Quantitative Molecular Infrared Spectroscopy of Minor Constituents of Planetary Atmospheres

 

Charles Chackerian, Jr., Lawrence P. Giver, and Darrell Goularte

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. Another possibility will be the identification of an extra-solar-system planet whose atmosphere would indicate an environment supportive of biologic processes. Prerequisite 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, -half widths and pressure-induced shifts. All of these quantities are needed for remote and in situ sensing techniques a) to establish limits of detection for as yet unobserved species, b) to quantify the abundance of those species which are observed and c) to 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.

Summary of Progress and Results

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. The astronomical observations give information on the deep atmosphere of Venus. A number of improved measurements have been made on IR intensities of various CO rovibrational transitions. We will use these results to improve the electric dipole moment function of the molecule and in turn will calculate the thousands of transition line intensities which are used in a variety of applications from combustion studies and stellar atmospheres to monitoring CO in the Earth’s atmosphere.

A design study was completed for 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. This instrument combines the specificity of magnetic rotation spectroscopy (MRS) for these reactive molecules along with the enhanced sensitivity afforded by cavity enhanced absorption.

On going work includes work on-line intensities of nitric acid the visible-near-infrared spectrum of water, a modern atlas of line intensities for carbon monoxide and the construction of a cavity-enhanced magnetic rotation spectrometer for the detection of free-radical molecular species.

Collaborators: Tom Blake, Battelle, Pacific Northwest Laboratory; Linda Brown, JPL/Cal. Tech.; Sumner Davis, University of California, Berkeley; Michael Di Rosa, SPRI; Mike Dulick, National Solar Observatory, Kitt Peak, AZ; Richard S. Freedman, SPRI; Guy Guelachvili, U. Paris XI

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