Sub-Doppler spectroscopy of ammonia near 570 GHz

We have employed the Cologne terahertz spectrometer [1, 2] to observe Lamb dip and crossover phenomena in rotational spectra at 572 GHz. Because of the large power output of the backward wave oscillators used in the system, no special means are required to induce this saturation effect; the coincidentally reflected radiation from the bolometer present in our quasi-optical absorption cell is sufficient to saturate the v(z), = 0 sub-group. This effect has allowed the observation of the three previously unresolved hyperfine components of the (NH3)-N-14 A <-- S, J(K) = 1(0) <-- 0(0) transition, and was also observed for the analogous (NH3)-N-15, transition in natural abundance (i.e. 0.366 %). These measurements indicate a possible improvement of molecular transition frequencies in the submillimeter region.