Supplementary Material

The Supplementary Material contains three additional figures and five tables. Figure SM 1 shows the mass spectra obtained upon ionization of the doped droplets by electron impact. Figures SM 2 and SM 3 show the pickup pressure dependences of the intensity of the infrared bands, Tables SM 4-SM 8 list calculated frequencies, infrared intensities, and mode assignments for allene, propyne, cis-vinylmethylene, trans-vinylmethylene and cyclopropene cations, respectively. Tables SM 9-SM 10 list the calculated electronic and thermodynamic parameters of the isomers and the cartesian coordinates of the atoms. Section SM 11 contains the discussion of the alternative assignment of the new bands. SM contains references 6, 19, 36-40.