AN EMPIRICAL EXPRESSION FOR LINEWIDTHS OF AMMONIA FROM FAR-INFRARED MEASUREMENTS

The hydrogen- and self-broadened linewidths of 116 (NH3)-N-14 ground state transitions with J,K = 1,0 to 10,10 have been measured at 0.006-cm(-1) resolution between 40 and 210 cm(-1) using a Bruker spectrometer. These experimental widths have been reproduced to 2.4 and 11%, respectively, using a heuristically derived expression of the form gamma = a(0) + a(1)J'' + a(2)K'' + a(3)J''(2) + a(4)J''K'', where J'' and K'' are the lower state symmetric-top quantum numbers. This function has also been applied to the measured widths of the 58 transitions of nu(1), each broadened by N-2, O-2, Ar, H-2, He [Pine el al., J. Quant. Spectrosc. Radiat. Transfer 50, 337-348 (1993)], and NH3 [Markov el al., J. Quant. Spectrosc Radiat. Transfer 50, 167-178 (1993)]. The rms of the observed minus calculated widths are +/-5% or better for the five foreign broadeners. For the self-broadening case, the expression fails to reproduce the K = 0 data. The values of the fitted constants suggest that for some broadeners the expression might also be written as gamma = a(0) + b(1)J'' - a(2)(J'' - K'') + b(3)J''(J'' - K''). (C) 1994 Academic Press, Inc.