The microwave frequencies, line parameters, and spectral constants for NH3-14

Abstract

By combining our precision measurements of 119 microwave inversion lines for 14NH3 with the reported millimeter and far-infrared spectral measurements, we have obtained the following improved values for the rotational and centrifugal distortion constants in the ground vibrational state (all values in cm^-1) : B = 9.94647, Dj = -8.50155 x 10^-4, Djk = 1.58517 x 10^-3, HJ6 = 2.464 x 10^-7, HjJK = -8.397 x 10^-7, HJKK = 9.351 x 10^-7, in the lower inversion state; and B = 9.94145, Dj = —8.3452 x 10^-4, DJK = 1.5412 x 10^-3, Hjq = 2.218 x 10^-7, HjJK = —7.318 x 10^-7, HJKK = —7.830 x 10^-7 in the upper inversion state. Fits of the microwave inversion lines were compared for 10, 15, and 21-term polynomial and exponential models. The 15-term exponential model seems to be the best representation, since any further increase in the number of terms results in only a minor improvement in the computed inversion frequencies. The fit of the A = 3 lines was improved by including an additional, /-dependent term in the Nielsen-Dennison expression. Computed values are also given for the line widths, absorption coefficients, and energy levels. The widths and absorption coefficients are computed from Anderson’s theory, and are in good agreement with the measure- ments of Legan et al The rotation-inversion energy levels are obtained from the analysis of the combined spectral data. All of the available related data on the ground state of NH3 have been collected and analyzed in this report.