Browsing by Author "Xing, Guoqiang"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Orientation effects in cross-beam ionization reactions between potassium and symmetric-top molecules(1993) Xing, Guoqiang; Brooks, Philip R.Symmetric-top molecules (CF$\sb3$Br, CF$\sb3$Cl, CF$\sb3$H and CH$\sb3$Br) in a seeded supersonic nozzle beam are orientation selected by a hexapole electric field, and collide at a right angle with fast (3-40 eV) potassium atoms. The ionization reactions at two different molecular orientations are studied: $$\eqalign{&\rm K + CX\sb3 - Y \to K\sp+ + CX\sb3 + Y\sp-\qquad (Tails\ Orientation)\cr &\rm K + Y - CX\sb3 \to K\sp+ + CX\sb3 + Y\sp-\qquad (Heads\ Orientation)\cr}$$ We observed that collision ionization reactions are influenced greatly by molecular orientations. For CF$\sb3$Br, CH$\sb3$Br and CF$\sb3$Cl, the reactivities are greater with the heads orientation than that with the tails orientation, but for CF$\sb3$H, the H end is unreactive. The steric effects are more pronounced at the low energy end near the thresholds, and almost disappear at energies above 20 eV. Most importantly, we also found that the energy thresholds of these reactions are different for heads and tails orientations, indicating that the electron affinity of a molecule should be considered as an anisotropic parameter. Some features of the experimental results are explained by the Harpoon Electron Transfer model. Further theoretical and experimental studies are required for the fully understanding of the reaction dynamics.Item Surface magnetic order of ultra thin epitaxial vanadium films on silver(1988) Xing, Guoqiang; Rau, CarlVanadium ultra-thin (1-7 monolayers) films are deposited epitaxially on well-defined single crystalline Ag(100) substrate. The topmost layers of the films are studied by electron capture spectroscopy(ECS). ECS is a surface-sensitive technique for the investigation of magnetic order existing at surfaces. It is found that the topmost atomic layer of V(100) films are ferromagnetic for all film thicknesses, in contrast to bulk vanadium which is paramagnetic at all temperatures. The films of thickness of 5 monolayers possess a surface Curie temperature T$\sb{\rm cs}$ = 475.1 K, and the critical behavior of the magnetization of the films is identical to that predicted by the well-known two-dimensional Ising Model of ferromagnet.