Effect bond elasticity

The enthalpy change related to associative process (A//ab) is due essentially to coulombic interactions and, subordinately, to polarization, repulsion, covalent bonding, elastic interactions, and vibrational effects. The latter two causes are generally negligible and may have some effects only at low T. 

We do not discuss elastic scattering here because it is not particularly interesting to most chemists, and it has been reviewed recently [l]. Comparisons of electron and positron scattering is treated only briefly here because it is the subject of a recent comprehensive review [2]. We limit the present discussion to topics of most interest to chemists. These inevitably involve molecular (not atomic) targets, and are concerned in particular with electronic (i.e., orbital) structure, vibrational effects, bond breaking, and the formation of compounds that contain positrons. 

In conventional physical vapor deposition, the material to be deposited is emitted in atomic or molecular form from a heated source and is allowed to impinge on a solid substrate. The process is carried out in a vacuum chamber in order to avoid contamination of the deposit and also to avoid the possibility that the emitted vapor may condense in the gas phase. The rate of evaporation from the source is controlled via the source temperature which is normally much higher than the substrate temperature. Atoms or molecules impinging on the substrate may be more or less elastically reflected back into the gas phase or, more commonly, they stick to the substrate where they are held by attractive forces. If the energy of attraction is very high relative to the thermal energy of the substrate, the deposited molecule is effectively bonded or chemisorbed to the substrate. 

A deformation due to bond bending and stretching which is effectively instantaneous and independent of temperature (ordinary elastic deformation, oe)-... 

In 65% sugar jellies the hydrogen bridges alone are active. Such gels are easily deformed and are elastic. In calcium pectinate gels, the most effective cross links are calcium ion bonds between the carboxyl groups. The bond distances are short therefore an inelastic, rather brittle, gel results. 

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