Quantitative Molecular Infrared Spectroscopy of Minor Constituents of Planetary Atmospheres

Research Staff: Charles Chackerian, Jr., Lawrence P. Giver, Darrell Goularte, James Podolske

Infrared spectroscopic techniques are extremely powerful for a number of observational objectives for "understanding" planetary atmospheres. Monitoring the health of the Earth's atmosphere is an important example of one class of such observations. In addition molecular line intensities of greenhouse gases are needed to interpret infrared spectroscopic field measurements as well as to make accurate predictions with various global warming models. Another possible use will be the identification of extra-solar-system planets whose atmospheres may indicate an environment supportive of biological processes. Prior to the design of appropriate instruments as well as the interpretation of observations, quantitative laboratory spectroscopic measurements must be done at physical conditions appropriate for the environment of interest. Our measurements provide molecular line and band intensity values as well as line-positions, line-half widths and pressure-induced shifts. All of these quantities are needed for remote and in situ sensing techniques to a) establish limits of detection for as yet unobserved species, b) quantify the abundance of those species which are observed and c) determine atmospheric physical conditions.

We use a complement of laboratory instruments to obtain the spectral measurements. These include a BOMEM DA8 interferometer, a 25-meter base-path multiple reflection absorption cell and 30, 10 and 5 cm absorption cells which are coolable to about 60-77 K. We also use spectroscopic facilities at the National Solar Observatory and the Battelle, Pacific Northwest Laboratory.

We are also seeking to develop a new class of detectors for halogen atoms at the low part per trillion level.

In 2000, papers were published on the infrared line intensities for CO2 bands, which are observed in the window regions of dark-side emission spectra of Venus, as well as H2O bands important for the climate forcing of the Earth's atmosphere. We continue to make progress on improving the electrical dipole moment function of the electronic ground state of the CO molecule which we are using to improve all the absolute intensities listed for this molecule in the HITRAN data base. These intensities are useful for interpreting the spectra of certain stars, for interpreting spectra of atmospheric contaminants when our sun is used as a light source and for in situ measurements of CO which use infrared absorption/emission monitoring.

New spectra were recorded from 750 cm-1 to 4000 cm-1, with a spectral resolution of 0.002 cm-1, of HNO3 at ambient temperature. These spectra are being analyzed for line intensities which are currently uncertain to about 30%.

Both low- (10 nanometer) and high-resolution long-absorbing-path spectra were obtained of near-infrared absorption bands of H2O. These measurements are being interpreted and compared to earlier calculations as well as other experimental work done at shorter absorbing path lengths.

First steps in the construction of a compact spectroscopic-based instrument, which will be capable of the detection of free-radical molecular species (in situ) in the part per trillion mixing ratio range, were completed. This instrument combines the specificity of magnetic rotation spectroscopy (MRS) for these reactive molecules along with the enhanced sensitivity afforded by cavity enhanced absorption. An automobile headlamp was used to produce magnetic rotation spectra of atomic iodine.

Collaborators: Richard Freedman, Space Physics Research Institute; Linda Brown, JPL/Cal. Tech.; Rohidas Kshirsagar, National Research Council Associate; Michael Di Rosa, Livermore National Laboratory; Mike Dulick, National Solar Observatory; Guy Guelachvili, U. Paris XI; Tom Blake, Battelle, Pacific Northwest Laboratory


Point of Contact: Charles Chackerian, Jr., 650/604-6300, cchackerian@mail.arc.nasa.gov