The unimolecular reaction of isolated CF3CN: The influence of laser fluence/intensity on the rovibronic excitation of CN(X 2Σ+) produced via infrared multiple photon dissociation

Abstract

Measurements of nascent CN(X 2Σ+) rovibronic state distributions following the unimolecular reaction CF3CN‡→CF3+CN are reported. Excitation under collision free conditions is provided by IR multiple photon excitation using the focused output from a CO2 TEA laser at fluences 3–150 J cm−2. At fluences 3–20 J cm−2, the CN(X 2Σ+,v′′ = 0) rotational temperature increases monotonically from 500 to 1200 K with increasing fluence, while at fluences 30–150 J cm−2, the rotational and vibrational temperatures do not change and are TR = 1200±100 K and TV = 2400±150 K. At low fluences, the increase in V, R, T excitations with increasing fluence reflects an increase in the vibrational excitation of the dissociating parent molecules. The ultimate level of excitation that the parent molecules attain depends both on the fluence and the intensity of the laser, and the appearance time of the CN fragments decreases with increasing fluence, since molecules excited significantly above dissociation threshold decompose more rapidly than those near reaction threshold. At fluences ≳30 J cm−2, the sample is depleted before the CO2 laser output reaches its peak intensity, thus causing the observed ’’saturation’’ effect in product excitation level. A straightforward model which includes effects due to laser pulse shapes can account for the experimental observations.