Structure of excitation and fluorescence spectra recorded at the ¹0_u⁺(5¹P₁)–X¹0_g⁺ transition of Cd₂

Excitation and fluorescence ultraviolet spectra of Cd₂ recorded at the ¹0_u⁺(5¹P₁)–X¹0_g⁺ transition are reported. The Cd₂ molecules (seeded in Ar) produced in a continuous free-jet supersonic beam were excited in a vacuum chamber with a pulsed dye-laser beam. A well-resolved vibrational structure of the ¹0_u⁺←X¹0_g⁺ excitation spectrum as well as the isotopic structure of the vibrational components were recorded. Analysis of the spectrum yielded vibrational constants for the ¹0_u⁺ state: ω_e^′=100.50±0.25cm^-1, ω_e^′x_e^′=0.325±0.003cm^-1, D₀^′=8638±15cm^-1, D_e^′=8688±15cm^-1, and ΔR_e=R_e^″-R_e^′=1.04±0.01Å derived for the ²²⁶Cd₂ isotopomer. The ¹0_u⁺ state potential-energy (PE) curve was obtained numerically using an inverse perturbation approach (IPA) procedure. Condon internal diffraction (CID) patterns in the ¹0_u⁺→X¹0_g⁺ fluorescence band, emitted upon the selective excitation of the v^′=38 and v^′=39 vibrational components of the ²²⁶Cd₂ isotopomer, were observed and improved the v^′ assignments derived from the analysis of the isotopic structure. Analysis of the fluorescence spectrum yielded information on the repulsive part of the ground-state interatomic potential. The result confirms a relatively soft repulsion between two Cd atoms in the short-range (2.53–4.05 Å) region and makes allowance for a covalent admixture to the ground-state van der Waals bonding. Quasirelativistic valence ab initio calculations on the PE curves for the investigated states have been performed at the complete-active-space multiconfiguration self-consistent-field (CASSCF/CAS) multireference second-order perturbation theory (CASPT2) level with the total of 40 correlated electrons. In the calculations, the Cd atom is considered as a 20-valence electron system whereas the Cd²⁰⁺ core is replaced by an energy-consistent pseudopotential which also accounts for scalar-relativistic effects and spin-orbit interaction within the valence shell. A comparison with results obtained from other experiments and ab initio calculations is presented.