Collision cascade high density

SIMS is a very surface-sensitive technique because the emitted particles originate from the uppermost one or two monolayers. The dimensions of the collision cascade are rather small and the particles are emitted within an area of a few nanometers diameter. Hence, SIMS can be used for microanalysis with very high lateral resolution (50 nm to 1 pm), provided such finely focused primary ion beams can be formed. Furthermore, SIMS is destructive in nature because particles are removed from the surface. This can be used to erode the solid in a controlled manner to obtain information on the in-depth distribution of elements.109 This dynamic SIMS mode is widely applied to analyze thin films, layer structures, and dopant profiles. To receive chemical information on the original undamaged surface, the primary ion dose density must be kept low enough (<1013 cm-2) to prevent a surface area from being hit more than once. This so-called static SIMS mode is used widely for the characterization of molecular surfaces (see Figure 3.10). 

The explanation appears to lie in the accumulation of lattice vacancies at the nucleation sites of the helium bubbles. The average energy of a neutron in the fast reactor is above 100 keV, greatly in excess of the 25 eV or so which is required to displace an atom from a lattice site. Neutron collisions, followed by multiple cascade processes, therefore lead to a high density of vacancies and interstitials. The interstitials have a higher mobility than the vacancies and tend to be more rapidly absorbed at grain boundaries and dislocations, where they lose their identity. The surplus vacancies are then available for the formation of voids at the nucleation centers of the helium produced by the (n, a) reactions. 

When excited and bath species are identical, resonant V-V exchange causes very rapid vibrational deactivation with very little rotational or translational energy involvement [61]. This can lead to a curious quasi-equilibrium of the vibrational modes in which translation and rotation remain cold [61]. Overall equilibration in these circumstances can then take many collisions. When excited and bath molecules have very different vibrational constants, the existence of near-resonant V-V pathways depends on such factors as the magnitude of anharmonicity and initial vibrational state and is highly partner specific. The mechanism can lead to rapid population of intermediate vibrational states in both excited and bath molecules from which there may only be very slow VRT pathways for relaxation, whereas in other instances, successive near-resonant paths can lead to a population cascade down to the lowest level. In the example shown above, N2 has fewer near-resonant V-V pathways than O2 on collision with OH (8 3) and so despite being the lesser partner in number density, O2 is overall a more efficient relaxer of OH than the more abundant N2. 

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