Measurement of HCl absorption coefficients with a DF laser

Absorption coefficients in the fundamental P-branch of HCl at several DF laser transitions from 2439.02 cm−1 to 2862.87 cm−1 have been measured experimentally. The 2-1 P(3) DF laser transition has been shown to overlap the P(6) HCl37 absorption line within the halfwidth of an atmospherically broadened line. The absorption coefficient k was measured to be 5.64 ± 0.28 (atm-cm)−1 for a 0.27% mixture of HCl in N2 at a total pressure of 760 Torr. A theoretical and experimental comparison of the pressure dependence of k showed that the 2-1 P(3) DF transition lies 1.32 ± 0.15 GHz from the center of the P(6) HCl absorption line. This line separation is in good agreement with published positions for this DF laser transition and the HCl absorption line. At least four other DF laser transitions in the spectral interval measured are reported showing measurable absorption (≥10%) for a 10.2% mixture of HCl in N2 across a 1-m pathlength. Applications of these results to differential absorption lidar (DIAL) and to heterodyne detection are also discussed.