During the past decades, considerable experimental and theoretical work has been done to characterize the deposits of chemical contaminants on the sensitive parts of spacecrafts and to quantify the detrimental effects of contamination on their properties and functions. In particular, specific attention is currently drawn on optical elements such as windows, mirrors and lenses which are very sensitive to the deposition of outgassed species from polymer materials and on the impact of solar radiation on deposited layers during the mission. In this context, there is a need for a deeper understanding of the wavelength-dependence of photolytic processes affecting contaminants and the modification of the spectral characteristics upon operational ageing. Having analyzed the constraints and limits of in-lab studies using chemicals defined as model contaminants in ECSS-Q-ST-70-05C standard, we have investigated the photochemical behavior of contaminants originating from epoxy adhesives using a dual approach, (i) a parametric study of thin films of uncured solid epoxy resin on optical grade substrates (ca. 150 nm), exposed to photoactivation under inert atmosphere at different wavelengths (254, 312 or 365 nm) with a controlled irradiance of about 1 mW.cm-2, (ii) the evolution upon photolysis at 254 nm of contaminant deposits outgassed from a cured epoxy adhesive, widely used for space application.

Monitoring by UV-visible and infrared spectroscopies over periods up to two days (about 50 J.cm-2) reveals notable changes taking place upon exposure to 254 nm or to a combination of irradiation wavelengths. Depending on the chemical nature and on the morphology of the deposits, the evolution of spectral properties was either due to physical relaxation of the irregular deposit or by photoactivated chemical transformations. The mechanistic aspects of the photochemical reactions involving the epoxies will be discussed based on the recorded spectroscopic information and complementary analyses of model compounds.