Submillimeter‐Wave Measurements and Analysis of the Ground and ν 2 = 1 States of Water

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Abstract
In order to facilitate further studies of water in the interstellar medium, the envelopes of late-type stars, jets, and shocked regions, the frequencies of 17 newly measured H2 16O transitions between 0.841 and 1.575 THz are reported. A complete update of the available water line frequencies and a detailed calculation of unmeasured rotational transitions and transition intensities as a function of temperature are presented for the ground and ν2 = 1 state levels below 3000 cm-1 of excitation energy. The new terahertz transitions were measured with a recently developed laser difference frequency spectrometer. Six of these transitions arise from the ν2 = 1 state, and the other 11 are in the ground state; all have lower state energies from 700 to 1750 cm-1 and should be accessible to Stratospheric Observatory For Infrared Astronomy (SOFIA) through the atmosphere. The transitions near 0.850 THz are accessible from the ground with existing receivers. Observations of the newly measured ν2 = 1 state transitions, which include the 11, 1-00, 0 fundamental at 1.2057 THz and five other very low J transitions, should provide valuable insights into role played by the ν2 = 1 state in the cooling dynamics of jets, shocks, masers, and strongly infrared-pumped regions. The line list is presented to assist in the planning of observational campaigns with the Far-Infrared Space Telescope (FIRST) and other proposed space missions with which a full suite of water observations can be carried out.