Both the anti-Stokes and Stokes emission properties of cubic Y 2 O 3 :Ho 3+ arising from excitation at a wavelength of 632.8 nm are reported over the concentration range 0.25-10 mol % Ho 3+ . Power dependence studies are used to assign the bands in the emission spectra. The most efficient yellow-green upconversion emission was observed at a Ho 3+ concentration of 0.5 mol %. This originates from an absorption process involving two photons and undergoes progressive quenching as the Ho 3+ concentration is increased. It is shown herein that as the concentration of Ho 3+ is increased, cross-relaxation processes become dominant. These processes involving Ho 3+ ions in the 5 F 5 levels and the metastable 5 I 7 states enable an alternative process for the population of the 3 K 8 , 5 F 3 , and 5 F 2 levels that gives rise to enhanced emission between 480 and 505 nm as the concentration of Ho 3+ ions is increased from 0.5 to 10 mol %. This cross-relaxation process is discussed in the text. In this work we also report the first discrimination between emission bands due to transitions originating from the 5 F 4 and 5 S 2 levels of Ho 3+ . The emission bands originating from the 5 F 4 level (viz. the yellow-green 530-570 nm and the near-infrared 740-780 nm bands) are shown (as expected) to have a similar dependence on the incident photon flux.

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Water drops and water-alcohol drops formed in a \Vilson cloud chamber were photographed with periodic illumination and their velocity was measured. The drop size was determined by means of Stokes' law, and the square of the radius of the drop was found to vary linearly with time as soon as the drop was large enough to register photographically. A 60-cycle neon light that moved with constant velocity during the exposure was turned on by the expansion valve. This light was photographed on the same frame, and its flashes were used to egtablish the zero of the time scale for the drop images. It was observed that the straight line obtained from the experimental points intersected the time axis about 0.1 sec. after the zero. This fact seems to indicate that the rate of growth of the drops is smaller during the first 0.1-0.2 sec. and then reaches a constant value. The slope of the straight part of the curve was found to be about 7.5 X 10-6 cm 2 /sec. for water and 4.4X 10-6 cm'/sec. for water-alcohol.

Ca 1-x Cd x S as fine particle layers with well defined processing behave reproducibly as bright ultraviolet (370 and 405 nm) at x ~ 0.001 -0.01 and broad band visible emitters with a wavelength maximum dependence on CdS concentration over the range x = 0.05 to 0.20. These characteristics occur under both electron beam and tuned 250-300 nm UV light excitation as phosphors compatible with a range of display configurations.