Infrared Heterodyne Spectroscopy Research Articles

Infrared line intensities of chlorine monoxide

Absolute infrared line intensities of several ClO lines in the rotational‐vibrational (1–0) band were measured using infrared heterodyne spectroscopy near 12 μm. A measurement technique using combined ultraviolet absorption and infrared line measurements near 9.5 μm and 12 μm permitted an accurate determination of the column densities of O3 and ClO in the absorption cell and thus improved ClO line intensities. Results indicate ClO line and band intensities ∼2.4 times lower than previous experimental results. Effects of possible failure of local thermodynamic equilibrium conditions in the absorption cell and the implication of the results for stratospheric ClO measurements in the infrared are discussed.

Thermal bifurcation in the upper photosphere inferred from heterodyne spectroscopy of OH rotational lines

We have obtained low noise (S/N > 103), high spectral resolution (ν/Δν ≈ 106) observations of two pure rotation transitions of OH from the solar photosphere. The observations were obtained using the technique of optically null-balanced infrared heterodyne spectroscopy, and consist of center-to-limb line profiles of aυ = 1 and a υ = 0 transition near 12 μm. These lines should be formed in local thermodynamic equilibrium (LTE), and are diagnostics of the thermal structure of the upper photosphere. We find that theυ = 0R22(24.5)e line strengthens at the solar limb, in contradiction to the predictions of current one-dimensional photospheric models. Our data for this line support a two-dimensional model in which horizontal thermal fluctuations of order ±800 K occur in the region τ5000 ≈ 10−3–10−2. This thermal bifurcation may be maintained by the presence of magnetic flux tubes, and may be related to the solar limb extensions observed in the 30–200 μm region.

Heterodyne spectrometer for the 10 micron region

We first consider the interest of infrared heterodyne spectroscopy to study the vertical distribution of atmospheric constituents such O3. Then an heterodyne spectrometer is described. It operates in the 10 micron region with a CO2 laser and gives a resolution of 5 MHz. A laboratory spectrum of C2H4 is presented as a preliminary result.

Remote sensing by IR heterodyne spectroscopy

The use of IR heterodyne spectroscopy for the study of planetary atmospheres is discussed. Infrared heterodyne spectroscopy provides a convenient and sensitive method for measuring the true intensity profiles of atmospheric spectral lines. Application of radiative transfer theory to measured line shapes can then permit the study of molecular abundances, temperatures, total pressures, excitation conditions, and dynamics of the regions of line formation. The theory of formation of atmospheric spectral lines and the retrieval of the information contained in these molecular lines are illustrated. Notable successes of such retrievals from IR heterodyne measurements on Venus, Mars, Jupiter, and earth are given. A discussion of new developments in IR heterodyne technology is also presented.

Infrared heterodyne spectroscopy of seven gases in the vicinity of chlorine monoxide lines

Chlorine monoxide (ClO) is thought to play an important role in a photochemical cycle which causes the destruction of ozone in the earth's stratosphere. Since lines of the (1,0) fundamental of ClO lie near C-14O2-16, laser lines, IR heterodyne spectroscopy is potentially an important technique for monitoring the ClO abundance. However, due to the presence of lines from other trace atmospheric gases in this spectral range, the interpretation of such observations is ambiguous unless high resolution laboratory measurements support the identifications. Measured frequencies are reported for spectral lines of seven trace atmospheric gases which absorb near the C-14O2-16 laser transitions relevant to the detection of ClO by IR heterodyne spectroscopy.

Measurements of stratospheric ethane in the Jovian South Polar Region from infrared heterodyne spectroscopy of the nu9 band near 12 microns

