An electron impact study of carbon dioxide dimers in a supersonic molecular beam: Appearance potentials of (CO2)2+, (CO CO2)+, and (Ar CO2)+

Electron impact ionization of (CO2)n clusters (1⩽n⩽5) was investigated in a molecular beam experiment coupled with a double focusing mass spectrometer. A special ion optical system and operating mode (deflection method) was used to discriminate between beam species and background species. For both neat and seeded (He, Ar, Kr) beams we have studied, ion beam profiles extracted from the ion source in order to obtain information about neutral cluster beam properties. All appearance potentials reported in this work have been referenced to rare gases in either the static gas or seeded beam mixtures. The following appearance potentials were obtained: (CO2)2+e→(CO2)2+, AP=13.1±0.2 eV, (CO2)2+e→(CO CO2)+, AP=19.1±0.5 eV, Ar CO2+e→(Ar CO2)+, AP=13.1±0.5 eV. The exothermicity of the reaction of CO2+ with CO2 to form (CO2)2+ was found to be 17.1±5 kcal/mol in good agreement with previous high pressure mass spectrometry determinations.