The electronic spectra of ThF and ThF(+) have been examined using laser induced fluorescence and resonant two-photon ionization techniques. The results from high-level ab initio calculations have been used to guide the assignment of these data. Spectra for ThF show that the molecule has an X (2)Δ(3/2) ground state. The upper spin-orbit component, X (2)Δ(5/2) was found at an energy of 2575(15) cm(-1). The low-lying states of ThF(+) were probed using dispersed fluorescence and pulsed field ionization-zero kinetic energy (PFI-ZEKE) photoelectron spectroscopy. Vibronic progressions belonging to four electronic states were identified. The lowest energy states were clearly (1)Σ(+) and (3)Δ(1). Although the energy ordering could not be rigorously determined, the evidence favors assignment of (1)Σ(+) as the ground state. The (3)Δ(1) state, of interest for investigation of the electron electric dipole moment, is just 315.0(5) cm(-1) above the ground state. The PFI-ZEKE measurements for ThF yielded an ionization energy of 51 581(3) cm(-1). Molecular constants show that the vibrational constant increases and the bond length shortens on ionization. This is consistent with removal of a non-bonding Th-centered 6d or 7s electron. Laser excitation of ThF(+) was used to probe electronically excited states in the range of 19,000-21,500 cm(-1).

Understanding the influence of electrons in partially filled f- and d-orbitals on bonding and reactivity is a key issue for actinide chemistry. This question can be investigated by using a combination of well-defined experimental measurements and theoretical calculations. Gas phase spectroscopic data are particularly valuable for the evaluation of theoretical models. Consequently, the primary objectives of our research have been to obtain gas phase spectra for small actinide molecules. To complement the experimental effort, we are investigating the potential for using relativistic ab initio calculations and semiempirical models to predict and interpret the electronic energy level patterns for f-element compounds. Multiple resonance spectroscopy and jet cooling techniques have been used to unravel the complex electronic spectra of Th and U compounds. Recent results for fluorides, sulfides, and nitrides are discussed.

Rotationally resolved spectra for Be(2) (+) have been recorded using the pulsed-field ionization zero kinetic energy photoelectron technique. Vibrational levels in the range v(+)=0-6 were observed. The rotational selection rules confirmed that the ground state is (2)Sigma(u) (+), resulting from the removal of an electron from the sigma(u) antibonding orbital of Be(2). The bond energy and equilibrium distance for Be(2) (+) were found to be D(e) (+)=16 438(5) cm(-1) and R(e) (+)=2.211(8) A. The ionization energy for Be(2) [59 824(2) cm(-1)] was also refined by these measurements. Comparisons with high-level theoretical results indicate that the bonding in Be(2) (+) is adequately described by multi reference singles and doubles configuration interaction (MRDCI) calculations that employ moderate to large scale basis sets.

The properties of the HfF+ cation are thought to be well-suited for investigations of the electron electric dipole moment (eEDM) and temporal variations of the fine structure constant. Precision spectroscopic measurements involving the X1Σ+ and low-lying 3Δ1 states have been proposed to measure both. Due to the lack of data for HfF+, the design of these experiments has relied entirely on the predictions of electronic structure calculations. Spectroscopic characterizations of the X1Σ+, 3Δ1, 3Δ2 and 3Δ3 states are reported. The results further support the contention that HfF+ is a viable candidate for eEDM measurements. The spacings between adjacent X1Σ+ and 3Δ1 levels are found to be less favorable for the proposed studies of the fine structure constant.

Visible and near-infrared illumination induces 5f-5f and ligand-to-metal charge-transfer (LMCT) transitions of the neptunyl tetrachloride anion in polycrystalline Cs 2 U(Np)O 2 Cl 4 , and results in near-infrared luminescence from the second electronically excited state to the ground state. This photoluminescence is used as a detection method to collect excitation spectra throughout the near-infrared and visible regions. The excitation spectra of LMCT transitions in excitation spectra were identified in previous work. Here the measurement and analysis is extended to include both LMCT and intra-5f transitions. The results manifest variation in structural properties of the neptunium-oxo bond among the lowlying electronic states. Vibronic intensity patterns and energy spacings are used to compare bond lengths and vibrational frequencies in the excited states, confirming significant characteristic differences between those excited by 5f-5f transitions from those due to LMCT transitions. Results are compared with recently published SO-RASPT2 calculations of [NpO 2 Cl 4 ] 2- .