Excited electronic states of Na2 and K2: The potential for long-lived “reservoir” states leading to collision induced population inversions

Potential energy curves (PECs) for the spin-free (ΛS) and spin-orbit (Ω) states associated with the four lowest-lying dissociation channels of Na2 and K2 were calculated at the SA-CASSCF/SO-CASPT2/aug-cc-pwCVQZ-DK level. The PECs of Na2 were consistent with the experimental data and with the FS-CCSD (2,0) calculations, reproducing the double-well and the "shelf" character for some of the potentials of the excited states. For K2, the PECs behaved in a similar way and the spectroscopic parameters for the ground and the excited states are in good agreement with the available experimental values. The dissociation energy of K2 was predicted to be De = 4454 cm-1, within an agreement of 5 cm-1 with the experiments. For Na2, De = 5789 cm-1 compared to the experimental value of 6022 cm-1. The inclusion of spin-orbit coupling effects resulted in avoided crossings, which affect the PECs. Spin-orbit changes the predicted curves for some excited Ω states arising from ΛS states that overlap each other, affecting their associated vibrational frequencies and bond distances. The current studies of the low-lying states in K2 reveal a similar structure to those of Na2, which suggests the accessibility of long-lived energy storing reservoir states and possible population inversions in K2 following prior experimental work on the reaction of halogen atoms with Na3 to produce excited states of Na2.