Comparison of the LDPI ReTOF mass spectrum of a clean substrate, obtained before the deposition of C₂H₂ ice at 15 K (black), a spectrum of 100 ML thick C₂H₂ ice obtained after the deposition at 15 K and warmed up to 60 K (red), and the spectrum obtained after annealing of the same C₂H₂ ice at 80 K (blue). The thermal desorption temperature of pure C₂H₂ ice under our applied UHV conditions is about 78 K. The presence of hydrocarbon features in the mass spectrum obtained at 60 K and their absence in the spectrum collected at 80 K is consistent with the assignment of signals above m/z=28 to the acetylene cluster ions (C₂H₂)_n⁺, where n=2,3, …, 7.