Measurements and Calculations of the Halfwidth of Two Rotational Transitions of Water Vapor Perturbed by N 2 ,O 2 , and Air Jean-Marcel Colmont,* Dominique Priem,* Georges Wlodarczak,* and Robert R. Gamache² *Laboratoire de Spectroscopie Hertzienne, URA CNRS 259, Universite ´ des Sciences et Technologies de Lille I, 59655 Villeneuve d’Ascq Cedex, France; and ² Department of Environmental, Earth, and Atmospheric Sciences, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854 Received March 18, 1998; in revised form June 17, 1998 In this paper we report the results of both an experimental and theoretical study of the halfwidths of two transitions of water vapor. Measurements on the lines of the H 2 16 O and H 2 18 O isotopomers located at 325.1 and 203.4 GHz, respectively, were carried out in the temperature range 300 –393 K, with N 2 and O 2 as perturbing gases. The foreign-broadening coefficients and their temperature-dependence parameters were determined assuming a Voigt profile and the usual temperature dependence for the halfwidth. The retrieved values are compared to values calculated using the complex semiclassical formalism of Robert and Bonamy. The assumed intermolecular potential is a combination of electrostatic and atom–atom components. This last contribution is defined as the sum of pairwise Lennard–Jones 6 –12 interactions between the atoms of H 2 O and the atoms of the perturbing molecules expanded to eighth order. Also calculated are the pressure-induced shifts of the spectral lines for temperatures from 200 to 400 K. Calculated and experimental results are in good agreement, within 3.2%, except for the N 2 -broadening temperature coefficients, for which there are discrepancies as high as 23%. Air-broadening parameters are determined following the classical relation:  (air) = 0.79 (N 2 ) + 0.21 (O 2 ). © 1999 Academic Press INTRODUCTION Extensive investigations have been devoted to the knowl- edge of the physical and chemical properties of our terrestrial atmosphere. One of the main goals is to observe the evolution, in time and in space, of the vertical distributions of radiatively active molecules, which have significant influence on the chemical cycles of the atmosphere and on climatic cycles. Millimeter-wave spectroscopy is a powerful technique uti- lized to study and to monitor the atmosphere through emission or absorption measurements obtained from ground-based fa- cilities or, more efficiently, from satellite platforms. To cor- rectly interpret remote sensing measurements, it is of prime importance to have at one’s disposal accurate values for the parameters which define the absorption (the central absorption frequency, the linestrength) and those which define the profile of the lines (the lineshape, the collisional broadening coeffi- cients, and their temperature dependence). At the present time, the air-broadened halfwidth and its temperature dependence are the major error sources in the retrieval of concentration profiles (1, 2). The European Space Agency plans to launch two satellite instruments, MASTER and SOPRANO, at the beginning of the next century. Several channels of MASTER are located in the millimeter-wave region, between 199 and 348 GHz, which is well suited to the study of the important minor atmospheric constituents. However, a lack of accurate data for the half- widths in the existing databases has been noted for numerous lines located in this region; in particular, nothing has been reported on the H 2 18 O isotopomer. Despite improvements in the theoretical calculations of such parameters, laboratory mea- surements of the collisional broadening effects remain neces- sary for analysis of the signals which will be recorded at limb viewing by these instruments. These measurements will also help confirm and/or improve the theoretical calculations. To have a more complete and up-to-date database in support of the instruments, measurements on selected lines in this region have been made. Here we report both experimental and theoretical results for N 2 -, O 2 -, and air-broadened halfwidths for the 3 1,3 4 2 2,0 transition of H 2 18 O located at 203.4 GHz, and the 5 1,4 4 4 2,2 transition of H 2 16 O located at 325.1 GHz. EXPERIMENTAL DETAILS Experimental Setup Spectrometers A heated 50-cm long, 25-cm diameter stainless steel cell, capped with lens-shaped windows, was used for the measure- ments. The temperature of the cell can be adjusted from am- bient to 393 K and is controlled within 0.1 K. The gas pressure is maintained using a bakeable capacitance manome- ter (Baratron 315) held at a temperature close to that of the cell. The microwave radiation is collimated by a Teflon lens, passed through the cell, and is focused onto a He-cooled InSb bolometer. Millimeter waves at 203.4 and 325.1 GHz related to Journal of Molecular Spectroscopy 193, 233–243 (1999) Article ID jmsp.1998.7747, available online at http://www.idealibrary.com on 233 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.