On the quenching of helium 2 3S: Potential energy curves for, and nonadiabatic, relativistic, and radiative couplings between, the a 3Σ+u, A 1Σ+u, b 3Πg, B 1Πg, c 3Σ+g, and C 1Σ+g states of He2

Abstract

This work considers the possible role of nonadiabatic effects in the collisional quenching of He(2 3S). The electronic structure aspects of a nonadiabatic-radiative decay mechanism are analyzed. In this mechanism the a 3Σ+u state is coupled by relativistic, rotational, and radiative interactions to the A 1Σ+u state which serves as a gateway to the X 1Σ+g (electronically quenched) state of He2 through the spin-allowed dipole-allowed bound–free transition A 1Σ+u →X 1Σ+g. State averaged MCSCF/second-order CI wave functions for the ground X 1Σ+g state, and the excited, a 3Σ+u, A 1Σ+u, b 3Πg, B 1Πg, c 3Σ+g, and C 1Σ+g states (referred to here as the primary space) of He2 were determined. Using these wave functions all interstate matrix elements of the form 〈Ψ0(J)‖Ô‖Ψ0(I)〉 were determined for (i)Ô=ĤBP≡Ĥso+Ĥss where Ĥso and Ĥss are, respectively, the spin–orbit and dipolar spin–spin interactions in the Breit–Pauli approximation, (ii) Ô=L̂e, where L̂e is the total electronic orbital angular momentum operator, and (iii) Ô=μ̂ where μ̂ is the dipole moment operator. In the nonrotating molecule these interactions give rise to the spin-forbidden dipole-allowed radiative transitions (b 3Πg, c 3Σ+g) →A 1Σ+u. However a complete description of these radiative decay processes requires consideration of interactions originating outside the primary space. Thus in this work the spin-forbidden, dipole-allowed perpendicular, μ⊥ (J, A 1Σ+u0+), J=c 3Σ+g1, b 3Πg1 and parallel, μ∥(b 3Πg0+, A 1Σ+u0+), transition moments were determined using quasidegenerate perturbation theory. The computed potential energy curves, coupling matrix elements, and dipole moments permit a fully quantum mechanical analysis of the nonadiabatic-radiative quenching mechanism. A preliminary phenomenological analysis of aspects of this process is provided.