THE INFRARED CONTINUUM OF PURE WATER-VAPOR - CALCULATIONS AND HIGH-TEMPERATURE MEASUREMENTS

We present experimental and theoretical studies of medium infrared abs orption by pure water vapor. Measurements have been made in the 1900-2 600 cm-1 and 3900-4600 cm-1 regions, for temperatures and pressures in the 500-900 K and 0-70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorpti on parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting f or the finite durations of collisions and line-mixing through wave-num ber dependent effective broadening parameters are introduced. The latt er have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direc t predictions from first principles using a statistical approach. Effe ctive broadening parameters obtained using these two different approac hes are in satisfactory agreement for both the temperature and wavenum ber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is sa tisfactory. The remaining discrepancies probably result from the influ ence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predic tions are obtained in wide temperature and spectral ranges when the to tal absorption at elevated density is considered. This agreement, whic h is due to the relatively weak continuum absorption and large contrib utions of nearby lines, makes the present models suitable for most pra ctical applications involving elevated densities.