Evaluation of Molecular Quadrupole Moments from Broadening of Microwave Spectral Lines. II. Calculation of the Quadrupole Moments and Discussion

Abstract

Molecular quadrupole moments of OCS, CHF₃, N₂O, NO, H₂, HD, D₂, N₂, and CO₂ have been obtained from the fit of the calculated line‐broadening constants ${\Lambda }_{\mathrm{aa}}$ for self‐broadening and ${\Lambda }_{\mathrm{ab}}$ for foreign‐gas broadening to the experimental results reported in a previous paper. The theoretical line‐broadening constants were calculated using Anderson's theory on pressure broadening in the modification of Tsao and Curnutte. The interaction mechanisms taken into account were: dipole–dipole, dipole–quadrupole, quadrupole–dipole, quadrupole–quadrupole, and London dispersion. The results for quadrupole moments in 10⁻²⁶ esu are summarized in the table below in which the rows are numbered by the absorbing molecule and the columns by the colliding (perturbing) molecule. The general conclusion is that Anderson's theory with the commonly used matching condition at low values of the impact parameter does not allow a reliable determination of molecular quadrupole moments, most probably because of overestimation of the dipole–dipole contribution. The theory fails completely for CHF₃ broadening and is very sensitive to experimental errors in the case of OCS, and the influence of $b_{\min }$ is rather questionable in the case of N₂O.