A theory of the infrared absorption spectrum of light dipolar diatomic molecules dissolved in simple liquid solvents is presented and discussed. This is done by keeping the quantum character of the orientational degrees of freedom of the absorbing molecules. Moreover, the low frequency part of the density fluctuations around this molecule is taken into account by introducing an almost static polarizing action which strongly disturbs the free rotation wavefunctions and therefore eliminates from the zeroth order of the calculation the usual selection rules of the diatomic rotator. The presence of this polarization is suggested by a study of the factorization of the density operator; the fluctuations of the true coupling of the active molecule with the liquid in respect to the quasistatic coupling considered above implies a broadening of all the possible transitions of the active molecule already polarized. Models are examined to represent the correlation of this fluctuation. An example of HCl molecule dissolved in liquid CCl4is then studied, and this leads to a satisfactory comparison of the theoretical results with the experimental band profiles.
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