Vibrationally excited populations from IR‐multiphoton absorption. I. Absorbed energy and reaction yield measurements

Abstract

The molecule 1,1,2‐trifluroethane (TFE) was used in experiments to determine the population distribution of excited molecules produced by infrared multiphoton absorption induced by high power TEA CO2 lasers operating at 1079.85 cm−1 [9.6 μm R(22) line]. Optoacoustic measurements of absorbed laser power provided a measure of the mean energy of the population distribution, while very low pressure photolysis measurements of the collision‐free decomposition yield gave information about the high‐energy tail of the distribution. The experimental results were accurately simulated using a Master Equation model that incorporated Quack’s statistical–dynamical theory of infrared multiphoton absorption (cases B and C), RRKM unimolecular reactions (three channels), and collisional energy transfer. The computer simulations included known TFE molecular properties and only four adjustable parameters, which were very highly constrained in order to fit the experimental data. From the simulations, we conclude that the optical coupling matrix elements are dramatically reduced in magnitude for energies above the reaction thresholds. This effect is symptomatic of the vibrational anharmonicity due to the presence of the reaction channels, even in molecules that have not yet reacted, resulting in vibrational frequency shifts of the absorption lines out of resonance with the laser line. This effect is expected to be present and observable in other highly vibrationally excited molecules.