Surface mechanisms governing long-term stability of GEM detectors in CO$_2$-based gaseous mixtures
Understanding the chemical stability of Gas Electron Multipliers (GEMs) operated in CO$_2$-based mixtures is essential for improving detector longevity and reliability. In this work, we investigate the interaction between CO$_2$ molecules and the copper electrodes of GEM foils through near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and complementary Raman mapping. The measurements reveal that CO$_2$ exposure promotes a mild reduction of CuO to Cu$_2$O on untreated surfaces, while sputter-cleaned foils remain metallic and chemically stable. Raman spectroscopy confirms the predominance of Cu$_2$O with spatially heterogeneous contributions from CuO at the micrometer scale, providing structural support for the oxidation-state evolution inferred from XPS. Carbon 1s spectra identify carbonyl (C=O), C-O, carbonate, and hydroxyl species, indicating that oxidized copper sites mediate surface reactions and the formation of oxygenated films. A spectral feature consistent with ionized gas phase CO$_2$ species is observed in the O 1s region, suggesting that a fraction of the gas phase may become ionized in the near-surface region during acquisition. This is relevant for GEM detectors, where CO$_2^{+}$ and other ionized species generated in the avalanche can interact with the copper electrodes. These findings indicate that CO$_2$ acts not only as a quencher but also as a weakly reactive component capable of establishing self-limiting redox equilibria that favor the formation of thin, inorganic oxygenated layers. Such layers are expected to be significantly less prone to charge accumulation than the polymeric or carbonaceous deposits typically formed in hydrocarbon-based mixtures. The results provide experimental insight into the mechanisms underlying GEM stability and contribute to a deeper understanding of aging phenomena in GEM-based systems.