Fragmentation of multiply charged hydrocarbon molecules C_nH^q+ (n ≤ 4, q ≤ 9) produced in high-velocity collisions: Branching ratios and kinetic energy release of the H⁺ fragment

Fragmentation branching ratios for channels involving H⁺ emission and associated kinetic energy release of the H⁺ fragment [KER(H⁺)] have been measured for multicharged C_nH^q+ molecules produced in high velocity (3.6 a.u.) collisions between C_nH⁺ projectiles and helium atoms. For CH^q+ (q ≤ 4) molecules, measured KER(H⁺) were found well below predictions of the simple point charge Coulomb model (PCCM) for all q values. Multireference configuration interaction (MRCI) calculations for ground as well as electronic excited states were performed which allowed a perfect interpretation of the CH^q+ experimental results for low charges (q = 2–3) as well as for the highest charge (q = 4). In this last case we could show, on the basis of ionization cross sections calculations and experimental measurements performed on the same systems at slightly higher velocity (4.5 a.u.), the prominent role played by inner-shell ionization followed by Auger relaxation and could extract the lifetime of this Auger relaxation giving rise to the best agreement between the experiment and the calculations. For dissociation of C₂H^q+ and C₃H^q+ with the highest charges (q ≥ 5), inner-shell ionization contributed in a prominent way to the ion production. In these two cases it was shown that measured KER(H⁺) were in good agreement with PCCM predictions when those were corrected for Auger relaxation with the same Auger lifetime value as in CH³⁺.