Ro-inversional Spectrum of Ammonia

The ground state J = 2 ←1 , K = 0 and K = 1 ro-inversional spectrum of 14NH3 and 15NH3 at 1.2 THz has been measured with an accuracy of 20 kHz using the Cologne terahertz spectrometer. The measured frequencies for the K = 0 components are:14NH3 (J, K) = a(2, 0) - s( 1,0): 1 214 852.942(20) MHz, 15NH3 (J, K) = a(2, 0 ) - s( 1,0) : 1 210 889.556(20) MHz.In addition we have determined from saturation dip measurements of the J = 1 - 0 transition the spin-rotation constant CN = 6.7(3) kHz and the unsplit line center frequency: 14NH3 (J, K) = s( 1, 0) - a(0, 0): 572 498.163( 10) MHz. The new results are in excellent agreement with existing high resolution Fourier transform data. The terahertz line frequencies are of considerably higher accuracy than the FT-data. by about two orders of magnitude. They will serve as future calibration lines. The ro-inversional transitions are of astrophysical interest