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Abstract

We present a simple analytic approximation for evaluating the ensemble-averaged orientation or alignment of a beam of molecules subjected to a strong static or radiative field. This approximation is based on the eigenproperties which polar or polarizable molecules exhibit in the strong-field, harmonic-librator limit, and on the Boltzmann statistics of the free rotor states which adiabatically correlate with the harmonic librator states. For either the permanent or induced dipole case, the resultant formula involves just two dimensionless parameters which characterize the strength of the molecule-field interaction and the rotational temperature. The net polarization of a molecular beam thus obtained is shown to be in an excellent agreement with the exact values computed numerically from first principles. The validity range of the approximation includes the large-interaction, high temperature regions of the parameter space where first-principle calculations are onerous.