Molecular dissociation, effect

Most of the energy associated with an incident x-ray or y-ray is absorbed by ejected electrons. These secondary electrons are ejected with sufficient energy to cause further ionizations or excitations. The consequences of excitations may not represent permanent change, as the molecule may just return to the ground state by emission or may dissipate the excess energy by radiationless decay. In the gas phase, excitations often lead to molecular dissociations. In condensed matter, new relaxation pathways combined with the cage effect greatly curtail permanent dissociation. Specifically in DNA, it is known that the quantum yields for fluorescence are very small and relaxation is very fast [6]. For these reasons, the present emphasis will be on the effects of ionizations. 

The effect of exciplex dissociation (process MC) on the over-all kinetics of molecular fluorescence decay has been examined by Ware and Richter34 for the system perylene-dimethylaniline in solvents with dielectric constants (e) varying from 2.3 to 37. In low dielectric media (e = 2.3-4) the perylene fluorescence response may be fitted to a two-component exponential curve and exciplex emission is also observed, whereas in more polar solvents (e > 12) exciplex fluorescence is absent (at ambient temperatures) and the molecular fluorescence decays exponentially. These observations are consistent with both an increase in exciplex stability toward molecular dissociation with solvent polarity (Eq. 13) and the increased probability of dissociation into solvated ions... 

The importance of lattice coupling in direct molecular dissociation is at present poorly understood. However, there are at least two ways in which inclusion of the lattice can affect direct dissociative adsorption. First, conversion of Et to Eq competes with translational activation in dissociation. Second, thermal distortion of lattice atoms from their equilibrium positions may affect the PES, e.g., the barriers to dissociation V ( ). These two effects can be most simply thought of as a phonon induced modulation of the barrier along the translational coordinate and in amplitude, respectively. 

The monochloride exhibits a slight tendency to molecular dissociation, but the effect is so small that both in the dissolved and in the gaseous condition the molecular weight agrees with the formula S2C12. As it does not combine with bromine, ebullioscopic determinations of... 

Bass, L. Moore, W. J. A model of nervous excitation based on the Wien dissociation effect. In "Structural Chemistry and Molecular Biology" A. Rich C. M. Davidson, Eds. Freeman ... 

The review covers in systematic form the literature data on the thermal decomposition of aliphatic nitrocompounds amassed over the past 25 years. Molecular structure effects on the rate and mechanism of gas phase reactions, transition state structures of bond dissociation and HNO2 elimination, the main features of decomposition in condensed phase, the data on C—N bond energy and its dependence from electronic, steric and conformational effects are considered. 

Results of molecular dynamics simulations of the short time behaviour of CO after the dissociation effect are presented by Straub and Karplus [106]... 

The anomalous photophysical properties of sulphur compounds continue to attract interest. Quenching studies of the luminescent S2 state of a number of aromatic thioketones show a range of lifetimes between 10 and 10 s. A variety of solvents shows the role of transient interactive effects and environmental viscosity. In the case of the S2 state of thiophosgene in perfluoro-solvents, molecular dissociation is found to be the most... 

In the following sections, we first consider molecules for which direct double photoionization seems to dominate, then we present clear examples of molecular Auger effects, and finally we discuss the dissociative double ionization mechanisms that have no counterpart in atoms. 

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