A new, definitive analysis of a very old spectrum: The highly perturbed A2Πu–X 2Πg band system of the chlorine cation (${\rm Cl}_2^ +$ Cl 2+)

Abstract

The laser-induced fluorescence spectrum of jet-cooled chlorine cation has been recorded in the 500-312 nm region with high sensitivity and rigorous vibrational and spin-orbit cooling. More than 80 bands of the highly vibrationally perturbed A (2)Π(u)-X (2)Π(g) electronic transition have been detected and shown to originate from the Ω = 3/2 spin-orbit component of v = 0 of the ground state. The spectrum extends some 3700 cm(-1) to the red of any previously published report and the 0-0 band has been identified for the first time. The bands have regular rotational structure but exhibit irregular vibrational intervals and isotope splittings. Our ab initio studies show that the perturbations are due to a ΔΩ = 0 spin-orbit interaction between the A(2)Π(3/2u) and B(2)Δ(3/2u) states which have an avoided crossing at ~2.5 Å, which corresponds to v ≈ 4 of the A state. The nonadiabatic coupled equations have been solved for this two-state interaction after constructing the diabatic potentials including only the diagonal (ΔΛ = 0) spin-orbit coupling, yielding low-lying vibrational energy levels, isotope splittings, and rotational constants in good agreement with experiment. The calculations show that many of the observed bands are actually transitions to predominantly B state vibrational levels, which borrow oscillator strength from the A-X transition through spin-orbit mixing. Starting from the coupled equations solutions, we have fitted the experimental data using an effective Hamiltonian matrix that includes the vibrational energy levels of the A and B states and a single electronic spin-orbit coupling term H(SO)(AB) which has a value of 240 cm(-1). Transitions up to v' = 32 in both states have been satisfactorily fitted for all three chlorine isotopologues, providing a quantitative description of the perturbations. Transitions to higher levels are complicated by interactions with other electronic states.