Analysis of collision induced far infrared spectrum of ethylene

Abstract

The theory for collision induced rotational absorption between nonpolar molecules is extended to near-symmetric top molecules using Wigner rotational matrices to represent the field and polarizability in the induced dipole moment. The resulting theory is applied to the collision induced spectrum of ethylene-ethylene. This spectrum is shown to be dominated by absorption due to the off-diagonal quadrupole moment in spherical coordinates, Q¯ ² ₂. Fitting the calculated to observed absorption coefficient at 140 cm^-1 leads to an accurate value of the quadrupole moment difference, Q¯ _xx - Q¯ _yy = -4·33 × 10^-26 esu·cm² in molecule fixed cartesian coordinates.