Knowledge Bank

Several absorption lines in the $6.3 \mu v2$-band of water vapor have been fully resolved ($resolution\sim 3\times 10-5cm-1)$ using tunable semiconductor lasers. Three atmospheric water vapor lines near $5.32\mu m$ were studied in detail using a $PbS0.6Se0.4$ laser transmitting over a 7.4m laboratory path at $22\circ C$ with a 3.6 Torr partial pressure of water vapor. These measurements yield accurate water vapor line parameters (see table at end of abstract) and reveal large discrepancies between the calculated and measured linewidths. The observed linewidths are two to four times narrower than the calculated widths given by Benedict and $Calfee.1$ These molecular absorptions are produced by transitions between high rotational states $(J\ge 12)$ of the $H2O$ molecule for which collision broadening due to $O2$ and $N2$ is expected to be relatively small. However, the line at $1879.01cm-1$ is surprisingly narrow, having a width of only three times the limiting low pressure Doppler width ($0.0055cm-1)$. The measured and calculated intensities are in fairly good agreement (within $\approx 20\%$) for these lines. Further measurements on water vapor lines near $6.5\ \mu $m will also be discussed. Our measurements have important implications concerning the transmission of the atmosphere in the middle infrared region and suggest the need for improvement in theoretical models for atmospheric absorption. This work was sponsored by the Department of the Air Force. [FIGURE]