A 2-DIMENSIONAL MODEL FOR COLLISIONAL ENERGY-TRANSFER IN BIMOLECULAR ION-MOLECULE DYNAMICS - M(-2, D-2, OR HD)-](MH(+)+H, MD(+)+D, MH(+)+D,OR MD(+)+H)()+(H)
Mr. Chacontaylor et J. Simons, A 2-DIMENSIONAL MODEL FOR COLLISIONAL ENERGY-TRANSFER IN BIMOLECULAR ION-MOLECULE DYNAMICS - M(-2, D-2, OR HD)-](MH(+)+H, MD(+)+D, MH(+)+D,OR MD(+)+H)()+(H), Theoretica Chimica Acta, 90(5-6), 1995, pp. 357-381
Guided ion beam kinetic energy thresholds in the ion-molecule reaction
s M(+) + H-2 -->) MH(+) + H, where M(+) is a closed-shell atomic ion B
+, Al+, or Ga+, were found to exceed by 0.4 to ca. 5 eV the thermodyna
mic energy requirements (or the theoretically computed barrier heights
) for these reactions. In addition, the formation of MD(+) occurs at a
significantly lower threshold than MH(+) when M(+) reacts with HD. Mo
reover, the measured reaction cross-sections for the production of MH(
+) or MD(+) product ions are very small (10(-17) to 10(-20) cm(2)), be
ing largest for B+ and smallest for Ga+. A previous paper from this gr
oup proposed that collisional-to-internal energy transfer is the rate-
limiting step for this class of reactions. It also suggested, based on
a dynamical resonance picture, that collisions occurring at or near C
-2v symmetry are more effective than other collisions even though C-2v
geometries provide no lower potential energy barriers than others. By
examining the collision paths characteristic of flux early in the bim
olecular collision and searching for geometries along such paths where
collisional-to-internal energy transfer is optimal, our earlier effor
ts predicted reaction thresholds in reasonable agreement with the (pre
viously perplexing) experimental data. In the present work, we introdu
ce a model Hamiltonian whose classical and quantum dynamics we apply t
o the M(+) + H-2, D-2, HD reactive collisions. We calculate the classi
cal collisional-to-internal energy transfer cross-sections and find en
ergy transfer thresholds that resemble the experimental reaction thres
holds but whose isotopic mass trends are not entirely consistent with
experiment. We then use a Green function method and a local quadratic
approximation to the potential surface to obtain analytical expression
s for the isotopic mass dependences of the collisional-to-vibrational
energy transfer and for the subsequent fragmentation of the three-atom
system. Finally, we analyze the origin of the threshold energy asymme
try in the M(+) + HD reactions.