. Together with a thermochemical cycle and the atomic ionization energy, these values provide the BDE of the cation, giving D0(Th2+) = 4.122(8) eV. Computations show that Th2 has three nearly degenerate low-lying electronic states (13Σu+, 11Σg+, and 13Δg) with bonding dominated by 7s and 6d orbitals, indicating predominantly transition-metal-like behavior. The 13Σu+ state exhibits a triple bond, whereas the 11Σg+ and 13Δg states possess quadruple-bond character and correspondingly shorter bonds. Although 13Σu+ is predicted to be the lowest state without spin-orbit coupling, the large spin-orbit stabilization of the 13Δg state makes its Ω = 1g component the ground state. The calculated dissociation energy (2.840 eV) and ionization energy of Th2 (5.098 eV) are both in excellent agreement with experiment.