Ferd E. Williams

A vacuum ultraviolet monochromator for absorption and photoconductivity measurements on luminescent solids.

A vacuum ultraviolet monochromator for the region 1000A to 2000A is described in detail. The limited range makes rotation of the grating about a point and simplified construction feasible, since the loss of resolution at the extreme of wave-length is only 0.1 percent of the measured wave-length. The spectrum of the hydrogen source and a typical absorption curve are given.

The Absorption Spectra of Manganese Fluoride, Zinc Fluoride, and Manganese‐Activated Zinc Fluoride

Thin films of MnF2, ZnF2, and ZnF2:Mn have been prepared by evaporation, and their optical absorption spectra have been measured from 1500 to 2600A. The absorption coefficient of MnF2 has a maximum of 8.64×104 cm−1 at 1613A, corresponding to an oscillator strength of 0.042; while there appears to be a maximum of lower value at 1470A for ZnF2. An unresolved weaker band at 2050A is also evident for MnF2. For the phosphor, ZnF2:Mn, the absorption coefficient is intermediate between those of ZnF2 and MnF2, but the peak characteristic of the Mn++ activator is shifted to 1666A and the oscillator strength of the transition is increased to 1.2.The relative excitation spectrum of ZnF2:Mn was determined, and maximum response occurred between 1600 and 1670A, indicating that radiation in the strong absorption band characteristic of the activator is most effective in exciting luminescence. Less efficient excitation occurs to 2200A.It is concluded that a transition to an excited atomic state of the activator, rather th...

Specific Magnetic Susceptibilities and Related Properties of Manganese‐Activated Zinc Fluoride

The specific magnetic susceptibilities of manganese‐activated zinc fluoride phosphors of diverse activator concentration, and prepared by various crystallization procedures, have been measured from −60°C to 200°C. A trivial Weiss constant, except for phosphors of inordinately high manganese concentration, demonstrates the absence of appreciable exchange demagnetization of adjacent paramagnetic activator ions and suggests that there exists negligible departure from a random distribution of manganese ions over cation sites. Utilizing this distribution, and recognizing that only manganese ions not having other manganese ions at any of the nearest cation sites are capable of luminescence, an expression for the dependence of luminescent efficiency on activator concentration is derived and found to be in quantitative agreement with experiment.

The Emission Spectrum of Manganese-Activated Magnesium Germanate***

An unusual fine structure of emission has been reported1 for some manganese-activated magnesium germanate phosphors. In order to determine the precise nature of the transitions involved, the temperature dependence of the actual intensities, half-widths and peak positions of the individual emission bands have been measured from −190°C to +90°C using ultraviolet excitation. The measurements were made in a constant temperature vacuum sample chamber and with a Littrow type spectrograph containing calibrated photographic plates. Five emission bands were clearly resolved between 6200 and 6800A. The intensity of each band increases with temperature up to a critical temperature above which the efficiency decreases rapidly. Also, the short wave-length bands practically disappear at −190°C, and the individual bands shift slightly to shorter wave-lengths and decrease in half-width at low temperatures. The experimental results are qualitatively interpreted, and tentative correlations with atomic spectra are proposed.

Some Magnetic Properties of Manganese‐Activated Luminescent Solids

The specific susceptibilities of a series of manganese‐activated zinc fluoride phosphors and their components have been measured. The unexcited manganese was found to be in the divalent 6S state. There is evidence for exchange demagnetization of adjacent paramagnetic activator ions.The change in paramagnetic susceptibility with excitation, predicted by Williams and Eyring [F. E. Williams and H. Eyring, J. Chem. Phys. 15, 289 (1947)] to explain the long non‐temperature‐dependent lifetimes of the emitting states of luminescent solids containing paramagnetic activators, has been measured for manganese‐activated zinc fluoride. Radiation from a high voltage discharge lamp was used to excite the phosphor in a vacuum magnetic balance having a sensitivity of 10−6 dyne per mm scale deflection. The observed change in susceptibility corresponds to a decrease of the order of magnitude of one Bohr magneton per excited activator ion.

Electronic Processes in Zinc Fluoride and in the Manganese‐Activated Zinc Fluoride Phosphor

Measurements have been made of the temperature‐dependence of electrical conductivity and thermal e.m.f. on ZnF2 and ZnF2:Mn, and of the Hall coefficient and rectifying power at 25°C on ZnF2. Both ZnF2 and ZnF2:Mn are N‐type impurity semiconductors. The presence of MnF2 in ZnF2 decreases the electronic conductivity by a factor of about 103, indicating a decrease in the concentration of electron donors by oxidation. A marked dependence of the temperature variation of conductivity on heat treatment has been observed. Contrary to the simple theory, the absolute values of the thermal e.m.f. increase with temperature. From the Hall coefficient and conductance at 25°C of heat‐treated ZnF2, a concentration of free electrons of 3×1016 cm−3 and a mean free path of 6.7×10−8 cm are deduced. ZnF2 rectifies in the expected direction.

Thermoluminescence of Manganese-Activated Zinc Fluoride Phosphors*

Thermoluminescence measurements at linear heating rates of 0.025 and 0.100 K°/sec. on the ZnF2:Mn phosphor with electron and ultraviolet excitation have been made as a function of activator concentration and crystallization temperature. Five individual glow peaks between 100°K and 700°K have been clearly resolved. The relative intensities and widths of the peaks are independent of the type of excitation and activator concentration but are strongly dependent on crystallization temperature and subsequent annealing. A detailed theoretical analysis of the glow peak at 350°K with the 0.025 K°/sec. heating rate indicates that the most probable mechanism of the kinetics is basically first order with a strong probability of retrapping. The depths of the electron traps responsible for the complete thermoluminescence are estimated, and the possibilities of the precise physical nature of individual electron traps in ZnF2:Mn are discussed in the light of the glow data.