State-to-state studies of the collisional quenching of electronically excited Cd(1<i>P</i>1) atoms

Abstract

A comprehensive study of the quenching of electronically excited Cd (5s5p 1P1) by a variety of simple molecules has been conducted using pulsed laser techniques. Except for He, Ar, and C2F6, deactivation occurs at essentially every encounter with all quenchers studied. The following process occurs with high efficiency with most of the quenching molecules studied, in striking violation of the Wigner spin rule and in spite of other available chemical or energy-transfer exit channels in many cases: Branching ratios for total Cd(5s5p3PJ) production were determined using an indirect method calibrated by earlier absolute measurements. Initial distributions of individual Cd(3P2,1,0) quantum states in process (1), determined by a pump-and-probe laser-induced fluorescence technique, show wide variations for different types of molecular quenchers. A qualitative model of bond-specific interactions of Cd(1P1) with quenching molecules has been developed which successfully rationalizes all the experimental results. For several of the molecules, and electronically statistical (5:3:1) distribution of Cd(3P2,1,0) is observed and attributed to a preponderance of bond sites at which there is a net attractive interaction of Cd(1P1), caused either by intersection of ionic surfaces (N2,CO) or by chemical bonding (alkyl C–H bonds), so that crossings occur with equal probability to all repulsive states correlating with Cd(3P2,1,0). Cases for which the Cd(3P2,1,0) distribution is skewed away from statistical towards Cd(3P2) [Ar, CH4, C2H6, C(CH3)4, C3H8] result from a less attractive potential with Cd(1P1) (due to stronger C–H bonds and/or weaker dispersion forces) and preferential crossings with the most repulsive ’’S’’-like states, which correlate with Cd(3P2,1,0). Cases for which the Cd(3P2,1,0) distribution is skewed away from statistical towards Cd(3P1) and Cd(3P0) (NO, C3F6, C2H2, C2H4, butadiene, propylene) are postulated to result from formation of long-lived complexes and the resultant formation of products which approach truly statistical distributions of vibrational, rotational, translational, and electronic energy. For the higher alkenes (isobutylene, 2-butene), Cd(1P1) interaction with the alkly C–H bonds completely dominates over the double bond interaction and electronically statistical Cd(3P2,1,0) distributions are observed.