LABORATORY MEASUREMENT OF THE N=1[-0 ROTATIONAL TRANSITION OF NH AT 1THZ

The N = 1 <-- 0 pure rotational transition of the NH radical (X-3 Sigm a(-)), located at 1 THz, was measured with the Cologne terahertz spect rometer using a frequency stabilized backward wave oscillator as radia tion source. The NH radical was produced in a de discharge of a flowin g mixture of ammonia and helium. Transition frequencies of all three f ine structure transitions with associated hyperfine components were pr ecisely measured and analyzed to determine accurate molecular constant s, including hyperfine coupling constants for both nuclei. As a result , the accuracy of the molecular constants has been improved by one to two orders of magnitude. The nuclear spin-rotation coupling constant f or the nitrogen nucleus has been determined for the first time. The pr ecise frequencies reported in this work are of importance to astronomi cal applications.