Electron Thermalization in the Condensed Phase

The brief review of the newest results in the theory of elementary chemical processes in the condensed phase given in this chapter shows that great progress has been achieved in this field during recent years, concerning the description of both the interaction of electrons with the polar medium and with the intramolecular vibrations and the interaction of the intramolecular vibrations and other reactive modes with each other and with the dissipative subsystem (thermal bath). The rapid development of the theory of the adiabatic reactions of the transfer of heavy particles with due account of the fluctuational character of the motion of the medium in the framework of both dynamic and stochastic approaches should be mentioned. The stochastic approach is described only briefly in this chapter. The number of papers in this field is so great that their detailed review would require a separate article. 

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

Owing to the electron-vibrational interaction in molecules, there is one more possible decay channel for SES. This is the nonradiative relaxation (internal conversion), in which the electron energy is transferred into vibrational energy of molecules (in the condensed phase, into thermal energy of the medium). If the molecule fluoresces, there may also occur fluorescence from the lowest excited state. (According to the empirical rule of Kasha,64 the molecular fluorescence occurs from the lowest excitation level irrespective of the wavelength of the exciting radiation.)... 

The other approach to calibration, by comparison with experimental data, also presents difficulties. Here, the problem is in ensuring that the calculations relate correctly to the experiment, which, in this field, most frequently refers to finite-temperature free energy differences in the condensed phase. As well as obtaining accurate electronic energy differences, therefore, zero-point energy (zpe), thermal or enthalpic (H), entropic (S), and medium ef-... 

The treatment of excited species as new chemical entities, and hence, the use of thermodynamics in dealing with their redox reactions is justified because in the condensed phase, the electronic relaxation times are usually several orders of magnitude longer than the time for thermal equilibration in all other degrees of freedom of a polyatomic molecule49, 50). 

For this reason it equilibrates its temperature in a short time in the condensed phase. It deactivates without radiation to the lowest vibrational state of the actual electronic state (marked in Fig. 1.1 as te). By isoelectronic internal conversion (ic) it can pass over to a very high vibrational state of the next lower electronic energy state. These deactivations normally take place until the molecule has reached the vibrational ground state of the first excited electronic state S,. This overall deactivation process is called thermal relaxation (sd), which can be divided into the isoenergetic deactivation internal conversion and the non-isoenergetic thermal equilibration . Its time scale is 10 ° to 10 s. The process can be symbolised by... 

A particularly interesting singlet carbene is cyclopropenylidene, 32 (Figure 5.29), which may be the most abundant cyclic hydrocarbon in interstellar space. The structure is thought to be stabilized because the nonbonded pair of electrons is in an orbital with considerable s character, while the p orbital of the carbenic center is stabilized by electron donation from the adjacent olefinic system. Cyclopropenylidene itself is not stable in the condensed phase, but the thermally stable bis(diisopropylamino) derivative 33 was reported to be stable. ... 

Metals. In simple metals such as the alkali and alkaline earth metals as well as A1 the valence electrons occupy only s and p levels. The rather extended shape of the atomic wave functions leads to a strong overlap and delocalization in the condensed phase. As a result, one obtains almost free-electron-like behavior for the electrons near the Fermi level and high electrical conductivity. Similarly, in the case of Cu and Ag, the electronic states at the Fermi level are derived from very delocalized s and p states, which explains the excellent electrical conductivity of these metals. Since the actual electrical conductivity of a material is strongly influenced by the scattering of the conduction electrons by impurities, lattice defects, and the thermal motion of the atomic nuclei, the quantitative prediction of electrical conductivity is difficult. 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

For the formation of a metallic film in addition to thick film silk-screen technique, thin film metallization is another means for the film deposition. Deposition of thin film can be accomplished by either physical or chemical means, and thin film technology has been extensively used in the microelectronics industry. Physical means is basically a vapor deposition, and there are various methods to carry out physical vapor deposition. In general, the process involves the following 1) the planned deposited metal is physically converted into vapor phase and 2) the metallic vapor is transported at reduced pressure and condensed onto the surface of the substrate. Physical vapor deposition includes thermal evaporation, electronic beam assisted evaporation, ion-beam and plasma sputtering method, and others. The physical depositions follow the steps described above. In essence, the metal is converted into molecules in the vapor phase and then condensed onto the substrate. Consequently, the deposition is based on molecules and is uniform and very smooth. 

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