Adsorption and adhesion on laser windows

A procedure to prepare uncontaminated calcium fluoride surfaces was developed. Characterization of the chemical composition and cleanliness of selected surfaces was performed with Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). Electron beam-induced decomposition of the calcium fluoride was observed when using typical AES electron beam parameters (5 keV, 5 microamperes). Threshold measurements for the onset of electron beam damage showed that AES analysis could be made without significantly altering the chemical composition of the surface by rastering a 2.5 keV electron beam with a current density less than 0.16 mA/sq cm. The adsorption of water on clean calcium fluoride surfaces was studied using ISS. Room-temperature exposure to >10L of water resulted in a saturated surface coverage of about a 1/4 monolayer. At a surface temperature of T sub SF = 180 K and a water pressure of 1.3 x ten to the -6th Pa, a coverage of one monolayer occurred almost instantaneously. Desorption of the adsorbed water was found to depend upon the surface temperature of the specimen prior to water exposure which suggests perhaps two different states of adsorption. The heat of adsorption of water on argon sputter cleaned calcium fluoride was determined to be 10.8 + or kcal/mole.