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

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 - X²Δ_3/2 band of ThF near 538.4 nm and the [18.2]Ω = 3/2 - X²Δ_3/2 band of ThCl near 551.0 nm have been recorded and analyzed. Stark shifts and splitting were analyzed to produce | μ⃗_el| values of 1.453(7) D and 0.588(9) D for the X²Δ_3/2 and [18.6]Ω = 3/2 states of ThF, respectively, and 2.022(35) D and 3.020(55) D for the X²Δ_3/2 and [18.2]Ω = 3/2 states of ThCl. Zeeman splittings and shifts were analyzed to produce g_e values of 1.038(4) and 1.079(4) for the X²Δ_3/2 and [18.6]Ω = 3/2 states of ThF and 1.130(4) and 1.638(4) for the X²Δ_3/2and [18.2]Ω = 3/2 states of ThCl. Analysis of g_e values demonstrates that the X²Δ_3/2 and [18.6]Ω = 3/2 states of ThF and the X²Δ_3/2 state of ThCl are predominately ²Δ_3/2 spin-orbit components, whereas the [18.2]Ω = 3/2 state of ThCl is an admixture of ²Δ_3/2 and ²Π_3/2 spin-orbit components. A molecular orbital description of the ground states is used to rationalize the observed | μ⃗_el|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.