Is Cd2 truly a van der Waals molecule? Analysis of rotational profiles recorded at the A0u+, B1u←X0g+ transitions

Rotational profiles of the 228Cd2 isotopomer recorded in the (υ′, υ″) = (26, 0), (27, 0), (42, 0), (45, 0), (46, 0), (48, 0) vibrational bands of the A0u+←X0g+ transition were analysed. As a result, the Bυ=26′=0.0311(5)cm-1, Bυ=27′=0.0309(5)cm-1, Bυ=42′=0.0279(5)cm-1, Bυ=45′=0.0278(5)cm-1, Bυ=46′=0.0275(5)cm-1 and Bυ=48′=0.0272(5)cm-1 excited- as well as the Bυ=0″=0.0207(5)cm-1 ground-state rotational constants of the (114Cd)2 were determined. The analysis allowed determining the absolute values for the Re′(A0u+)=2.71(7)Å and Re″=3.76(4)Å excited- and ground-state bond lengths, respectively. The obtained result - the Re″ - distinctly shorter than that obtained with assumption of pure ground-state van der Waals bonding, supports a theoretical prediction of a covalent admixture to the bonding. Analysis of the partially-resolved rotational profile recorded in the (υ′, υ″) = (38, 0) band of the same isotopomer recorded at the B1u←X10g+ transition allowed estimating the Bυ=38′=0.0317(2)cm-1 rotational constant in the B1u state.