A general theory for collision-induced absorption in molecules

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Abstract
We present a general theory for the dipole moment induced electrostatically in a pair of colliding molecules. Each molecule is placed in a weak, uniform external field and its charge distribution expanded in that field. The Coulombic interaction between these molecules is then expressed as an expansion in the external field; the coefficient of the linear term gives the induced pair dipole. The contributions from field gradients arise naturally in our approach. The spectral integrals γ and α have been calculated; our general expression can be used for overlapping as well as separated molecules. The asymptotic limit is evaluated explicitly for molecules of D∞h and Td symmetry. Our results show the role played by field gradients and raise serious doubts about the validity of previous determinations of higher-order multipole moments from collision-induced absorption spectra.