A selection rule for M-dependent transitions in collisional excitation of open shell diatomics

A selection rule is derived within the collision frame for M=0→M′=0 transitions in collisions between an open shell diatomic of odd multiplicity, e.g., 1Π or 3Σ, and a structureless particle. For 1Π and 3Σ states, the scattering amplitude for the J→J′ transition vanishes unless e/f symmetry is conserved. We then discuss how this selection rule for a 1Π state could be observed experimentally by measurement of the polarized fluorescence intensity ratio I⊥/I∥ in an optical–optical double resonance experiment. This raises the possibility of exploiting our selection rule to study the comparability of cross sections in the collision and laboratory frames.