Measurements and Analysis of High Resolution Terahertz Spectra of Water

The advent of future far-infrared and terahertz observatories like SOFIA -, the Herschel Space Observatory or APEX/ALMA -- challenges the spectroscopic community to provide accurate transition rest frequencies in this domain to allow for interstellar line identification. Water is the third most abundant molecule in the interstellar medium, and the search for rotational water lines is one of the primary objectives of airborne and satellite-based telescope projects, since its observation from ground-based sites is hampered by its strong atmospheric absorptions. The majority of rotational transitions of H₂O and its deuterated forms HDO and D₂O appear in the Terahertz regime, a frequency domain which is almost completely unexplored by high resolution spectroscopy.