The Interaction of Intense Picosecond Infrared Pulses with Isolated Molecules
In the past decade there has been much interest in the dynamics of highly vibrationally excited and dissociating molecules. Selectivity at high levels of excitation may eventually lead to the realization of laser-controlled photochemistry, with broad applications in such diverse areas as laser-assisted chemical vapor deposition, isotope separation, and photosynthesis. Polyatomic molecules in the ground electronic state can reach levels of excitation up to the dissociation threshold by absorbing a large number of photons from a resonant high-power infrared laser. Despite the selectivity of infrared excitation at low energy, however, at high excitation the excitation energy is no longer confined to one ‘mode’. It has been shown experimentally that for molecules excited close to or above the dissociation threshold equilibration of energy occurs, in agreement with theoretical predictions. There is no agreement, however, as to the validity of theoretical models that presuppose equipartitioning of energy in the region below the dissociation threshold. Recent spontaneous Raman spectroscopy experiments on infrared multiphoton excited molecules in our laboratory provide information on the intramolecular vibrational energy distributions of highly vibrationally excited molecules in this region. The experimental results show that an excess of energy can remain in the pumped mode up to levels of excitation close to the dissociation threshold. This paper provides a review of the results that were obtained in the past three years, part of which were published previously.
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