Theoretical study of the reactions of Ar++HX(v=0) and Ar+HX+(v) (X=H and D) at E=0.1 eV using the trajectory surface hopping method

Abstract

Trajectory surface hopping calculations have been carried out for collisions of Ar++H2 (v=0), Ar++HD (v=0), H2+(v)+Ar, and HD+(v)+Ar, where v=0, 1, and 2 on the Kuntz–Roach diatomics-in-molecules potential surfaces at a relative energy of 0.1 eV. The importance of the mutual “capture” of the two particles on the attractive ground potential energy surface is shown clearly. The fact that capture does not occur on every collision is attributed to an effect of the vibrational phase of the H2 or HD molecule. This vibrational phase effect can explain the drop in the experimental rate constant seen at very low temperatures in the Ar++H2 system. For H2+(v=2)+Ar and HD+(v=2)+Ar we also find that many trajectories hop to the first excited potential surface as the particles approach. Since these trajectories cannot reach small separations, this further reduces the reactive cross section for v=2 and higher levels. The ground potential energy surface has a fairly deep well, particularly when the Ar–H–H angle is near 90°. Hence, once capture occurs in the (Ar–H–D)+ system, the Ar–H and Ar–D distances rapidly interchange. The product ArD+ is always favored over ArH+ because the H atom can more easily escape the complex. Finally, the reactivity of Ar++H2 (v=0) is seen to be intermediate between that of H2+ (v=1) and H2+ (v=2) with Ar.