The inversion spectrum of ammonia at centimetre wave-lengths

Abstract

A technique is described for the investigation of gaseous spectra at high radio-frequencies. The sensitivity of the method is such that an absorption of 2 x 10 -6 per cm. of path can be detected at wave-lengths around 1 cm. The inversion spectrum of ammonia in the ground vibrational state has been examined at wave-lengths between 1.1 and 1.6 cm. and found to contain an extensive fine structure, arising from centrifugal distortion of the molecule. Twenty-nine lines have been identified, each corresponding to a different rotational quantum state. The wave numbers of these lines can be accurately represented by the formula v = 0.7935 + [-0.0050 5 J 2 +J+0.0070 4 K 2 ] + 0.63[-0.0050J 2 +J+ 0.0070K 2 ] 2 cm. -1 (vacuo) where J, K are the rotational quantum numbers of the symmetrical top molecule. Detailed measurements of 18 lines at a pressure of 0.5 mm. Hg show that their half widths, due to collision broadening, lie between 2 and 5 x 10 -4 cm. -1 , while their intensities agree with the theoretical values, within the experimental error of ± 5%. The integrated intensity at a pressure of 4.5 mm. Hg, over the interval 0.67 to 0.87 cm. -1 , agrees with the calculated intensity to better than 1%.