Peter D. Johnson

The Interpretation of the Dependence of Luminescent Efficiency on Activator Concentration

The previously published analysis (see reference 1) of the dependence of efficiency of substitutionally activated inorganic phosphors on activator concentration is generalized to include a wide variety of inorganic phosphors under various types of excitation. In addition to the optical excitation of the activator, the creation of conduction electrons and excitons, and the transfer of their energy to the luminescent and non‐luminescent activator ions and to other impurities and lattice defects are included in the formulation. The dependence of efficiency on activator concentration of ZnF2:Mn under c.r., x‐ray, vacuum u.v. and near u.v. has been measured and interpreted on the basis of the theory. Preliminary photo‐conductivity measurements clarify the role of the conduction band in the luminescence of ZnF2:Mn. Also the dependence of the efficiency vs. activator concentration of both ZnF2:Mn and KCl:Tl on temperature has been measured and interpreted on the basis of the variation in activation energy for co...

Absolute Optical Absorption from Diffuse Reflectance

An equation relating the diffuse reflectance of crystalline powders to particle size, index of refraction, and absolute optical absorption coefficient is derived. Absorption spectra calculated by means of this equation from diffuse reflectance of KCl:Tl, KBr:Tl, and didymium glass are shown to be in satisfactory agreement with absolute absorption measurements on clear crystals of the same materials.

Energy Levels and Rate Processes in the Thallium Activated Potassium Chloride Phosphor

The dependence of spectral distribution and intensity of emission of KCl: Tl on temperature and excitation wavelength are correlated with absorption data and theoretical calculations to provide a detailed picture of the energy relationships in this phosphor. The two principal excited states are the 1P10 and 3P10 states of Tl+ and are found to be approximately in thermal equilibrium with each other. The equilibrium is disturbed by emission, and the 4750A emission is favored by 1960A excitation and the 3050A emission is favored by 2470A excitation. The former processes are attributed to 1P10↔1S0 transitions; the latter, to 3P10↔1S0 transitions. The rate processes are formulated and the specific rate constants deduced. The presence of Tl+ or Cd++ adjacent to the activator induces additional excitation and emission bands. The activation energy for radiationless de‐excitation has been computed theoretically and found to be in accord with experiment.

Electron Traps in the Thallium‐Activated Potassium Chloride Phosphor

The effects of activator concentration, added impurities, infrared stimulation, and temperature of excitation on the thermoluminescence of KCl:Tl permit the identification of the two principal trapping levels as the 3P00 and 3P20 states of isolated Tl+. The energy of activation necessary to fill these metastable states, following excitation to the 3P10 and 1P10 states, is provided by the polarization energy. The concentration of traps simultaneously filled is limited primarily by re‐excitation of metastable Tl+. F and V centers are created by multiple excitation of Tl+. These interpretations are quantitatively correlated by a configuration coordinate model and are shown to be in accord with the band theory.

Spectral change mechanism in the pulsed high‐pressure sodium arc

The enhancement of radiative output of the pulsed high‐pressure sodium discharge in the blue region of the visible spectrum is due principally to radiation from the nd2D→3p2P transitions of the sodium atom where n=4–15. Excitation of these upper energy levels results from a transient non‐steady‐state increase in plasma temperature to about 5500 °K in the center of the arc. This temperature decays to a value near that encountered in steady‐state excitation conditions in a time comparable to the empirically optimized pulse duration and to the previously determined time for plasma temperature relaxation.

Light Production in the Aluminum-Oxygen Reaction

The mechanisms of excitation of the continuum and the AlO spectra in the aluminum-oxygen combustion reaction have been investigated. Aluminum oxide is found to be excited thermally. The color temperature and intensity of the continuum are limited to the boiling temperature of Al2O3. In aluminum-oxygen photoflash lamps, the principal light emission is blackbody radiation from AlO and Al2O3 at or below 3800°K. It is concluded that the possibility of substantial alteration of the color temperature or intensity of the radiation resulting from this reaction is remote.

Optical Absorption and Diffuse Reflectance of Powders

Two different models for the dependence of diffuse reflectance of powders on optical absorption coefficient, particle size, and index of refraction are discussed. For particles large compared to the wavelength of incident radiation a model in which the path of this radiation is controlled mainly by specular internal and external reflections and refraction is preferable to one in which path deviation by Lambert law scattering dominates.

Time‐dependent plasma temperature measurements of the high‐pressure sodium arc

The plasma temperature of the high‐pressure sodium arc has been measured as a function of time and power input on 60‐Hz sine‐wave operation. A maximum temperature of 4370 °K is found at power input above about 100 W/cm3 of arc tube volume. At zero current the temperature falls to 3300 ° K. Square‐wave operation indicates that temperature decay to this value occurs in 0.3 msec. Since local thermodynamic equilibrium (LTE) exists in the hot central core of the plasma, the electron density can be calculated using the Saha equation to decay from 1016 to 1015 cm−3 during this period. This decay in electron density is accompanied by an increase of arc reignition voltage of more than a factor of 4.

Energy States and Luminescent Processes in the ZnF2: Mn Phosphor

Single crystals of ZnF2: Mn have a single symmetrical thermoluminescent peak when excited by monochromatic radiation of wavelength 1800 to 2300 A. The electron trap responsible for this peak is a metastable state of the activator. Excitation occurs first to a higher excited state from which both the emitting and trapping states are accessible. With 1850 A excitation, transfer to the emitting state from the initial excited state is approximately 4 times as probable as transfer to the trapping state. Untrapping occurs by a monomolecular process modified by thermally activated retrapping from the state governing the rate of emission. Energy level diagrams are proposed for the states of Mn++ responsible for absorption emission and trapping.

Mercury Resonance Radiation in the High-Current, Low-Pressure Discharge

The dependence of the intensities of the 1849- and 2537-A resonance lines on current density, in the range 0.2–2 A/cm2, Hg pressure 10−3–0.5 torr, and discharge-tube diameter 10–25 mm, have been measured. The intensities nearly saturate at about 2 A/cm2. Absolute output at 1849 A, measured photochemically, is consistent with the results of B. T. Barnes. The dependence of this output on the measured parameters make this discharge a good absolute flux-density standard at 1849 A.