The linestrengths for 40 absorption lines of H 2 16 O water vapor that were located between 813 and 820 nm were measured; most of these lines were selected for their potential usefulness in laser remote measurements of atmospheric humidity using the differential absorption lidar technique. The air-induced pressure-broadening coefficients were also measured for 32 of these lines and the air-induced pressure shift coefficients were measured for 29 lines. These spectroscopic parameters were derived from spectra obtained with an AlGaAs diode laser and two long-path absorption cells. Collisional narrowing effects were observed and were accurately described by a Galatry profile. Comparisons were made with previous experimental work or theoretical calculations as available. แญง 1997 Academic Press
I. INTRODUCTION
as the availability of high-power diode lasers required for injection seeding.
The successful interpretation of the lidar profiles obtained Water vapor plays a major role in many atmospheric prowith this instrument relies strongly on an accurate knowledge cesses and it is a highly variable molecular constituent of of specific water vapor absorption line parameters: linethe atmosphere. Therefore, the ability to measure its spatial strength, pressure-broadening coefficient, pressure-induced and temporal distribution is extremely valuable to atmoshift coefficient, and the respective temperature-dependence spheric scientists in many situations. Remote measurements factors (3). With these parameters in hand, one can precisely from airborne platforms have the potential to provide the compute the effective water vapor absorption cross section required information over extended geographical scales, and at a given altitude and for a given wavelength. In support the differential absorption lidar (DIAL) technique has the of the LASE project, a series of spectroscopic measurements proven ability to carry out such measurements: for these were conducted that primarily focused on the 16 absorption reasons, airborne water vapor DIAL systems have all the lines that have been identified for use in the DIAL measureattributes of highly productive instruments that could, with a ments, since the experimental data for this water vapor abmore extensive development, greatly benefit the atmospheric sorption region were limited-to our knowledge-to the sciences community. The DIAL technique has already been linestrengths and to the line positions. These 16 lines were successfully applied to the remote measurement of atmoselected for atmospheric measurements on the basis of temspheric water vapor profiles from airborne platforms (1, 2).
perature insensitivity of the absorption cross section at the Extending these efforts, NASA Langley Research Center has line center (4). In the case of number density measurements, taken the challenge to demonstrate the autonomous operation this criterion restricts the choice of lines to transitions with of a water vapor DIAL system from a high-altitude extended Jะ ranging from 2 to 4, and in the case of mixing ratio range U-2 (ER2) aircraft. Although the Lidar Atmospheric measurements, to transitions with Jะ ranging from 3 to 6. Sensing Experiment (LASE) had to push the limits of pres-
The water vapor absorption in the region 780-860 nm, ent-day technology to succeed in this demonstration, it is also referred to as the 3n 3 / n 2 or 3n / d spectral region, viewed as an important intermediate step in a long-term results from the vibration-rotation transitions of mainly five effort to develop a spaceborne system. The transmitter of combination bands. These transitions occur from the ground this DIAL system is a double-pulse Nd:YAG-pumped state to the (310), ( 211), ( 131), (112), and (013) vibra-Ti:sapphire laser seeded by a diode laser. The wavelength tional levels (5). In the region 813-820 nm, the absorption region 813-818 nm was selected for operation since it offers is due primarily to the (211) -( 000) band with some contria large distribution of strengths for temperature-insensitive butions from the (310) -( 000) and ( 112) -(000) bands. water vapor absorption lines and combines the technical advantages of a high laser gain for the Ti:sapphire as well The experimental investigation of this region has been