The ND Radical: Laboratory Measurement of the N = 2-1 Rotational Transition at 1 THz.

The N = 2-1 pure rotational transition of the ND radical (X3Sigma-) near 1 THz was measured with the Cologne terahertz spectrometer. The ND radical was produced in a dc discharge of a flowing mixture of deuterated ammonia and helium. Frequencies of five strong fine-structure transitions with associated hyperfine components were precisely measured and analyzed to determine a complete set of accurate molecular constants, e.g., the rotational and centrifugal distortion constants B0 = 263265.4735(45) MHz and D0 = 14.62876(74) MHz, together with the fine and hyperfine constants. Based on these constants, the rotational frequencies of ND up to 4 THz were predicted with the aim to guide future astronomical searches. For completeness we also include similar predictions for NH. Copyright 1998 Academic Press.