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)A and Re''=3.76(4)A 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