We present a rigorous calculation of the contribution of water dimers to the absorption coefficient α(ν,T) in the millimeter and far infrared domains, over a wide range (276–310 K) of temperatures. This calculation relies on the explicit consideration of all possible transitions within the entire rovibrational bound state manifold of the dimer. The water dimer is described by the flexible 12-dimensional potential energy surface previously fitted to far IR transitions [ C. Leforestier et al., J. Chem. Phys. 117, 8710 (2002) ], and which was recently further validated by the good agreement obtained for the calculated equilibrium constant Kp(T) with experimental data [ Y. Scribano et al., J. Phys. Chem. A. 110, 5411 (2006) ]. Transition dipole matrix elements were computed between all rovibrational states up to an excitation energy of 750 cm−1, and J = K = 5 rotational quantum numbers. It was shown by explicit calculations that these matrix elements could be extrapolated to much higher J values (J = 30). Transitions to vibrational states located higher in energy were obtained from interpolation of computed matrix elements between a set of initial states spanning the 0–750 cm−1 range and all vibrational states up to the dissociation limit ( ∼ 1200 cm−1). We compare our calculations with available experimental measurements of the water continuum absorption in the considered range. It appears that water dimers account for an important fraction of the observed continuum absorption in the millimeter region (0–10 cm−1). As frequency increases, their relative contribution decreases, becoming small ( ∼ 3%) at the highest frequency considered ν = 944 cm−1.
The purpose of The Journal of Chemical Physics is to bridge a gap between journals of physics and journals of chemistry by publishing quantitative research based on physical principles and techniques, as applied to "chemical" systems. Just as the fields of chemistry and physics have expanded, so have chemical physics subject areas, which include polymers, materials, surfaces/interfaces, and biological macromolecules, along with the traditional small molecule and condensed phase systems. The Journal of Chemical Physics (JCP) is published four times per month (48 issues per year) by the American Institute of Physics.
The American Institute of Physics (AIP) is a 501(c)(3) not-for-profit membership corporation created for the purpose of promoting the advancement and diffusion of the knowledge of physics and its application to human welfare. It is the mission of the Institute to serve the sciences of physics and astronomy by serving its member societies, by serving individual scientists, and by serving students and the general public.
As a "society of societies," AIP supports ten Member Societies and provides a spectrum of services and programs devoted to advancing the science and profession of physics. A pioneer in digital publishing, AIP is also one of the world's largest publishers of physics journals and produces the publications of more than 25 scientific and engineering societies through its New York-based publishing division.
Laboratory absorption measurements of the water-vapor continuum in the far infrared region from 12 to 55 cm-1 (0.4 – 1.83 THz) were obtained using a multipass absorption cell, a Fourier transform spectrometer and a liquid-He-cooled bolometer detector. Measurements were made at a temperature, T = 297 K with water vapor and nitrogen pressures up to 2.2 and 81 kPa, respectively. The effects of the choice of lineshape function and far-wing cut-off factors on the reported continuum absorption are analyzed by modeling the resonant water-vapor spectrum using van Vleck–Weisskopf and Lorentzian lineshapes. Comparisons with available microwave data and model calculations are also presented.
