Benoist E. Grossmann

Water-vapor line broadening and shifting by air, nitrogen, oxygen, and argon in the 720-nm wavelength region

Abstract Pressure broadening and pressure shift in air, nitrogen, oxygen, and argon have been measured for water-vapor lines in the 720-nm wavelength region using a ring dye laser in conjunction with two long path absorption cells. Deviations resulting from the Dicke-narrowing effect were observed between the measured profiles and the standard Voigt profile. The corresponding collision-narrowing coefficients were computed using two Dicke-narrowed profiles, the soft- and the hard-collision profiles. The water-vapor absorption lines broadened by oxygen or argon, where the pressure shifts become comparable to the broadenings, were found to be asymmetric. These asymmetric lines are attributed to statistical dependence or correlation between velocity- and state-changing collisions. A vibrational dependence of the broadening and shifting was observed. Widths and shifts (in magnitude) were found to be significantly smaller for lines associated with upper vibrational states ( v ′ 1 v ′ 2 v ′ 3 ) = (221) as compared to ( v ′ 1 v ′ 2 v ′ 3 ) = (301). For each buffer gas under study, a linear relationship between the widths and shifts was measured, the broader lines having the smallest pressure shifts (in magnitude). The average air-to-nitrogen broadening ratio was measured to be 0.907 ± 0.011. We show that the air-broadening coefficient could also be retrieved from the independent measurements of the nitrogen- and oxygen-broadening coefficients. The average line shift in oxygen was found to be twice as large as the shift in nitrogen, indicating that collisions with oxygen molecules contribute significantly to the shift in air. Water-vapor line broadening and shifting in air were investigated in the temperature range from 300 to 400 K. The temperature exponent for air broadening was found to be J -dependent, the broader lines (i.e., low- J lines) having the higher exponents. The average exponent value was measured to be 0.670. The temperature-dependence exponent for the line shift in air was found also to be J -dependent but it showed the opposite behavior (i.e., the high- J lines have the higher exponents). The temperature exponent for line shifting was measured to range between 0.4 and 1.2.

Spectroscopy of water vapor in the 720-nm wavelength region: Line strengths, self-induced pressure broadenings and shifts, and temperature dependence of linewidths and shifts

Abstract A high-resolution spectrometer incorporating a narrow linewidth tunable dye laser and two long pass absorption cells has been used to provide measurements of water vapor absorption line parameters in the 720-nm wavelength region. Measurements were made of line strengths, self-induced pressure broadenings and shifts, and broadening and shift temperature dependences in the range 300–400 K for 270 water vapor lines between 13 550–13 950 cm −1 . Deviations resulting from the Dicke narrowing effect were observed between the measured profiles and the standard Voigt profile. The corresponding collision narrowing coefficients were computed using both the soft and hard collision profiles. Self-induced pressure shift coefficients were found to range between −0.08 and +0.05 cm −1 /atm. The temperature dependence exponent for self-broadening was found to be J -dependent with an average value of 0.75 and the average temperature exponent for line-shifting was measured to be 1.57.

Line-shape asymmetry of water vapor absorption lines in the 720-nm wavelength region

Spectral line-shape analyses were performed for water vapor lines broadened by argon, oxygen, and xenon in the 720-nm wavelength region. A line-shape asymmetry was observed, which is attributed to statistical dependence or correlation between velocity- and state-changing collisions. The generalized (asymmetric) Galatry profile, which results from the soft-collision profile and includes correlation between velocity- and state-changing collisions, was fitted to the observed line shapes and was found to compare favorably with the observed data. The most prominent asymmetries were observed with xenon as the buffer gas.

Raman-shifted dye laser for water vapor DIAL measurements

For improved DIAL measurements of water vapor in the upper troposphere or lower stratosphere, we have generated narrowband (~0.03-cm(-1)) laser radiation at 720- and 940-nm wavelengths by stimulated Raman scattering (SRS) using the narrow linewidth (~0.02-cm(-1)) output of a Nd:YAG-pumped dye laser. For a hydrogen pressure of 350 psi, the first Stokes conversion efficiencies to 940 nm were 20% and 35% when using a conventional and waveguide Raman cell, respectively. We measured the linewidth of the first Stokes line at high cell pressures and inferred collisional broadening coefficients that agree well with those previously measured in spontaneous Raman scattering.

Spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar system

A narrow-linewidth pulsed alexandrite laser has been greatly modified for improved spectral stability in an aircraft environment, and its operation has been evaluated in the laboratory for making water-vapor differential absorption lidar measurements. An alignment technique is described to achieve the optimum free spectral range ratio for the two étalons inserted in the alexandrite laser cavity, and the sensitivity of this ratio is analyzed. This technique drastically decreases the occurrence of mode hopping, which is commonly observed in a tunable, two-intracavity-étalon laser system. High spectral purity (> 99.85%) at 730 nm is demonstrated by the use of a water-vapor absorption line as a notch filter. The effective cross sections of 760-nm oxygen and 730-nm water-vapor absorption lines are measured at different pressures by usingthis laser, which has a finite linewidth of 0.02 cm(-1) (FWHM). It is found that for water-vapor absorption linewidths greater than 0.04 cm(-1) (HWHM), or for altitudes below 10 km, the laser line can be considered monochromatic because the measured effective absorption cross section is within 1% of the calculated monochromatic cross section. An analysis of the environmental sensitivity of the two intracavity étalons is presented, and a closed-loop computer control for active stabilization of the two intracavity étalons in the alexandrite laser is described. Using a water-vapor absorption line as a wavelength reference, we measure a long-term frequency drift (≈ 1.5 h) of less than 0.7 pm in the laboratory.

Optimization of the alexandrite laser tuning elements for a water vapor lidar

An overview of some of the developments completed on an alexandrite laser for making water vapor DIAL measurements is presented in this paper. A computer control for active stabilization of the two intracavity etalons has been implemented and recently tested in an aircraft environment. Long-term frequency drift (i.e., 2 hours) of less than 0.7 pm has been observed in the laboratory. An alignment technique to get the optimum free spectral range ratio for the two etalons is also developed.

Airborne water vapor DIAL system development

A differential absorption lidar (DIAL) system developed at NASA Langley Research Center for the remote measurement of atmospheric H2O and aerosols from an aircraft is briefly discussed. This DIAL system utilizes a Nd:YAG laser-pumped dye laser as the off-line transmitter and a narrowband, tunable Alexandrite laser as the on-line transmitter. A 1-m monochromator and a multipass absorption cell are used to position the on-line laser to the center of the H2O line. The receiver system has a 14-in. diameter, f/7 Celestron telescope to collect the backscattered laser light and focus in into the detector optics. Return signals are converted to electrical signals by the optical detector and are digitalized and stored on magnetic tape. The results of fligh tests of the system are shown.

Alexandrite laser characterization and airborne lidar developments for water vapor DIAL measurements

The spectral characteristics of an alexandrite laser used for making water vapor DIAL measurements are evaluated. The optical servo-system used to lock the laser wavelength on a water vapor absorption line is described. A brief description of the DIAL system is given and the data obtained with this lidar during flight tests in March 1990 are also presented.