Collisional cooling investigation of THz rotational transitions of water

An investigation of the pressure broadening by helium and hydrogen of six rotational transitions of water has been completed. The six transitions studied included two para water transitions (0₀₀–1₁₁ and 1₁₁–2₀₂) and four ortho water transitions (1₀₁–1₁₀, 2₂₁–3₁₂, 3₀₃–3₁₂ and 3₁₂–3₂₁) in the frequency region 0.55–1.17 THz. This survey was accomplished using the collisional cooling technique which allowed the broadening of each transition to be studied below the water condensation temperature. For each of the transitions studied, the temperature dependence of the pressure broadening by helium showed little dependence on temperature, while the broadening by hydrogen showed a sharp decrease at the lowest temperatures. This behavior was modeled, for each transition broadened by helium and hydrogen, with a power law, or a power law modified with a Boltzmann-like step function, and the results of these fits will be presented. In addition, an extensive investigation of the systematic error in the temperature of the water vapor in the collisional cooling experiment will be discussed. Finally, the impact of these new broadening measurements on models of star formation in the interstellar medium will be outlined. ©2010 The American Physical Society