Dissociation mechanisms of excited CH3X (X = Cl, Br, and I) formed via high-energy electron transfer using alkali metal targets.

High-energy electron transfer dissociation (HE-ETD) on collisions with alkali metal targets (Cs, K, and Na) was investigated for CH(3)X(+) (X = Cl, Br, and I) ions by a charge inversion mass spectrometry. Relative peak intensities of the negative ions formed via HE-ETD strongly depend on the precursor ions and the target alkali metals. The dependency is explained by the exothermicities of the respective dissociation processes. Peak shapes of the negative ions, especially of the X(-) ions, which comprise a triangle and a trapezoid, also strongly depend on the precursor ions and the target alkali metals. The trapezoidal part of the I(-) peak observed with the Na target is more dominant and much broader than that with the Cs target. This dependence on the targets shows an inverse relation between the peak width and the available energy, which corresponds to the exothermicity assuming formation of fragment pair in their ground internal states. From a comparison of the kinetic energy release value calculated from the trapezoidal shape of I(-) with the available energy of the near-resonant level on the CH(3)I potential energy curve reported by ab initio calculations, the trapezoidal part is attributed to the dissociation to CH(3) + I((2)P(3/2)) via the repulsive (3)Q(1) state of CH(3)I, which is not dominant in the photo-dissociation of CH(3)I. The observation of trapezoid shape of the CH(2)I(-) peak with the Cs target indicates spontaneous dissociation via repulsive potential from the (3)R(2) Rydberg state, although the correlation between the (3)R(2) Rydberg state and relevant repulsive states has not been reported by any theoretical calculation.