Propagation Calculations — Molecular Absorption Calculational Models. Molecular absorption is calculated for a given laser wavelength X by the computer code laser (51). Absorption line parameters for atmospheric molecular species are taken from the AFGL line-parameter compilation (52). The average molecular absorption coefficient for each of 32 atmospheric layers is calculated for the following atmospheric models: U.S. standard, tropical, midlatitude summer, mid-latitude winter, subarctic summer, and subarctic winter. Transmission Efficiencies. The transmission efficiency for space-to-Earth propagation was calculated for a number of laser lines in the 2, 9, and 11 gm spectral regions. The 2 and 11 gun regions were chosen because they may afford an improvement in transmission through various meteorological aerosols. The 9 gm region was chosen because the laser lines of certain isotopic species of CO2 may offer higher transmission efficiencies than their naturally occurring counterpart, 12C16O2; 12C16O2 is uniformly distributed in the atmosphere and its strong absorption lines should be avoided by selection of alternate laser wavelengths. All calculations are for propagation to receptor site elevations of 0.0, 0.5, and 3.0 km for a zenith angle of 50°. The spectral region from 2.100 to 2.315 gem offers an excellent high-transparency window with relatively few strong absorption features. Calculated transmission efficiencies of all laser lines examined in this spectral region exceed 99.9% for all site elevations and are insensitive to seasonal variations. Since the individual windows between absorption features are wide (in many cases > 10 cm '), there is hope that a scalable, high-power laser operating at a wavelength in one of these windows can be developed. The transmission efficiency of several mid-rotational P- and /?-branch laser lines of the isotopic-species l2CIKO2 laser operating on the 10°0—*02°0 band are given in Table 1. Operation of a CO2 laser in this mode results in a significant improvement in the transmission efficiency compared with operation on “standard” lines of the 00°l-»10°0 band of 12Cl6O2; seasonal variations, however, are pronounced and the highest (average) annual transmission efficiency to typical receptor sites (h = 0.5 km) is only 87.7% for the 9.124-gim line. Because of the potential importance of the 11-gim region, the interval from 10 to 12 gm was closely examined for high transparency windows. This spectral region is
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