Ultrashort Light and Voltage Pulses Applied to Silver Halide Crystals by Turbine-Driven Mirror and Spark-Gap Switch

Abstract

Studies of electron mobility in silver halide crystals have been carried out by Haynes and Shockley by means of ultrashort light and voltage pulses applied simultaneously to large, single crystals of these materials. Their apparatus made use of a radar-type, pulse-forming network discharged through mercury spark-gap tubes. An apparatus for carrying out this same type of experiment has been developed using an air-driven turbine with a mirror and a spark-gap switch. Repetitive light pulses of 3.33-microsecond duration and voltage pulses of 10 kv and about 20-microsecond duration can be applied to a crystal at a rate of 500 per second. The light and voltage pulses can be initiated simultaneously or at controlled time intervals. The simplicity and practicality of this type of apparatus for electron mobility and photolysis studies on the silver halides are illustrated by experimental results. Light pulses from a mercury arc release photoconduction electrons in the surface of a silver chloride crystal. The voltage pulse pulls the liberated electrons in a straight column for varying distances through the crystal, where they become trapped. This is shown by the direct photolysis along the column in the crystal. Also, print-out silver specks produced in large photographic grains are displaced to one side of the grains by the voltage pulses applied during exposure. The displacement is in the direction that electrons are pulled in the applied field. By separation of the light pulse and voltage pulse in time, the mean lifetime of the photoliberated electrons can be studied. The lifetime of the electrons was found to be approximately 10 microseconds, of the same order as that of electrons in large, wellannealed single crystals of silver chloride.