Observation of high-energy backward scattered ions in a beam study of threshold energy collision-induced dissociation: dynamics of a long-lived excited state of the acetone molecular ion

Abstract

Collision-induced dissociation of acetone molecular ions with helium as a target gas has been studied at 0.65and 0.45-eV center-of-mass collision energies. Scattering contour maps show that fragment ions, CH3CO+, are predominantly backward scattered with intensity maxima lying well outside the elastic scattering circle. This can occur only by the conversion of internal energy into translational energy, demonstrating the presence of long-lived (r > 30 µ$) excited states of the acetone molecular ion. From the observed reaction dynamics we infer that the first electronically excited state Of acetone ion has the unique characteristic that on collision it releases all the excess energy into the recoil kinetic energy of the resulting internally excited ground state ion which then rapidly fragments into CH3CO+ and CH3.

Tandem mass spectrometry: dissociation of ions by collisional activation

“…The molecular beam experiments which directly measure the energy transfer in a single collision and the kinetic energy dependence of energy transfer support the concept that CID for small polyatomic molecular ions is best described as a curve‐crossing mechanism which mainly involves impulsive collisions 29, 39. In many ways this is conceptually similar to endothermic charge‐transfer processes.…”

“…Energy exchange is maximized when the collision time is matched to the level width of the transition and declines essentially exponentially at higher velocity. As demonstrated from several studies, for CID processes the photoelectron spectrum frequently provides clues to the existence of isolated states which result in deviation from the simplest formalism of energy transfer 29, 39…”

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

“…gradually shifts the average energy deposition to higher values, saturating at about 6 eV average energy deposition); 2. ions initially in the A state are excited to higher vibronic states (and the dissociation also occurs on the electronically excited surfaces) or, at very low collision energies, are deexcited to the X state with sufficient vibrational excitation to dissociate on the ground-state hypersurface {that is, at very low collision energies (Eon < 2 eV), collision with rare gas atoms triggers the release of stored electronic energy (the adiabatic X ..... A difference; 2.2 eV) into recoil kinetic energy of the acetone ion which then rapidly dissociates into MeCO+ and Me' on the ground-state potential surface because vibrational energy in the acetone ion exceeds its dissociation limit on that surface [33,34]}.…”

“…Qian et al [34,41] recently reviewed the available information about the unimolecular dissociation and CAD of the acetone, methyl nitrite, and nitromethane cations. They indicated that several of these studies had concluded that the dissociation of the nitromethane [42,43] and methyl nitrite [43,44] cations (to NO+, MeO+, and Me+) and acetone cation (to MeCO+ [17,23,25,27,[30][31][32][33][34] and Me+ (29]) cannot be described by QET. J Am Sue Mass Spectrum 1992,3,427-444…”

“…These MezCO+' (A) ions are vibrationally . excited and survive more than 38/Ls prior to dissociation [33,34] (i.e.. an isolated electronic state [18a]). Table 1 shows the absolute branching ratios for the CAD of (C 3H 6 0 +: ) from acetone measured under single-collision conditions at the center-of-mass collision energies (E em ) indicated.…”