Search

In the presence of a relatively high percentage of lead or certain other agents and after the usual excitation, zinc sulfide phosphors become capable of storing luminescence energy at such a high potential that, unlike phosphorescence, it can be released only by radiation with the near infra-red. There are two stimulating bands—a strong one at 1.25–1.60μ and a weaker one adjoining the visible. The resulting luminescence is increased by a small content of copper. Its decay, under continued stimulation, is monomolecular in contrast to the decay of phosphorescence, which is bimolecular. In the case of lead, the color of the stimulated luminescence is green, that of the shorter of its two fluorescence bands. These fluorescence bands, in the green and yellow, are excited by different portions of the spectrum, and their phosphorescence decay is at different rates. After excitation at 77°K, there is also a bright emission observed on radiating with infra-red of 1–3μ, but this is due to the release of stored luminescence energy which is normally latent at 77°K, appearing at higher temperatures as phosphorescence. It does not affect the magnitude of the luminescence which is subsequently obtainable, after warming to 25°C, by stimulation with infra-red of the same range.

Calcium silicate phosphors activated with varying amounts of manganese luminesce yellow to orange under cathode-ray excitation, but are not excited by ultraviolet λ2537. If a small amount of lead is added with the manganese, the resulting material responds to λ2537 to give the same color as that produced by cathode rays, and in addition to the visible luminescence, there is an ultraviolet band at λ3400. The spectral distribution in the visible is dependent upon manganese concentration and on temperature.If lead alone is used as an activator, the product gives emission entirely in the ultraviolet. Three emission bands are evident at approximately 3000A, 3340A, and 3500A, any one of which may be predominant, depending upon the conditions of preparation. The excitation spectrum also varies widely, having several maxima between 2000A and 3000A. When the phosphor is excited by 2288A line from a low pressure cadmium arc, a fourth broad band appears in the emission, which peaks about 3850A and extends far enough to the longer wave-lengths to be easily observed visually.X-ray analysis with a Philips Geiger counter spectrometer demonstrates that the emission band at 3000A is due to the α-metasilicate, the one at 3500A to the β-metasilicate, while the 3340A band is due to the β-orthosilicate. During the firing process the orthosilicate is formed first by the reaction between the calcium oxide and silica in equi-molar proportions, and it reacts thereafter with additional silica to form metasilicate.

Unexpected emission bands have been observed for ZnCdS: Ag high in cadmium. With high Ag, the main emission band under cathode-ray excitation at 10 kv appeared to be shifted to shorter wavelength on increase in current density. Under ultraviolet excitation at −196°C, the apparent shift was resolved into two emission bands, one of which corresponded to the new band that appeared under cathode rays; the other, to the normal band of the unactivated phosphor. With low Ag, edge emission bands appeared under high current density cathode rays and sodium bands under ultraviolet at −196°C.

Two groups of oxygen-containing phosphors are examined for the effect of activator concentration and temperature upon the location of their peak emission. The first group includes five phosphors whose emission is distributed between two main spectral bands to an extent dependent on activator concentration. Sufficient decrease in activator content causes all of the emission to appear in the shorter wave-length band. An increase in temperature has the same effect. In the second group, represented by three phosphors, the emission cannot be resolved into component bands by change in activator content. An increase in temperature shifts the emission to shorter wave-length.

When activated with manganese alone, zinc fluoride emits an orange fluorescence under excitation by 2537A whose brightness is very weak because its peak excitation lies at 2200A. Other activators, cerium, tungsten, and titanium, shift the peak excitation to 2500–2600A and produce a higher absorption in the range 2000–2600A. The emission of tungsten and titanium lies mainly in the visible; that of cerium consists of a series of bands in the ultraviolet.Combinations of these elements with manganese produce greater fluorescence brightness. With tungsten or titanium, the normal orange emission of manganese is increased manifold with no change in the spectral distribution nor in the unique exponential decay of its afterglow except that the decay is raised to a higher brightness level. With cerium, the individual emission of each activator persists, with no reduction in intensity of the ultraviolet cerium bands and with only a slight increase in the intensity of the manganese band.As explanation of this apparent discrepancy, evidence was found that only tungsten and titanium are present in the same phase as manganese, namely, zinc fluoride. Cerium combines to form a separate phase, an aggregate of zinc and cerium fluorides.

The spectral distribution for the excitation of phosphors is so erratic that it seems at present impossible to predict the location of strong excitation bands. Out of many phosphors examined in rare gas discharges, none was found to exceed in brightness the calcium tungstate and zinc silicate already in commercial use. The light output of the latter, however, was considerably improved by the elimination of the free silica usually present, which evidently has an appreciable absorption to radiation in the Schumann region. On the reasonable assumption that the resonance radiation is the source of excitation, the efficiency of fluorescence emitted by the zinc orthosilicate phosphor in the neon discharge denotes a quantum conversion near unity, just as in the low pressure mercury discharge.

Discharge lamps with sodium alloys have been made in bulbs about 5 cm in diameter, a size chosen purposely small in order to attain readily the high current density required by the low vapor pressure of alloys. The most desirable elements, from the standpoint of both color and efficiency, proved to be cadmium and mercury in about equal proportions by weight with somewhat less than 10 percent sodium in solid solution with them. Such an alloy gave a white discharge, with good color values throughout the spectrum with the exception of reds, which were only fair. The overall efficiency was reduced by 30–40 percent, from 43 LPW for pure sodium to 25–30 LPW. Larger diameter bulbs would allow proportionately higher values. A reasonable maintenance was held for over 1000 hours. It could be improved by avoidance of the stem chamber, into which the alloy tends fractionally to diffuse.

The spectral emission of phosphors varies widely with the composition of the basic compound, even under activation by the same element. In any series of isomorphous compounds, however, the variations can be coordinated. They are found to follow a definite rule that is based on corresponding variations in the field strength to which the activator ion is subjected by the anions of its environment. The field strength is shown to depend on the size of the crystal lattice and on the polarizability of the elements composing the crystal.