Knowledge Bank

According to Randall and Wright who discovered the inversion doubling of the $NH3$ rotational lines in the far infrared, the splitting is 1.33 to $1.36cm-1$. More recent measurements of Oetjen et al. and of McCubbin and Sinton confirmed this result. The line splitting should be just twice the splitting of the inversion terms which was found to be $0.794cm-1$ from microwave measurements. Two times 0.794, however, disagrees with 1.34 beyond the limit of error. On the other hand Bleancy and Loubser found an anomalous pressure dependence of width and position of microwave inversion lines which the theories of Margenau and of Anderson explained semi-quantitatively by the dipole interaction of the molecules at higher pressures. Up to pressures of 1 atmosphere Margenau’s theory agrees well with the experiments which were recently extended to $ND3$ by Birnbaum and Maryott. It seemed of interest to examine whether this anomalous pressure-shift of the inversion spectrum shows up in the splitting of the rotational line doublets in the far infrared, and whether the discrepancy mentioned above can be explained by the pressure effect. Measurements were made on the $NH3$-lines of $J=1,J=2$ and $J=0$. The latter line is not doubled, but its position should be shifted to lower frequencies by one half of the inversion splitting. Our experiment shows in fact a decrease of the inversion splitting with increasing pressure, but the effect differs for the different J-values. We should like to interpret the microwave results tentatively as an averaged effect with respect to the different J (and K) values.