Relative Optical Collision Diameters from the Pressure Broadening of Individual Infrared Absorption Lines

A matching method for the study of the widths of individual infrared absorption lines is used to obtain relative values of optical collision diameters, and to determine qualitative features of molecular interactions in pressure broadening phenomena. Isolated vibration-rotation lines of HCl and C${\mathrm{H}}_4$ are successively broadened by the same fourteen gases, and collision diameters derived are compared with each other and with previously published values of optical collision diameters. Striking correlations are found between the HCl relative diameters and absolute values of optical collision diameters obtained by other investigators using microwave techniques on the inversion spectrum of N${\mathrm{H}}_3$. The experimental data here presented are consistent with the presence of a pressure broadening interaction between the induced dipole of a broadener molecule and some undefined property of the absorber, for any broadener used. An additional interaction between the quadrupole moment of the broadener and the dipole of the absorber appears to exist when unsymmetrical molecules are employed to broaden the spectral lines of polar absorbers.