Abstract: This paper describes a molecular accommodation (sticking coefficient) model for predicting molecular deposition characteristics on optical surfaces under various incident-flux and surface-temperature conditions. The model is based on the simple, but general, detailed balancing concept that the deposition flux is equal to the difference between the incident flux and the desorption/re-evaporation flux, the latter assumed to be in an Arrhenius form involving an effective activation energy (a combination of the heats of desorption, vaporization, migration, etc.). By defining sticking coefficient as the ratio of deposition flux to incident flux, a general sticking coefficient formula in terms of the incident flux, the activation energy, and the surface temperature can be derived. This model can be expressed in a functional form containing the activation energy term and two other empirically determined gas-surface interaction parameters. Hence, sticking coefficient can be used as a correlational physical quantity to determine the deposition flux. The model has been applied to the MSX (Mid-Course Space Experiment) UVISI (Ultraviolet and Visible Imager and Spectrographic Imager) internal contamination problem. The main concern here was possible molecular deposition on UVISI mirrors due to Chemglaze Z306 paint outgassing during MS mission flight. Our approach was to derive a general Chemglaze Z306 sticking coefficient formula based on correlation with Chemglaze Z306 outgassing/deposition data at high source temperatures (75$DGR@C, 125$DGR@C, and 300$DGR@F). This formula was then used to predict UVISI deposit buildup under relatively low source/mirror temperature conditions ($MIN@40$DGR@C to $PLU@20$DGR@C) during MSX mission flight.!6
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