Ethane emission-line profiles from the South Polar Region of Jupiter were measured at sub-Doppler resolving powers (..nu../..delta nu..roughly-equal10/sup 6/) using the Goddard Space Flight Center infrared heterodyne spectrometer. Lines in the /sup R/Q/sub 3/-branch of the ..nu../sub 9/ fundamental of ethane near the P42 /sup 14/C/sup 16/O/sub 2/ laser transition at 829.9273 cm/sup -1/ were observed. Comparison with diode laser laboratory spectroscopy showed that the planetary line widths were limited by velocity broadening (approx.200 MHz) due to Jovian rotation within our field of view (FWHM = 2''). Peak brightness temperatures of the lines were measured to be approx.160 K implying monochromatic (unbroadened) line brightnesses approaching 170 K. Analytical fits to our data were made by direct modeling of the radiative transfer equation using a Voyager 2 polar temperature profile and various vertical profiles for C/sub 2/H/sub 6/, H/sub 2/, and NH/sub 3/. The best fit required a constant C/sub 2/H/sub 6/:H/sub 2/ mixing ratio above the 100 mbar level of 1.2 x 10/sup -6/. These results are compared with previous work and Voyager infrared interferometer spectrometer results.

Infrared Heterodyne Spectroscopy

Infrared heterodyne spectroscopy is an extremely useful tool for Doppler-limited studies of atomic and molecular lines in diverse astrophysical regions. The current state of the art is reviewed, and the analysis of CO2 lines in the atmosphere of Mars is outlined. Doppler-limited observations have enabled the discovery of natural laser emission in the mesosphere of Mars and the discovery of failure of local thermodynamic equilibrium near the surface of Mars.

A 10 Micron Heterodyne Receiver for Ultra High Resolution Astronomical Spectroscopy

Infrared heterodyne spectroscopy is extremely useful in determining by precise measurements of atomic and molecular infrared line spectra the molecular abundances, velocity structure, and excitation conditions of the interstellar medium, and the temperature- and pressure-profiles of planetary atmospheres (cf. Kostiuk et al. 1977, Betz et al. 1976, 1977, Abbas et al. 1978).

Stratospheric sounding by infrared heterodyne spectroscopy

Intensity profiles of infrared spectral lines of stratospheric constituents can be fully resolved with a heterodyne spectrometer of sufficiently high resolution (∼5 MHz or 0.000167 cm−1 at 10 µ). The constituents' vertical distributions can then be evaluated accurately by analytic inversion of the measured line profiles. Estimates of the detection sensitivity of a heterodyne receiver are given in terms of minimum detectable volume mixing ratios of stratospheric constituents, indicating a large number of minor constituents which can be studied. Stratospheric spectral line shapes and the resolution required to measure them are discussed in light of calculated synthetic line profiles for some stratospheric molecules in a model atmosphere. The inversion technique for evaluation of gas concentration profiles is briefly described, and applications to synthetic lines of O3, CO2, CH4, and N2O are given. Some recent heterodyne measurements of CO2 and O3 absorption lines are analytically inverted, and the vertical distributions of the two gases are determined.

Laser heterodyne spectroscopy measures planetary winds

A new technique involving quantum electronics and lasers is being used for high‐resolution astronomical observations. The technique—infrared heterodyne spectroscopy—has detected wind patterns on Venus to roughly 5 meters/sec (almost as good as the police do with radar detection of speeders). In addition, the technique has been used to detect such things as nonthermal emission from planets and the distribution of various gases in Earth's atmosphere.

A 10µm Laser Heterodyne Spectrometer for Remote Detection of Trace Gases

Infrared heterodyne spectroscopy provides a means of measuring the intensity profiles of individual rotation-vibration spectral lines with high sensitivity. Considerable effort has been expended on optimizing these instruments for remote measurements of gases in planetary atmospheres with the result that present generation spectrometers now are beginning to provide new and startling results on the planets. In this paper, the fundamental principles of laser heterodyne spectroscopy are discussed. Detailed considerations of the optical design and of the electronic design of the spectral line receiver are given. Representative results obtained with this spectrometer are discussed, including precision frequency measurements of NH3 (1)2 ) lines, detection of auroral emission from Jupiter, and measurements of terrestrial 03 and CO2.

Infrared heterodyne spectroscopy of astronomical and laboratory sources at 8.5 µm

THE first successful infrared heterodyne spectrometer featuring semi-tuneable semiconductor diode lasers was constructed and used near 8.5 µm to make laboratory measurements of line profiles in N2O and to detect thermal emission from Mars and from the Moon. This experiment was conducted at the coude focus of the 30-inch telescope at the Goddard Optical Research Facility in January and February 1974.