Pressure Broadening of Symmetric-Top Molecules in the Millimeter-Wavelength Region

Self‐broadened and foreign‐gas‐broadened widths of a number of millimeter lines of the symmetric‐top molecules CH₃F and CHF₃ have been measured and compared with values calculated by Anderson's and Murphy and Boggs' theories of pressure broadening. The lines studied are the $\text{J\hspace{0.17em}=\hspace{0.17em}2\hspace{0.17em}→\hspace{0.17em}3, K\hspace{0.17em}=\hspace{0.17em}0, 1, 2}$ lines of CH₃F, and the $\text{J\hspace{0.17em}=\hspace{0.17em}6\hspace{0.17em}→\hspace{0.17em}7, K\hspace{0.17em}=\hspace{0.17em}3, 4, 5, 6}$ lines of CHF₃ at 300°K and the $\text{K\hspace{0.17em}=\hspace{0.17em}5}$ and 6 lines at 200°K. The broadening of the $\text{J\hspace{0.17em}=\hspace{0.17em}2\hspace{0.17em}→\hspace{0.17em}3, K\hspace{0.17em}=\hspace{0.17em}2}$ line of CH₃F by CO₂ at 300°K was also studied. With one exception where Anderson's theory and experiment are in good agreement, viz., the $\text{J\hspace{0.17em}=\hspace{0.17em}2\hspace{0.17em}→\hspace{0.17em}3, K\hspace{0.17em}=\hspace{0.17em}2}$ line of CH₃F (300°K), this theory gives results greater than the experimental values. Although the theory of Murphy and Boggs is over all in better agreement with experiment, here too a number of discrepancies appear. Neither theory gives the observed dependence of the linewidth on the rotational quantum number $K$ .