Theory of Absolute Intensities for Vibration-Rotation Transitions in Methane

Transition dipole moments have been calculated for lines in the ${\nu }_3$, ${\nu }_4$, $2{\nu }_3$, and $3{\nu }_3$ vibration-rotation bands of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${\mathrm{H}}_4$. The values of ${\mu }^2$ are 0.76 ± 0.10, 0.69 ± 0.04, (1.86 ± 0.07) × ${10}^{-3\mathrm{}}$, and (2.96 ± 0.16) × ${10}^{-6\mathrm{}}$ ${\mathrm{D}}^2$, respectively. These results have been applied to predict absolute line intensities. In particular, for the $P7F$ line at 2947.912 ${\mathrm{cm}}^{-1\mathrm{}}$, the calculated intensity is (1.34 ± 0.17) ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{atm}}^{-1\mathrm{}}$ at 300° K. Furthermore, ultraweak absorption features observed in the $3{\nu }_3$ region may be accounted for on the basis of transitions with $J\approx 30$.