A hollow-cathode THz spectrometer for the study of astrophysical ions and radicals: Benchmarking with N2H+ and extended measurements for N2D+

- A DC-discharge hollow-cathode THz spectrometer was developed.
- Rotational spectral lines of N2H+ and N2D+ were measured.
- Known N2H+ lines were used to optimize ion formation conditions.
- Measurements for N2D+ were extended up to ∼1 THz.
- N2D+ molecular parameters were refined and the spectrum predicted above 1 THz.

Here we present the first results from a DC-discharge hollow-cathode spectrometer developed for the detection of transient species of astrophysical interest in the THz spectral regime. The new instrument was benchmarked using rotational spectral lines of N2H+ and N2D+. The known rotational lines of N2H+ were used to optimize ion formation conditions in the discharge. We then used the known spectral lines at frequencies below 700 GHz to further optimize the production efficiency of N2D+, and extended spectral measurements for this ion up to ∼1 THz. This resulted in the detection of four additional N2D+ rotational lines that have not been previously measured experimentally. We have used the observed line positions for N2D+ to refine the molecular parameters and extend a more accurate spectral prediction above 1 THz. In addition to being excellent target molecules for benchmarking our spectrometer, N2D+ and N2H+ are commonly used as tracers of isotopic fractionation in dense interstellar clouds. Therefore, these new measurements are important for guiding astronomical observations using the high frequency bands of the new Atacama Large Millimeter Array (ALMA) and other far-IR observatories.