Excited chemical bonds

Observation of the ion angular distribution after electron stimulated desorption of chemisorbed species (ESDIAD) can provide direct quantitative information on the orientation of adsorbed molecules on surfaces. Electrons incident on the surface can excite chemical bonds into non-bonding states, causing molecular decomposition. The excess energy can be converted into kinetic energy, which accelerates an ionic fragment of the molecule along the axis... 

Cluster systems composed of several rare-gas atoms weakly bound to a molecule acting as a chromophore have attracted much attention in the last recent years [1-6], A gradual increase of the cluster size makes it possible to bridge the gas-phase limit and the condensed-matter regime. This allows one to investigate fundamental questions such as the mechanisms of energy tranfer from the electronically or vibrationally excited chemical bond (or bonds) to the solvent the influence of the weak solvation interactions on the molecule dissociation and the effect of caging and recombination of the chemical impurity induced by the solvent [6]. 

In this paper, general principles of physical kinetics are used for the descnption of creep, relaxation of stress and Young s modulus, and fracture of a special group of polymers The rates of change of the mechanical properties as a function of temperature and time, for stressed or strained highly oriented polymers, is described by Arrhenius type equations The kinetics of the above-mentioned processes is found to be determined hy the probability of formation of excited chemical bonds in macromolecules. The statistics of certain modes of the fundamental vibrations of macromolecules influence the kinetics of their formation decisively If the quantum statistics of fundamental vibrations is taken into account, an Arrhenius type equation adequately describes the changes in the kinetics of deformation and fracture over a wide temperature range. Relaxation transitions m the polymers studied are explained by the substitution of classical statistics by quantum statistics of the fundamental vibrations. 

Kinetics of Generation and Evolution of Excited Chemical Bonds in Macromolecnles as a Bacl onnd for the Kinetics... 

Correlation between Kinetics of Generation of Excited Chemical Bonds and Kinetics of Deformation. . . ... 

The regions embracing the excited chemical bonds are certain specific formations, the so called, dilatons [30, 32, 33, 39]. In Sect. 4.3, the kinetics of their generation and evolution will be described on the basis of spectroscopic investigations. 

Excited Chemical Bonds in Macromolecules Their Observation and Investigation... 

In recent years, excited chemical bonds have been studied experimentally by IR and Raman spectroscopy. To observe the excited chemical bonds, Vettegren [30,32,33, 39], Vettegren et al. [24-29,31, 34-38], and others [56-58] analyzed the shape of the regularity bands in spectra of different polymers. These bands correspond to the vibrauons of sequence of chemical bonds in the macromolecule, forming a helix or a planar zigzag. Deformation of valence angles and bonds causes a frequency shift, Av. The latter is related to the deformation of a hehx by the equation [39] ... 

Table 2. Some characteristics of the regularity bands and deformation of the excited chemical bonds in macTomolecules of non-stressed polymers at 300 K...

In accordance with Eq. 11, the average extension of the excited chemical bonds, is evaluated from the measured frequency shift of the satellite peak, Avd, by ... 

The relative concentration of the extended excited chemical bonds is determined as a ratio of the intensity of the low-frequency satellite, 1, and that of the main band, Iq... 

The spectroscopic methods permit to evaluate the vibrational energy in the regions containing the excited chemical bonds. 

Figure 8 shows the evolution in time of ea and Ca in strained PP. It is clear that the relaxation of stress (and of Young s modulus) is accompanied by an increase in both the deformation and concentration of the excited chemical bonds. 

Fig. 8. The increase in concentration, Aca (A), and deformation, Ca(0), of the excited chemical bonds versus time, t, as well as stress relaxation (. ), for PP at 300 K and Eg = 2.5%...

The experiments carried out for the non-stressed polyimide fibers (at a = 0) shows that Eq. (18) is valid too. Therefore, excited chemical bonds are generated in the absence of stress. 

As can be seen from Table 1, the activation energy for the generation of excited chemical bonds Uoa, coincides with the activation energy Uq for any macroscopic deformation process, Uod = Uo- This equality is the principal conclusion. 

To correlate the kinetics of macroscopical processes (fracture and relaxation of Young s modulus) with the kinetics of microscopical processes (extension of the excited chemical bonds), let us substitute Eq. (19) into Eqs. (5) and (9). If the equalities Uop = Uod and Xop = Xod are supposed to be valid, then Eqs. (5) and (9) become ... 

We also wish to postulate that the rates of both deformation and fracture are caused by the rate of excited-bond generation rather than the rate of their dissociation. Therefore, the activation energy, Uq, characterizes the generation of the excited chemical bonds rather than their relaxation. 

In Sect. 3, the kinetics of deformation, relaxation, and fracture was investigated only between two specific temperatures of the main relaxation transitions. In Sect. 4, the kinetics of these processes, described by Eqs. (3), (5), (7), and (8) was shown to be strongly related to the generation of excited chemical bonds, caused in turn by energy fluctuations. Nevertheless, the experimental results in a wide temperature range show that, in general, Eq. (1) is not obeyed. 

Research has been carried out in Britain on the use of tunable CO2 lasers to excite chemical bonds in the production of chemical compounds and in the U.S.A. lasers are being used for the separation of isotopes of materials such as Uranium. 

The complex is described by A-B -C where A-B is a vibrationally excited chemical bonded molecule attached by a van der Waals bond to atom (or molecule) C. Energy from the excited chemical bond is transferred to the... 

We imagine that the chemical bond within the complex has become vibrationally excited through collisions or absorption of light to produce A-H B. (The asterisck on A-H usually means that it is in its u = l level.) After a time t, energy from the vibrationally excited chemical bond spills over into the weak van der Waals bond and causes its rupture. The fragments A-H (now in its u-0 state) and B then fly apart with relative translational energy AE. 

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