entially excite certain vibrational modes in polyatomic molecules. Dehydration of cluster molecules is a probable reaction because of the low bond energy of H2O to the cluster, i.e., followed by V-T collisional relaxation of the reaction products. Second, rapid heating of an aerosol particle exposed to intense ir laser radiation causes internal pressures sufficient for explosive breakup (56). Since the fundamental constituents of the aerosols remain, after passing through the beam, then renucleation will probably occur and depletion of the Junge layer is not believed possible. The irradiated layer volume will be insignificant compared with the whole, and anthropogenic increases in the total mass of sulfate particles will completely outweigh any depletion rate associated with laser-SPS proliferation. Receptor Thermal Pollution. From the geophysical perspective, the exact method of waste heat disposal from the receptor power plant is probably not very important, but persistent local weather and biological effects can be quite different for dry cooling towers, wet towers, or cooling to bodies of water. The environmental impact of thermal pollution from receptors and conventional or nuclear power plants is qualitatively similar and will not be discussed further here. Environmental Impact on Wildlife In addition to providing protection to the general public, the perimeter “fence” will protect most indigenous animals from exposure to dangerous irradiance levels; the principal exception is birds. In traversing the primary beam, birds and insects will certainly be incinerated. There is some controversy over the ability of birds to sense the dangerous irradiance levels and to avoid the beam. It is difficult to resolve this point without experimental studies, and studies involving high-power radar transmitters are not applicable to the present problem. Laser-Plasma Interactions in the Ionosphere Various mechanisms of linear and nonlinear absorption of laser light by the ionosphere were examined in detail. Because the laser beam is propagating in a highly underdense plasma (electron plasma frequency < laser frequency), dissipative heating due to linear (Ohmic) and nonlinear (anomalous) absorption is completely negligible (57). Furthermore, the laser intensity is far below threshold for excitation of any known parametric or backscatter instability. Modification of either the electron density or temperature structure of the ionosphere will not occur. Perturbation of the Plasma Chemistry of the Mesosphere and Thermosphere Preliminary examination has revealed that certain reactions in the mesosphere and thermosphere may be induced by the presence of an intense IR-photon flux and chemical reaction channels may be altered. In the upper atmosphere, mid-IR photons have sufficient energy only for vibrational-state photoexcitation and a limited number of charged-species reactions, such as positive- and negative-ion photodissociation and electron photodetachment. Such laser photons have insufficient energy for any photoionization or neutral photodissociation reactions of interest to upperatmosphere constituents. Multiphoton processes are only important when the colli-
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