Publisher Summary This chapter focuses on absorption and emission by atmospheric gases. At centimeter and shorter wavelengths, absorption and emission by atmospheric gases can significantly affect the propagation of electromagnetic radiation through the atmosphere. At frequencies from 1 to 300 GHz, referred to as “microwave frequencies,” absorption by atmospheric gases is dominated by water vapor lines at 22 and 183 GHz, oxygen lines near 60 GHz and at 118 GHz, and relatively narrow and weaker ozone lines above 100 GHz. Non-resonant absorption by water vapor and oxygen has significant effects in the window regions away from the dominant lines. The radiative transfer expressions appropriate for calculating absorption and emission by atmospheric gases are formulated. The general expressions for the spectral-line absorption coefficient are provided, followed by specific expressions for calculating absorption by water vapor and oxygen. Problems associated with the calculation of absorption by these two molecules are discussed. The absorption by microwave lines of ozone and other minor constituents is considered. Finally, the results of calculations of atmospheric absorption and emission, as well as the measured values are presented in the chapter.

2.3. Absorption and Emission by Atmospheric Gases

THIS letter reports the preliminary results of some recent measurements of the thermal emission spectrum of the stratosphere in the submillimetre region. This follows analysis of previous measurements from aircraft1–3 using Fourier spectroscopic techniques which snowed how, using a Michelson interferometer with a Golay cell detector, spectra may be obtained to a maximum resolution of about 0.2 cm−1. From such results, the mixing ratios of H2O and O3 may be determined relative to O2. Several emission lines due to H2O, O2 and O3 were also observed for the first time in the atmosphere, which raised the possibility that other much weaker lines due to minor atmospheric gases might be observed.

High Resolution Observations of the Submillimetre Stratospheric Emission Spectrum

Using the pressure dependence of absorption, absorption coefficients and detunings were measured for CO 2 pump lines and the strong fat infrared emission in optically pumped D 2 O. The P (32) CO 2 line was found to be detuned ∼1.5 GHz from the ν 2 band transitions 6_{6}, 6_{5} \rightarrow 5_{5}, 5_{4} . The resulting emission lines at 50.3 μm and 66 μm were found to be detuned from their respective transitions by about the same amount. On the basis of these measurements and gain estimates for the far infrared, the resulting emission lines are identified as stimulated Raman emission.

Stimulated Raman emission in infrared excited gases

The ground state binding energy (BE), rotational and vibrational energy levels, and line strengths for radiative transitions between some of these energy levels are calculated for the [H2O]2 molecule. These quantities are computed for three intermolecular potentials published for the [H2O]2 molecule, two of which were calculated by MO SCF techniques and one which was derived from an empirical point charge model for the water molecule. The value of BE calculated for two of the potentials is approximately 6 kcal/mole, and for the third is approximately 3 kcal/mole. The total concentration of [H2O]2 in equilibrium with H2O vapor is calculated for the sets of energy levels determined for these potentials. Results are compared with available experimental data. Using calculated values of line strengths and experimental data for the integrated absorptions for two rotational transitions, an independent value is deduced for the equilibrium concentration of [H2O]2 in fair agreement with values calculated from two of the intermolecular potentials. The theoretical calculations for BE and the equilibrium concentration of [H2O]2 are also compared with those obtained from analysis of experimental data for the second virial coefficient B2(T) of H2O vapor. The theoretical values for BE bracket the value of 3–4 kcal/mole obtained from analysis of the data for B2(T). A discussion of our results is presented, along with suggestions for experimental work to search for the spectrum of [H2O]2 in the absorption spectrum of H2O vapor.

Rotation and vibration spectra for the H2O dimer: Theory and comparison with experimental data

Optical fiber based supercontinuum light sources combine the brightness of lasers with the broad bandwidth of incandescent lamps and thus are promising candidates for sources in spectroscopic applications requiring high brightness and broad bandwidth. Herein, near-infrared (IR) Fourier transform (FT) spectrometry with a supercontinuum (SC) light source is investigated. The efficient, collimated propagation of broad bandwidth SC light through an 18 m path length multipass cell is demonstrated. A normalized spectral difference is calculated for the SC spectrum on consecutive FT mirror scans and is found to vary by less than 0.5%, indicating excellent spectral stability. The rms noise on zero absorbance lines is obtained as a function of the number of mirror scans at 0.125, 2, and 16 cm−1 resolution for both the SC and conventional tungsten lamp source. The SC source has approximately a factor of ten times more noise than the lamp under comparable conditions for each resolution and data acquisition time. This clearly indicates that spectral acquisition with the SC source is not detector noise limited. NIR-FT spectra of methane and methyl salicylate, acquired with both the SC and lamp source, are reported. These spectra illustrate the advantage the SC source has over the incandescent source in that it can efficiently traverse long path lengths, thus providing a sensitivity advantage. The spectra also demonstrate the disadvantage of the SC source with respect to the lamp in the increased level of amplitude noise. Prospects for the future use of SC sources in absorption spectroscopy, including possible noise mitigation strategies, are briefly discussed.

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W.J. Burroughs

Papers

Submillimetre-wave solar observations using a double-output michelson interferometer.

Observations of millimetre wavelength solar radiation at sea level

Observation of atmospheric absorption using submillimetre maser sources

Observation of pressure broadening effects in D2O using a CN maser

On the use of apodization in submillimetre fourier transform spectroscopy