New Selection Rule, Illustrated by Collision Cross Section Studies on the B 1Σ+ ← X 1Σ+ and the C 1Σ+ ← X 1Σ+ Transitions in Carbon Monoxide at Kinetic Energies of 300, 400, and 500 eV

Generalized oscillator strengths have been determined at 300, 400, and 500 eV for the two transitions B 1Σ+ ← X 1Σ+ and C 1Σ+ ← X 1Σ+ in carbon monoxide. The Born approximation does not hold accurately for either transition. The largest deviations (percentage wise) are found for the first of the above transitions and the trend of oscillator strength with momentum change (of the colliding electron) is very different from anything previously observed. This abnormal behavior is also found in relative intensity measurements done at low kinetic energies (to 23 eV). These transitions were studied because recent experimental and theoretical investigations [J. Chem. Phys. 53, 3801, 3806 (1970)] indicated that deviations from the Born approximation at high kinetic energies are subject to a selection rule; the deviations are maximum when term symbols are the same in ground and excited states. Previous experimental tests have been done only on forbidden transitions. The experiments reported in the present paper show that the deviations also occur for allowed transitions in which term symbols are the same in ground and excited states. Previous experiments are also reconsidered with a view to testing the selection rule hypothesis.