## 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 $\times $ 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$\cdot $1 and 1$\cdot $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 $\overline{\nu}$ = 0$\cdot $7935 + [- 0$\cdot $0050$_{5}\overline{J^{2}+J}$ + 0$\cdot $0070$_{4}$K$^{2}$] + 0$\cdot $63[-0$\cdot $0050$\overline{J^{2}+J}$ + 0$\cdot $0070K$^{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$\cdot $5 mm. Hg show that their half widths, due to collision broadening, lie between 2 and 5 $\times $ 10$^{-4}$ cm.$^{-1}$, while their intensities agree with the theoretical values, within the experimental error of $\pm $ 5%. The integrated intensity at a pressure of 4$\cdot $5 mm. Hg, over the interval 0$\cdot $67 to 0$\cdot $87 cm.$^{-1}$, agrees with the calculated intensity to better than 1%.