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

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Abstract
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.