Photochemical processes in a two-component molecular contaminant film

Previously, significant laboratory work has been performed on the photochemical deposition and darkening of molecular contaminant films. Much of this work addresses single, purified molecular species to understand fundamental photochemical processes. However, some of this work disagrees with other studies involving mixed, real spacecraft materials. There are also points of disagreement with contaminated returned optics from the Hubble Space Telescope where mixed contaminants were found. In this paper, we describe a method for vacuum depositing a controlled, reproducible contaminant film containing two molecular species: tetramethyl-tetraphenyl trisiloxanc (DC 704) and dioctyl phthalate (DOP). We use this film to show differences in photochemical processes compared to a pure film of DC 704. We show that some photopolymerization processes occur more slowly in a two-component, mixed film during accelerated exposure to vacuum ultraviolet (VUV) radiation.