Threshold behaviour energy

Isotopic variations of H + H2, obtained by replacing hydrogens by deuteriums, were considered (Wu et al., 1973a) within a different computational scheme (Johnson, 1972) and, in connection with threshold behaviours and resonances for varying isotopic combinations. No significant reaction probabilities were found for total energies below the static barrier potential V s). One-dimensional barriers provided reasonable probabilities only... 

Walker and Wyatt (1972) have also performed a distorted-wave calculation for H + H2, based on the Porter-Karplus surface. They constructed reactant and product distortion potentials assuming adiabatic vibrational motion in each case, and obtained numerical solutions for the relative motions. Their results show that by choosing adequate potential parameters it is possible to reproduce the threshold behaviour, but that probabilities grow above unity soon after the threshold energy. 

Photoelectrons released from a surface by light impact have very low kinetic energy, and thus they only travel any appreciable distance in a high-vacuum environment. The photoelectric effect exhibits threshold behaviour, i.e. light below a certain frequency, which depends on the material of the cathode being exposed, will not result in the emission of electrons, regardless of how high the irradiance is. 

Resonance photoemission measurements have been recently made for U metal , and show indeed a resonant enhancement of the satelUte at 2.3 eV only for the threshold energy (5 A i2. hv = 94 eV) (Fig. 15). In addition the main peak at Ep shows the expected off-resonance behaviour. Further support for such an interpretation of the satellite is given by the analysis of the photon excited Auger emission. This is shown to be composed of two different bands also separated by 2.3 eV and due to the two screening channels by 5 f or 6 d states ... 

Fig. 2.1. Schematic illustration of the behaviour of the positron-helium and electron-helium total scattering cross sections. Notable are the large differences in magnitude of the cross sections at low energies, their merging at approximately 200 eV and the onset of inelastic processes at the positronium formation threshold EPS in the positron curve.

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