COLLISION-INDUCED ANISOTROPIC RELAZATION IN GASES

An expression is derived for the cross sections, σj(x), for the collision-induced relaxation of the multipole moments of an ensemble of excited atoms in a state of total angular momentum j. Various general sum rules are obtained for the σj(x), as well as for the Λj(x)σj(x)-σj(0), the cross sections for reorienting collisions. For a j=1 state, one obtains Λ1(1)Λ1(2)=53. This result implies that the cross section for collision induced |Δm|=2 transitions is always twice that for |Δm|=1 transitions, independent of the assumed interaction potential. Absolute cross sections are obtained using the van der Waals collision model. Translational motion is treated in a classical linear path approximation. It is shown that the cross sections can be written in the form σj(x)=Dφj(x)v35v, where v is the relative velocity between the colliding atoms, D depends on the particular atoms and states considered, and φj(x) is a geometric factor which is tabulated for j values up to j=8. The Λj(x) are related to the cross sections for transfers between magnetic sublevels. It is shown that in the present model a transfer from sublevel α to -α is forbidden for states with half-integer j values. © 1969 The American Physical Society.