Fragment ion-photon coincidence study of dissociative ionization accompanying light emission by electron impact on acetylene

Fragment ion-photon coincidence (FIPCO) spectra by 120 eV electron impact on acetylene, C₂H₂, have been observed. Correlated detection of CH(A-X) emission and H⁺ and C⁺ ions, as well as between the CH⁺ ion and its own emission, was observed and verified with the help of Monte Carlo simulation of the band shapes observed. The translational energy distribution of the H⁺ ion is determined to be 1000±150 K (0.13±0.02 eV). Those for CH⁺ and C⁺ are 0.25 and 0.15 eV, respectively; the former translational energy accompanies a Gaussian distribution with a full-width at half-maximum (FWHM) of 0.2 eV, whereas the latter FWHM is 0.15 eV. A strong C₂H_n⁺ (n = 0, 1) band is also observed in the FIPCO spectra and ascribed to dissociative processes following light emission from C₂H⁺ and C₂H₂⁺. Similar experiments have been carried out at 350 eV electron impact energy, to examine the dissociative processes accompanying the light emission caused by the ionization of a C_1s electron. No significant difference is observed between the FIPCO spectra at 120 and 350 eV impact energy.