Absorption in the Low-Frequency Wing of the NH3 Inversion Spectrum

The absorption due to the low-frequency wings of the NH3 inversion lines was measured accurately at 2800 Mc for a pressure of 10-cm Hg. At this pressure, where the shift in inversion frequency has not set in, the absorption was about 40 percent larger than that calculated by summing the Van Vleck-Weisskopf expression over the rotational states using known resonance frequencies and line widths. A similar discrepancy was found in comparing the theoretical absorption with previously obtained data at 9000 Mc, while at resonance, 24 000-Mc theory and experiment were in very good agreement. To investigate various features of pressure broadening, the variation of absorption with pressure (5 to 30-cm Hg), temperature (−12 to 100°C), and frequency (1200 to 2800 Mc) was examined. These functional variations are interpretable, for the most part, by the Van Vleck-Weisskopf relation.