In-situ ellipsometric analysis of the photofixing of Volatile Contaminant Materials

The outgassing and deposition of Volatile Condensable Materials (VCM´s) onto optically-sensitive surfaces of satellites is of significant interest to spacecraft-contamination engineers. The effluent of RTV CV-2568 has been photofixed onto a temperature controlled glass substrate with a 90 nm MgF2 anti reflection layer deposited on the surface. The deposition/photofixing has been monitored in-situ by both quartz crystal microbalances placed near the substrate and transmission spectroscopic ellipsometry taken directly on the depositing film. We will show the deposition rate under one sun of illumination as a function of impingement rate at −40 C saturates at approximately 1nm/24 hours, for an impingement rate of approximately ten times the deposition rate. Impingement rates higher than this value place the process in the photon flux limited regime, while rates at or below this value place the process in the flux limited regime. Our results indicate deposition is driven by photons of wavelength greater than 250nm, while darkening of the film is driven by photons of wavelength shorter than 250nm. We will present the optical constants, n and k, for the photofixed film. Finally, the activation energy for the deposition process can also be extracted by measuring the deposition rate as a function of substrate temperature.