Knowledge Bank

The $2.7\mu$ band of $H2O$ has been observed at a number of optical densities with the 7200-line vacuum grating spectrometer. The water content was varied by reducing the pressure within the spectrometer, and by varying the optical path at atmospheric pressure outside the fully evacuated spectrometer. The effective spectral slits were of the order of $0.16cm-1$, and the scanning rate was reduced so that the records should give the true line shape as passed by the exit slits. By measuring the dependence on pressure and water content of the peak absorption of a number of well-separated lines, and of the minima between close lines due to the absorption in their wings, it is possible to determine both the strengths and widths of individual lines. The line strengths show some deviations from those calculated for the rigid asymmetric rotator, in that P-branch lines are relatively stronger, R-branch lines are relatively weaker, than the theoretical strengths, the anomaly increasing at higher J. There also appears to be a variation of width from line to line with those arising from levels of lowest energy being in general the broadest. The results will be discussed in the light of other recent experimental $studies1$ on $H2O$.