Linewidths of the Microwave Inversion Spectrum of Ammonia

Abstract

The linewidths of some 25 inversion spectral lines of ammonia in the frequency region of 18–29 kMc/sec have been measured. Corrections due to various effects such as source modulation, cell‐length broadening, and Doppler shift were applied to the experimental data to evaluate the linewidth parameters. Self‐broadening and foreign‐gas broadening by some 15 different gases have been studied. In the case of self‐broadening of the series of lines with J=K for which the effect of rotational resonance is negligible, the width parameters determined experimentally agree well with those calculated by Anderson's theory from (2, 2) to (5, 5), but appreciable deviations are found at higher J. For the lines with J≠K, the discrepancy becomes larger and increases with increasing difference between J and K. The quantum number dependence of the foreign‐gas broadening by methyl chloride and by carbon dioxide has been investigated in detail. For methyl chloride broadening the theoretical and measured widths agree with each other to about the same extent as in self‐broadening, whereas the agreement is somewhat less satisfactory in the case of carbon dioxide. An alternative formulation of the collision theory of line broadening is presented. Here the theory of radiation interruption is treated as a problem of multichannel collision. The essential feature of this formulation is that it includes the higher‐order effects of the collision process rather than just the usual first‐order terms. This analysis leads to suggestions as to possible directions of improving the calculations.