Laser-Induced Unimolecular Reactions
The vibrational energy levels of a diatomic molecule are schematically shown in Fig.l. Due to anharmonicity they are not evenly spaced as they would be for a strictly harmonic oscillator. It is clear that the nearly even spacing is advantageous in reaching high vibrational excitation. For monochromatic excitation below the resonant frequency for the fundamental vibrational transition, the energy mismatch from real intermediate states is much smaller than in the electronic case. Nevertheless, the power level required for monochromatic radiation to excite a diatomic molecule, such as HF, to a level with fourteen vibrational quanta close to the dissociation limit is very high. It would be difficult to avoid dielectric breakdown of the gas. In fact, dissociation of a diatomic molecule by monochromatic infrared radiation has not been convincingly demonstrated. Perhaps some emphasis is needed to say that the interest is here focused on collisionless mechanisms. One can, of course, heat a molecular vapor to sufficiently high temperature so that collisionally induced dissociation sets in. It is possible to reach such temperatures by one-photon absorption processes in a c.w. laser beam, if the frequency corresponds to a strong absorption line of the molecular species.
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