Theoretical Model for the Differential Quenching Rates of CO Fluorescence by Ortho- and Parahydrogen

Abstract

The experimental measurements of Millikan and co-workers, which show that para-H2 quenches vibrationally excited CO faster than normal-H2 (14para, 34ortho) at temperatures below 500°K, are analyzed in the Born–Bethe approximation. Beginning with the observation that the rotational transition j = 2 → j = 6 in H2 has almost the same energy as the vibrational transition υ = 1 → υ = 0 in CO and invoking the assumption that long-range forces are solely responsible for the difference in quenching rates between the two spin species of H2, we find that the temperature dependence of the measured rate differences can be reproduced almost quantitatively by truncating the Born–Bethe series at second order and by using an interaction potential derived from a multipole expansion of nonoverlapping charge distributions. The implications of this result, and the assumptions on which it is based, are discussed in considerable detail.