Far-Infrared Laser Sideband Spectroscopy of the NH2 Radical.

We have constructed a far-infrared (FIR) laser sideband spectrometer for carrying out pure rotational spectroscopy of fundamental radicals and ions in the FIR or "terahertz" frequency region. The tunable FIR source is obtained by sideband generation of a carbon dioxide laser-pumped FIR laser with a GaAs Schottky barrier diode. The minimum detectable absorption coefficient of the spectrometer achieved so far is 1 x 10(-6) cm-1 at 1600 GHz. Observation of the pure rotational spectrum of the NH2 radical in the 2B1 ground electronic state was demonstrated as the first example. Spectra were measured by utilizing the double modulation technique; i.e., the Zeeman effect was used in addition to the normal frequency modulation technique in order to flatten the baseline and to appreciate full system sensitivity. The radical was generated by a DC glow discharge of a mixture of ammonia and helium. Nine Q- and R-branch rotational transitions were measured in the frequency range between 1300 and 2600 GHz. Zero-field transition frequencies were obtained with an accuracy of around 1 MHz. The line frequencies were analyzed with previous microwave spectroscopic results, and the rotational and spin-rotation constants with their centrifugal correction terms were revised. With the aid of the revised rotational constants, it becomes possible to predict the line frequency of certain low N transitions of the radical to within about 1 MHz for the first time. Copyright 1998 Academic Press.