Carbonaceous deposits produced on Ru-capped multilayer mirrors under extreme ultra violet irradiation in the presence of adventitious gaseous hydrocarbons are a major obstacle to process implementation of EUV lithography. Here, by means of synchrotron radiation and laboratory measurements we show how carbon contamination occurs as a result of photoelectron-induced surface chemistry. We also demonstrate how a device based on an oxygen ion conducting solid electrolyte can act as a sensitive and reproducible sensor for detection of trace amounts of hydrocarbons in high vacuum environments.