Thorium Monosilicide, ThSi: An Experimental and Theoretical Study

The present theoretical and experimental combination study investigates the ThSi molecule in detail. Computationally, we utilized high-level multireference and coupled-cluster levels of theory conjoined with large correlation consistent basis sets to study a series of electronic and spin-orbit states of ThSi. We report potential energy curves (PECs), electron configurations at equilibrium distances, spectroscopic constants, energetics, and spin-orbit coupling effects for 16 electronic states of ThSi. The studied 16 electronic states are arranged tightly within 0.9 eV, highlighting the complexity of the electronic spectrum of ThSi. The ground electronic state of ThSi is a single-reference 1¹Σ⁺ state that derives from the 1σ²2σ²1π⁴ electronic configuration. The Ω = 0⁺ spin-orbit ground state of ThSi is composed of 1¹Σ⁺ (47%) and 1³Π (44%) electronic states. Our measured bond energy (D₀) of ThSi, obtained using resonant two-photon ionization (R2PI) spectroscopy is 3.146(4) eV, where the assigned error limit is given in parentheses in units of the last quoted digits. The computed D₀ of ThSi (Ω = 0⁺) at the CBS-C-CCSD(T)-δT(Q)-δDK-δSO level (3.181 eV) is in good agreement with the experimental value. Our derived enthalpy of formation for ThSi, ΔfH0Ko(ThSi(g)), is 971.8(6.0) kJ/mol. Finally, we have performed density functional theory (DFT) calculations for ThSi(1¹Σ⁺) using 16 exchange correlation functionals that span multiple rungs of "Jacob's ladder" of density functional approximation (DFA) to assess the DFT errors on D₀, re, and ωe of ThSi with respect to experimental and ab initio coupled-cluster values.