Beau J. Barker

Spectroscopic investigations of ThF and ThF

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).

Spectroscopy and Structure of the Simplest Actinide Bonds

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.

Spectroscopic characterization of Be2+X Σ2u+ and the ionization energy of Be2

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.

Communication: spectroscopic measurements for HfF+ of relevance to the investigation of fundamental constants.

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 X(1)Σ(+) and low-lying (3)Δ(1) 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 X(1)Σ(+), (3)Δ(1), (3)Δ(2) and (3)Δ(3) states are reported. The results further support the contention that HfF(+) is a viable candidate for eEDM measurements. The spacings between adjacent X(1)Σ(+) and (3)Δ(1) levels are found to be less favorable for the proposed studies of the fine structure constant.

Experimental and theoretical studies of the electronic transitions of BeC

Electronic spectra for BeC have been recorded over the range 30,500-40,000 cm(-1). Laser ablation and jet-cooling techniques were used to obtain rotationally resolved data. The vibronic structure consists of a series of bands with erratic energy spacings. Two-color photoionization threshold measurements were used to show that the majority of these features originated from the ground state zero-point level. The rotational structures were consistent with the bands of (3)Π-X(3)Σ(-) transitions. Theoretical calculations indicate that the erratic vibronic structure results from strong interactions between the four lowest energy (3)Π states. Adiabatic potential energy curves were obtained from dynamically weighted MRCI calculations. Diabatic potentials and coupling matrix elements were then reconstructed from these results, and used to compute the vibronic energy levels for the four interacting (3)Π states. The predictions were sufficiently close to the observed structure to permit partial assignment of the spectra. Bands originating from the low-lying 1(5)Σ(-) state were also identified, yielding a (5)Σ(-) to X(3)Σ(-) energy interval of 2302 ± 80 cm(-1) and molecular constants for the 1(5)Π state. The ionization energy of BeC was found to be 70,779(40) cm(-1).

Pulsed-field ionization zero electron kinetic energy spectrum of the ground electronic state of BeOBe+.

The ground electronic state of BeOBe(+) was probed using the pulsed-field ionization zero electron kinetic energy photoelectron technique. Spectra were rotationally resolved and transitions to the zero-point level, the symmetric stretch fundamental and first two bending vibrational levels were observed. The rotational state symmetry selection rules confirm that the ground electronic state of the cation is (2)Σ(g)(+). Detachment of an electron from the HOMO of neutral BeOBe results in little change in the vibrational or rotational constants, indicating that this orbital is nonbonding in nature. The ionization energy of BeOBe [65480(4) cm(-1)] was refined over previous measurements. Results from recent theoretical calculations for BeOBe(+) (multireference configuration interaction) were found to be in good agreement with the experimental data.