Supplement Number 1

The supplementary material includes detailed description of experimental procedures and conditions; Cartesian coordinates (Å) and harmonic vibrational wavenumbers (cm-1) of the DHP minimum optimized at the MP2/cc-pVTZ level; the original mass spectra of DHP measured after electron collisions (Fig. S1); the ab-initio MD (PBE0/SVP) simulated mass spectra of DHP at the different impact energies (Figs. S2-S4); the assignments of all cations and their percentages (in %) from MD simulations (Table S1); the ionization cross sections for the specified cations of DHP measured in the electron energy range of 5-140 eV (Fig. S5); experimental cross-sections and simulations near thresholds (Table S2).