Pressure-broadening of water transitions near 7180 cm(-1) by helium isotopes.

In this study, pressure-broadening parameters for several H2O transitions near 7180 cm(-1) are obtained which describe collisions with (3)He and (4)He. The sensitivity of those parameters to choice of theoretical line profile (Galatry vs. Voigt) is investigated. H2O is an important species in atmospheric chemistry and astronomy. Because of this, basic fundamental research, which explores the nature of the H2O spectrum in the presence of different gases of varying physical properties, can provide useful reference data which can be applied in the fields of atmospheric and planetary remote sensing. Measurements were made using an intensity-modulated laser photoacoustic spectrometer. Results from the present work show that Galatry line profiles, with a constrained narrowing parameter, more accurately describe experimental spectra than Voigt profiles over a wide range of experimental pressure conditions. Average pressure-broadening parameters were found to be 0.0216 cm(-1)/atm and 0.0209 cm(-1)/atm for H2O in (3)He and (4)He, respectively, and were compared to a literature model for the mass-dependence of line broadening. Specific values were obtained for each transition with nominal combined uncertainties of 2-6%.

Spectroscopic measurement of the vapour pressure of ice.

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180 cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.

Measurement of H2O broadening of O2 A-band transitions and implications for atmospheric remote sensing.

We present laboratory measurements of H(2)O-broadened (16)O(2) A-band (b(1)Σ(g)(+) ← X(3)Σ(g)(-)(0,0)) absorption spectra acquired with a laser-based photoacoustic spectroscopy method. This absorption band is widely used in a variety of high-precision atmospheric remote sensing applications. We report H(2)O broadening parameters for six of the strongest transitions in this band, and we show that these measured values are nominally 1.5-2 times greater than the corresponding air-broadening parameters. Simulations of atmospheric transmission spectra in the O(2) A-band that incorporate our measured H(2)O broadening parameters indicate that H(2)O present at concentrations typically found in the Earth's atmosphere can influence the column-integrated transmission relative to the dry air case. Further, because of spatial and seasonal variations in humidity, failure to account for the enhanced H(2)O pressure broadening effects can lead to concomitant biases in atmospheric O(2) A-band retrievals of quantities such as surface pressure and path length in greenhouse gas retrievals.

H2O broadening of a CO2 line and its nearest neighbors near 6360 cm(-1).

Remote sensing of CO(2) requires high-fidelity reference data of spectral line parameters to be successful. The 6360 cm(-1) region is commonly used by satellites, field campaigns, and point-source gas sensors because it contains well-characterized and relatively isolated transitions of appropriate line strengths for atmospheric applications. However, the presence of gases other than CO(2), N(2), and O(2) can be a source of uncertainty for atmospheric measurements. Near 6360 cm(-1), there are numerous H(2)O and HDO transitions. Water makes up approximately 1-4% of Earth's lower atmosphere and can interfere with remote sensing measurements by (1) appearing as a direct spectral interference or (2) acting as a foreign broadener for CO(2) lines. The primary goal of this work was to quantify H(2)O broadening of CO(2) through precision spectroscopy measurements on the R16e transition at 6359.967 cm(-1) and its two nearest neighbors. A secondary goal was to assess the accuracy of H(2)O reference line parameters in the HITRAN 2008 database for spectrally removing typical levels of moisture from air samples containing approximately 400 ppm of CO(2).

Standard photoacoustic spectrometer: model and validation using O2 A-band spectra.

We model and measure the absolute response of an intensity-modulated photoacoustic spectrometer comprising a 10 cm long resonator and having a Q-factor of approximately 30. We present a detailed theoretical analysis of the system and predict its response as a function of gas properties, resonance frequency, and sample energy transfer relaxation rates. We use a low-power continuous wave laser to probe O(2) A-band absorption transitions using atmospheric, humidified air as the sample gas to calibrate the system. This approach provides a convenient and well-characterized method for calibrating the absolute response of the system provided that water-vapor-mediated relaxation effects are properly taken into account. We show that for photoacoustic spectroscopy (PAS) of the O(2) A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O(2). PAS also shows great potential for high-resolution line shape measurements: calculated and experimental values for the PAS system response differ by about 1%.

High-resolution spectroscopy of H2SO4, HDSO4, and D2SO4 vapor in the region 1200-10,000 cm(-1).

The high-resolution (0.05 cm(-1)) spectra of gas-phase H2SO4, HDSO4, and D2SO4 were measured over the frequency region 1200-10,000 cm(-1) using Fourier-transform infrared spectroscopy. The increased resolution of this work compared with previous studies has lead to an improved vibrational analysis of H2SO4. This study has answered unresolved questions about combination bands and overlapping features from previous gas-phase spectroscopic studies of H2SO4 and marks the first experimental measurement of the nu8 and nu15 torsional vibrations in this molecule. This work leads to a brief discussion on vibrational mode mixing in sulfuric acid.

Gas phase spectroscopy of HNO3 in the region 2000-8500 cm(-1).

Spectra of gas phase HNO3 were collected in the region 2000-8500 cm(-1) using Fourier-transform infrared spectroscopy. This region is dominated by the nu1 O-H stretching mode but also contains many previously unreported combination bands and overtones. This work marks the first observation of Fermi resonance the 2nu1 O-H stretching overtone. Previously unobserved bands were assigned and integrated intensities were obtained. For bands already reported in the literature, comparisons of relative intensities are presented when possible. This work gives a brief discussion on the trends in overtone intensities and on mode mixing in HNO3 in relation to previous experimental and theoretical studies.