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Efficient generation of the second harmonic of CO 2 laser pulses was achieved in GaSe single crystals at repetition frequencies up to 100 Hz. The phase-matching angles were measured for GaSe at pump wavelengths of 9.3, 9.6,10.3, and 10.6/zm. The threshold power densities for surface optical damage and for the formation of a plasma jet on the surface of GaSe were determined. The efficiency of second harmonic generation in a GaSe crystal 0.65 cm thick was up to 9%. A comparison was made of GaSe and ZnGeP 2 crystals as materials suitable for the generation of the second harmonic of CO 2 laser radiation.

The emission of positronium negative ions from Cs deposited W(100) surfaces has been studied. A dramatic change in the emission efficiency was observed upon coating a W(100) surface with Cs. The conversion efficiency (the fraction of incident slow positrons yielding the ions) of the W(100) target with a 2.2 × 10 14 atoms cm −2 Cs coverage, measured over a time interval of 3 × 10 3 s immediately after deposition, was found to be 1.25%, which is two orders of magnitude higher than that obtained for the clean, uncoated W(100) surface and 45 times greater than the highest efficiency reported thus far.

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