Ultra-sensitive laser-based contamination detection for space applications

Molecular contamination due to outgassing of organic materials in vacuum can be a source of significant degradation for space equipment, especially for sensitive optical instruments. Thus, on-ground contamination measurements in vacuum are essential to ensure the flight hardware cleanliness and for making in-flight performance predictions. A suitable detection technique shall work in-situ and in real-time, i.e. it shall be possible to measure the real-time build-up of contamination layers of a few nanometers and less on surfaces in vacuum in the presence of an outgassing source. In this paper we address a new experimental setup for controlled contamination and high-sensitivity spectroscopic analysis of the contaminants on representative material for typical space optics. The central method of choice for the detection of organic contaminants incorporated in this setup is laser-induced fluorescence (LIF), which provides the demanded distinguishability of the applied contamination species. Different excitation wavelengths in the UV spectral range are used. LIF measurements are accompanied by in operando verification of amount and species of the contaminants by a thermoelectric quartz crystal microbalance (TQCM) and a mass spectrometer (MS). This parameter study aims for exploring the laser parameter dependent detection limits and the temperature-dependent condensation process with respect to various substrate-contaminant combinations, in order to reveal strategies to prevent contamination for space applications.