Probing actinide bonding character with REMPI, MATI, and PFI-ZEKE spectroscopy

Electronic structure of small actinide compounds (UO, UO+, UO2, ThO, ThO+) in the gas phase was investigated using Resonantly Enhanced Multi-Photon Ionization (REMPI), Mass-Analyzed Threshold Ionization (MATI) and Pulsed Field Ionization-Zero Kinetic Energy (PFI-ZEKE) techniques.; The ionization energies (IE) for UO, UO2 and ThO have been determined using photo-ionization efficiency, MATI and PFI-ZEKE measurements. The IEs were found to be appreciably higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations. The ability to selectively ionize ground state molecules was the key to obtaining accurate IEs. Previous determinations had underestimated the IEs due to ionization of thermally excited molecules in high temperature environments.; The electronic spectroscopy of UO2 has been examined using REMPI with mass-selected detection of the UO2+ ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO2+ were observed in the range from 17400 to 32000 cm-1. These bands originated from the U(5f&phis;u7ssigmag)O 2 X3phi2u and 3phi 3u3 states. Comparisons of transition energies with theoretical predictions for excited states of UO2 yielded tentative assignments for the upper states of the prominent band systems.; The REMPI spectrum of ThO was examined in the range 315-370 nm. Rotationally resolved bands were recorded for three new electronic states. In addition, transitions to the A' (v=1-3) levels and the N (v=2) level were observed for the first time.; The electronic and rovibronic structure of ThO+ has been examined using PFI-ZEKE technique. Rotationally resolved spectra were recorded for the X2Sigma +, 1 2Delta and 12pi states. Surprisingly, it was found that ionization of ThO weakens the bond, while increasing the vibrational frequency and decreasing the bond length.; PFI-ZEKE spectroscopy has been used to observe the low-lying energy levels of UO+. Rotationally, resolved spectra were recorded for the ground state and the first 9 electronically excited states. Term energies, vibrational frequencies and anharmonicity constants for low-lying energy levels of UO+ are reported. The pattern of low-lying electronic states for UO+ indicates that they originate from the U3+(5 f3)O2- configuration and 5 f electrons do not participate in bonding for UO+.