Nonadiabatic theory of collision-broadened atomic line profiles

The close-coupled theory of atomic scattering in a radiation field can be used to calculate nonadiabatic effects on collision-broadened atomic line profiles. When the strength of the radiation field is not too large, reduced free-free dipole matrix elements, which are independent of the field strength and are analogous to the free-free Franck-Condon factors of line profile theory, can be defined in terms of the $S$-matrix elements for light-induced atomic scattering. The profile can then be calculated even when the molecular states are mixed by off-diagonal terms in the molecular Hamiltonian due to the breakdown of the Born-Oppenheimer approximation. Numerical close-coupled scattering calculations are used to calculate the profile for the asymptotically forbidden, collision-induced radiative transition $\mathrm{O}{}_{}{}^1{}_{}{}^{}S+\mathrm{Ar}\to \mathrm{O}{}_{}{}^1{}_{}{}^{}D+\mathrm{Ar}+h\nu$. The profile was calculated in two ways: (1) with the use of the normal Born-Oppenheimer approximation for the final states and (2) with the use of the new technique to treat the nonadiabatic mixing among the ${}_{}{}^1{}_{}{}^{}\Sigma$, ${}_{}{}^1{}_{}{}^{}\Pi$, and ${}_{}{}^1{}_{}{}^{}\Delta$ final states. The Coriolis interaction mixes the Hund's case-($a$) $\Lambda$ states asymptotically to give Hund's case-($e$) states. The central and red-wing parts of the profile which originate primarily from large internuclear separations are strongly affected by this mixing. The calculated profile which takes this mixing into account agrees well with the experimental profile but differs significantly from the Born-Oppenheimer profile. The differences are explained in terms of intensity borrowing and Hund's case-($e$) selection rules.