Hopping motion

Consider the following diagram, given as 4.7.1. on the next page, showing two types of self-diffusion. Self diffusion can occur by at least two mechanisms, vacancy and interstitial. Both are "hopping" motions, as described above. 

In a diffusion-limited reaction, movement of species occurs by a "hopping motion. 

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

Early field ion emission studies of gas-surface interactions use field ionization mass spectrometry. Gas molecules are supplied continuously to the tip surface by a polarization force and by the hopping motion of the molecules on the tip surface and along the tip shank. These molecules are subsequently field ionized. The role of the emitter surface in chemical reactions is not transparent and has not been investigated in detail. Only in recent pulsed-laser stimulated field desorption studies with atom-probes are these questions addressed in detail. We now discuss briefly a preliminary study of reaction intermediates in NH3 formation in pulsed-laser stimulated field desorption of co-adsorbed hydrogen and nitrogen,... 

Wakahara, T., Yamada, M., Takahashi, S. etal. (2007) Two-dimensional hopping motion of encapsulated La atoms in silylated La2 C8o. Chemical Communications, 2680-2682. 

In the case where hopping motion is involved, the density operator j is written as... 

Figure 18. Hopping motion of the excess proton in the system of polaron wells. 2J is width of the narrow polaron band is the pulling electric held. (From Ref. 37.)...

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