dorsal/arxiv
View SchemaInstantaneous Pair Theory for High-Frequency Vibrational Energy Relaxation in Fluids
| Authors | Ross E. Larsen, Richard M. Stratt |
|---|---|
| Categories | |
| ArXiv ID | physics/9810050 |
| URL | https://arxiv.org/abs/physics/9810050 |
| DOI | 10.1063/1.478185 |
Abstract
Notwithstanding the long and distinguished history of studies of vibrational energy relaxation, exactly how it is that high frequency vibrations manage to relax in a liquid remains somewhat of a mystery. Both experimental and theoretical approaches seem to say that there is a natural frequency range associated with intermolecular motions in liquids, typically spanning no more than a few hundred cm^{-1}. Landau-Teller-like theories explain how a solvent can absorb any vibrational energy within this "band", but how is it that molecules can rid themselves of superfluous vibrational energies significantly in excess of these values? We develop a theory for such processes based on the idea that the crucial liquid motions are those that most rapidly modulate the force on the vibrating coordinate -- and that by far the most important of these motions are those involving what we have called the mutual nearest neighbors of the vibrating solute. Specifically, we suggest that whenever there is a single solvent molecule sufficiently close to the solute that the solvent and solute are each other's nearest neighbors, then the instantaneous scattering dynamics of the solute-solvent pair alone suffices to explain the high frequency relaxation. The many-body features of the liquid only appear in the guise of a purely equilibrium problem, that of finding the likelihood of particularly effective solvent arrangements around the solute. These results are tested numerically on model diatomic solutes dissolved in atomic fluids (including the experimentally and theoretically interesting case of I_2 in Xe). The instantaneous pair theory leads to results in quantitative agreement with those obtained from far more laborious exact molecular dynamics simulations.
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"abstract": "Notwithstanding the long and distinguished history of studies of vibrational\nenergy relaxation, exactly how it is that high frequency vibrations manage to\nrelax in a liquid remains somewhat of a mystery. Both experimental and\ntheoretical approaches seem to say that there is a natural frequency range\nassociated with intermolecular motions in liquids, typically spanning no more\nthan a few hundred cm^{-1}. Landau-Teller-like theories explain how a solvent\ncan absorb any vibrational energy within this \"band\", but how is it that\nmolecules can rid themselves of superfluous vibrational energies significantly\nin excess of these values? We develop a theory for such processes based on the\nidea that the crucial liquid motions are those that most rapidly modulate the\nforce on the vibrating coordinate -- and that by far the most important of\nthese motions are those involving what we have called the mutual nearest\nneighbors of the vibrating solute. Specifically, we suggest that whenever there\nis a single solvent molecule sufficiently close to the solute that the solvent\nand solute are each other\u0027s nearest neighbors, then the instantaneous\nscattering dynamics of the solute-solvent pair alone suffices to explain the\nhigh frequency relaxation. The many-body features of the liquid only appear in\nthe guise of a purely equilibrium problem, that of finding the likelihood of\nparticularly effective solvent arrangements around the solute. These results\nare tested numerically on model diatomic solutes dissolved in atomic fluids\n(including the experimentally and theoretically interesting case of I_2 in Xe).\nThe instantaneous pair theory leads to results in quantitative agreement with\nthose obtained from far more laborious exact molecular dynamics simulations.",
"arxiv_id": "physics/9810050",
"authors": [
"Ross E. Larsen",
"Richard M. Stratt"
],
"categories": [
"physics.chem-ph"
],
"doi": "10.1063/1.478185",
"title": "Instantaneous Pair Theory for High-Frequency Vibrational Energy Relaxation in Fluids",
"url": "https://arxiv.org/abs/physics/9810050"
},
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