Linewidth dependence of quasi-stationary states in large molecules

We study the quasi-stationary states, or eigenstates, of the decay matrix for a system described by one single vibronic state |s〉 coupled to a manifold {l}, both |s〉 and {l} levels being coupled to separate continua which endow them with finite linewidths γ_s, γ_l. When all the zero-order states have the same linewidth (not necessarily zero) the quasi-stationary states are identical to the molecular eigenstates; when the difference between the linewidths γ_s ? γ_l is large enough, the zero-order sets (s, {l}) are completely decoupled, a result which has been established for the two-level system. We indicate some consequences of collisional broadening involving pressure effects in the gas phase.