Rotational relaxation matrix for fast non-Markovian collisions

By using the Zwanzig-Mori theory, the rotational relaxation {Γ} matrix of the sudden approximation is corrected for rotational adiabaticity, off-energy shell scattering, and initial bath-molecule correlations. Expressions derived for the case of a linear rotator perturbed by fast binary collisions are shown to satisfy all known fundamental relations; in addition, the {Γ} matrix is symmetric and positive definite in line space. The general case of non-Markovian relaxation of an arbitrary molecular rth rank tensor is treated, which substantially expands the scopes of the impact scalar $(r=0)$ relaxation. A model of {Γ} is developed compatible with the known data on the N₂-N₂ potential and is shown to fit well a variety of different relaxation characteristics $(r=0,1,2)$ measured in compressed room-temperature nitrogen.