Optical Stark and Zeeman Spectroscopy of Thorium Fluoride (ThF) and Thorium Chloride (ThCl).

Abstract

Experimentally and theoretically determined magnetic and electric dipole moments, bond distances, and vibrational spacings are used for a comparative study of the bonding in ThF and ThCl. Numerous bands in the visible electronic spectra between 16 400 and 18 800 cm-1 of supersonically cooled molecular beam samples have been detected using medium-resolution (Δν ≈ 0.1 cm-1) 2D spectroscopy. High-resolution (Δν < 20 MHz) field-free, Stark, and Zeeman spectroscopy of the detected [18.6]Ω = 3/2 - X2Δ3/2 band of ThF near 538.4 nm and the [18.2]Ω = 3/2 - X2Δ3/2 band of ThCl near 551.0 nm have been recorded and analyzed. Stark shifts and splitting were analyzed to produce | μ⃗el| values of 1.453(7) D and 0.588(9) D for the X2Δ3/2 and [18.6]Ω = 3/2 states of ThF, respectively, and 2.022(35) D and 3.020(55) D for the X2Δ3/2 and [18.2]Ω = 3/2 states of ThCl. Zeeman splittings and shifts were analyzed to produce ge values of 1.038(4) and 1.079(4) for the X2Δ3/2 and [18.6]Ω = 3/2 states of ThF and 1.130(4) and 1.638(4) for the X2Δ3/2and [18.2]Ω = 3/2 states of ThCl. Analysis of ge values demonstrates that the X2Δ3/2 and [18.6]Ω = 3/2 states of ThF and the X2Δ3/2 state of ThCl are predominately 2Δ3/2 spin-orbit components, whereas the [18.2]Ω = 3/2 state of ThCl is an admixture of 2Δ3/2 and 2Π3/2 spin-orbit components. A molecular orbital description of the ground states is used to rationalize the observed | μ⃗el|values for the ThX (X = F, Cl, O, and S) series and garner insight into the bonding mechanism. The dipole moments in the ground state of ThF and ThCl have been calculated using relativistic coupled-cluster methods. It is demonstrated that the systematic inclusion of electron-correlation contributions plays an essential role in obtaining accurate predictions for the dipole-moment values in ThF and ThCl.