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 o n acetylene, C2H2, have been observed. Correlated detection of CH(A-X) emis sion and H+ and C+ ions, as well as between the CH+ ion and its own emissio n, 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 accomp anies 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 C2Hn+ (n = 0, 1) band i s also observed in the FIPCO spectra and ascribed to dissociative processes following light emission from C2H+ and C2H(2)(+). Similar experiments have been carried out at 350 eV electron impact energy, to examine the dissocia tive processes accompanying the light emission caused by the ionization of a C-1s electron. No significant difference is observed between the FIPCO sp ectra at 120 and 350 eV impact energy.