Kinetics and mechanism of the collision-activated dissociation of the acetone cation

For center-of-mass collision energies E_cm = 1–60 eV, the major fragment ions for the collision-activated dissociation (CAD) of the acetone cation are the acetyl cation (m / z 43; absolute branching ratios of 0.96–0.60) and the methyl cation (m/ z 15; absolute branching ratios of 0.02–0.26); the absolute total cross-sections were 24–35) Ų. The breakdown curves (viz, plots of the absolute branching ratios versus E_cm) show complex, complementary energy dependences for production of MeCO⁺ and Me⁺, indicating apparent closure of the Me⁺ channel for E_cm > 30 eV. Our observations are consistent with a competition between three fast, primary (direct) reactions, each of which opens sequentially at its respective threshold energy (viz, reactions 8, 10, and 8′). 1 $$Me_2 CO^ + \cdot \to MeCO^ + + Me \cdot (X^2 A''_2 ) \Delta H = 0.82 eV$$ 1 $$ \to MeCO^ + + Me \cdot (B, 1^2 A'_1 ) \Delta H = 6.55 eV$$ 1 $$ \to Me^ + + Me \cdot + CO \Delta H = 4.24 eV$$ That is, the breakdown curves for MeCO⁺ and Me⁺ (and other CAD fragments) are consistent with the interpretation by other authors that the collisional activation of the acetone cation involves electronic transitions, so that CAD occurs primarily from isolated electronic states (i.e., non-quasi-equilibrium theory (QET) behavior). For acetone we found a correspondence between the photoelectron-photoion-coincidence and CAD breakdown curves. This may indicate that collisional activation in non-QET systems corresponds to scattering angles that emphasize optically allowed transitions accessed by photoionization.