Photocathodes of quantum efficiency above 1% at the doubled YAG frequency of 532 nM are very sensitive to the local vacuum environment. These cathodes must have a band gap of less than 2.3 eV, and a work function that is also on the order of {approximately}2 volts or less. As such, these surfaces are very reactive as they provide many surface states for the residual gases that have positive electron affinities such as oxygen and omnipotent water. Attendant to this problem is that the optimal operating point for some of these cesium based cathodes is unstable. Three of the cesium series were tried, the Cs-Ag-Bi-O, the Cs{sub 3}Sb and the K{sub 2}CsSb. The most stable material found is the K{sub 2}CsSb. The vacuum conditions can be met by a variety of pumping schemes. The vacuum is achieved by using sputter ion diode pumps, and baking at 250{degrees}C or less for whatever time is required to reduce the pump currents to below 1 uA at room temperature. To obtain the required partial pressure of cesium, a simple very sensitive diagnostic gauge has been developed that can discriminate between free alkali and other gases present. This Pressure Alkali Monitor (PAM) can bemore » used cesium sources to provide a low partial pressure using standard feedback techniques. Photocathodes of arbitrary composition have been transferred to a separate vacuum system and preserved for over 10 days with less than a 25% loss to the QE at 543.5 nM.« less

Photocathode transfer and storage techniques using alkali vapor feedback control

A thermionic diode is described for the measurement of surface potentials during the adsorption of gases or sodium on evaporated metal films. The method, which enables rapid surface potential changes to be followed, uses a stabilized current supply and allows characteristics curves to be plotted out automatically. A new filament stabilizer is also described. Some test surface potential data are given.

http://iopscience.iop.org/article/10.1088/0022-3735/4/11/015/pdf

A recording diode for the measurement of the surface potential of adsorbed layers on evaporated metal films

The Kelvin and the retarding‐field contact potential difference techniques have been used to follow the work‐function changes during the adsorption of barium onto both polycrystalline and single‐crystal tungsten surfaces. A double‐evaporation technique was devised in order to obtain conditions under which the barium could be evaporated without the pressure exceeding, at best, 3 × 10−10 torr. Contrary to the results of a previous similar investigation which used a thermionic method, the work functions of barium covered (110) and (100) tungsten surfaces differed considerably. For films, estimated to be greater than five monolayers in thickness, the final work functions on the (110) and (100) surfaces, respectively, were 2.955 ± 0.005 eV and 2.560 ± 0.005 eV. A model is suggested in which the barium on the (100) surface forms a polycrystalline deposit, while that on the (110) surface compares closely with a single‐crystal (110) plane in barium metal but with a high density of defects. The shape of the work f...

Adsorption of Barium onto Oriented Tungsten Surfaces

/pdf/photocathode-transfer-and-storage-techniques-using-alkali-ecjcmlrosh.pdf

The use of a quartz crystal microbalance to study the oxidation of aluminum during vacuum deposition

A retarding field technique for measuring the instantaneous evaporation rate of barium and barium oxide from dispenser and oxide cathodes is described. The method is based upon measuring the time rate of change of the anode work function caused by the deposition of sublimate from the cathode surface. A simple planar diode, employing a molybdenum anode, is used. In contrast to the typical curve for barium films on molybdenum, showing a minimum work function at approximately a monolayer coverage, the results for barium oxide films indicate a similar decrease in work function up to a monolayer coverage; further deposition leads to a still lower work function.

Retarding Field Technique for Measuring Sublimation from Dispenser and Oxide Cathodes

A quartz‐crystal microbalance technique has been developed which measures the chemical composition of an alkali‐antimonide film during photocathode formation at the normal photocathode processing temperatures. It is possible to measure continuously and simultaneously the chemical composition, photoelectric emission, resistance, and optical transmission of the alkali‐antimonide during photocathode formation. A thermal ionization source, which acts as a relative partial pressure indicator, yields additional data on the kinetics of the reaction mechanisms. Results are reported for the Cs–Sb and the K–Sb systems. The data indicate the importance of the experimental parameters (antimony film thickness, temperature, and alkali‐metal. vapor pressure) upon the reaction rates. Correlation of electro‐optical properties with composition depends on a knowledge of these parameters.

John V. Florio

Papers

Retarding Field Technique for Measuring Sublimation from Dispenser and Oxide Cathodes

Quartz‐Crystal Microbalance Technique for the Preparation of Alkali‐Antimonide Photocathodes