Infrared Band Contours. I. Spherical Top Molecules

Abstract

The relationships between the absorption coefficient, the true average absorption coefficient, and the pressure independent ``average'' absorption coefficient are considered. It is found that the absorption coefficient will equal the pressure independent average absorption coefficient provided that certain conditions are fulfilled. These conditions may be satisfied for heavy spherical tops but not for light molecules. The average absorption coefficient is obtained for fundamental bands of spherical tops including the interaction between the rotational angular momentum and the vibrational angular momentum of degenerate modes. The form of the band envelope is described and found to be in general agreement with experimental bands. The separation between the P and R maxima is found to be proportional to 1—ζ and this relation may be used for computing Coriolis constants. The band envelopes for CF₄ are measured and zeta values obtained from the P–R separation. They are found to be in good agreement with the sum rule. Finally, the zeta value for the CO stretching vibration of Ni(CO)₄ is obtained from the 5μ band envelope and conclusions are drawn regarding the form of the vibration.