The construction of a pulsed electron gun for ultrafast reflection high-energy electron diffraction experiments at surfaces is reported. Special emphasis is placed on the characterization of the electron source: a photocathode, consisting of a 10 nm thin Au film deposited onto a sapphire substrate. Electron pulses are generated by the illumination of the film with ultraviolet laser pulses of femtosecond duration. The photoelectrons are emitted homogeneously across the photocathode with an energy distribution of 0.1 eV width. After leaving the Au film, the electrons are accelerated to kinetic energies of up to 15 keV. Focusing is accomplished by an electrostatic lens. The temporal resolution of the experiment is determined by the probing time of the electrons traveling across the surface which is about 30 ps. However, the duration of the electron pulses can be reduced to less than 6 ps.

By means of a crossed-beam technique, the absolute cross section for charge transfer between lithium ions and lithium atoms has been measured for incident ion energies ranging from 14 to 1000 eV. The lithium ions are produced thermally in a P-eucryptite source and are accelerated and focused into a ribbon beam in a Pierce gun. The total energy spread of the beam is less than 3% at all energies; mass analysis shows that above 1000'C emitter temperature the impurity ions in the beam are below 0.2%. This ion beam traverses a high-density, collimated lithium vapor beam issuing from an oven. A typical density of neutral lithium atoms in the interaction region is 10"atoms/cms. The slow ions produced in the charge-transfer interaction are collected on a negatively biased cage surrounding the interaction volume of the two beams. The slow-ion charge-transfer current completely accounts for the ion-beam attenuation, thus indicating that all scattered beam ions are collected. The vapor-beam density was determined by condensing the vapor on a cold target, dissolving the metallic lithium thus collected, and titrating the resulting solution. The cross section for charge transfer varies from 240)&10 '6 cm2 at 14 eV incident ion energy to 80)(10 "cm' at 1000 eV, and has the expected energy dependence for resonant charge transfer Q'ts 193&&10 -s 3 3&&10 -slog L where the incident ion energy E is in eV and Q is in cm'.

No abstract

No abstract

An ion gun using separate acceleration and deceleration stages to produce ion beams in the 1-100 eV energy region has been constructed and tested. The gun is based on an existing electron gun design but uses a thermal emitter Li20' AJ,Oa'nSi02 (n =2 and n =4) as a source of Li+ ions. Current densities of 1O-L I0-6 A/cm 2 at beam energies of 2-100 eV were obtained and both the spatial and the energy distributions of the ions were investigated. Minimum beam radii (typically about 1 mm) occurred a few centimeters from the last aperture. Beam profiles were measured as a function of axial distance and well collimated beams up to 16 cm in length were obtained. The energy spread in the beam was found to be 0.22 eV (total width at half maximum intensity), in good agreement with the expected thermal energy spread from the source which operates at about 1200°C. Mass analysis shows that beam purities are in excess of 99% 7Li+ for isotopically enriched emitter material. Theoretical calculations of space charge effects on image distance and image size are presented as well as design parameters and graphs tor the operation of the gun under various conditions.