Spectroscopy of the ground and low-lying excited states of ThO+

The ThO(+) cation is of interest as it is a useful prototype for experimental and theoretical studies of bonding in a simple actinide compound. Formally the ground state of ThO(+) has the configuration Th(3+)(7s)O(2-), where there is a single unpaired electron associated with a closed-shell Th(4+)-ion core. The first tier of excited states above the X (2)Sigma(+) ground state is expected to be 1 (2)Delta, 1 (2)Pi, and 2 (2)Sigma(+) derived from the Th(3+)(6d)O(2-) configuration. Spectroscopic observations of ThO(+) using the pulsed field ionization-zero kinetic-energy photoelectron technique are reported here. Rotationally resolved spectra were recorded for the X (2)Sigma(+), 1 (2)Delta, and 1 (2)Pi states. Extensive vibrational progressions were observed. Surprisingly, it was found that ionization of ThO decreases the dissociation energy, while increasing the vibrational frequency and decreasing the bond length. Accurate values for the ionization energies of ThO [53 253.8(2) cm(-1)] and Th [50 868.71(8) cm(-1)] were determined as part of this investigation.