Full State-Resolved Energy Gain Profiles of CO₂ (<i>J</i> = 2−80) from Collisions of Highly Vibrationally Excited Molecules. 1. Relaxation of Pyrazine (<i>E</i> = 37900 cm⁻¹)

Abstract

State-resolved energy gain profiles for "strong" and "weak" collisions of pyrazine (E = 37900 cm(-1)) and CO(2) are reported. Nascent energy profiles for scattered CO(2) (00(0)0, J = 2-64) were measured using high-resolution transient IR absorption spectroscopy at lambda = 2.7 microm. The data are combined with earlier data for CO(2) (J = 58-80) to yield the full state-resolved distribution of scattered CO(2) (00(0)0). The scattered CO(2) (00(0)0, J = 2-80) molecules have a biexponential rotational distribution with T(lowJ) = 437 +/- 50 K for J < 50 and T(highJ) = 1145 +/- 110 K for J > 50. Fitting the data with a two-component exponential model yields CO(2) product distributions with T(rot) = 329 and 1241 K. The cooler distribution accounts for 78% of the scattered population and results from elastic or weakly inelastic collisions that induce very little rotational excitation in CO(2). The hotter distribution involves large changes in CO(2) rotational energy and accounts for 22% of collisions. The relative translational energy of the scattered molecules increases as a function of final CO(2) rotational state with T(rel) = 860 K for J = 2, 1650 K for J = 60, and 5500 K for J = 80. The total rate constant for appearance of scattered CO(2) (00(0)0) is k(app) = (4.8 +/- 1.4) x 10(-10) cm(3) molecule(-1) s(-1), which is approximately 85% of the Lennard-Jones collision rate. Population depletion measurements yield a collision rate of k(dep) = (5.7 +/- 1.8) x 10(-10) cm(3) molecule(-1) s(-1), which is in very good agreement with the Lennard-Jones collision rate. The full energy transfer probability distribution function P(DeltaE) for pyrazine(E)-CO(2) collisions is presented and compared to results from other studies.