Quasi-molecule

The technique of INS is probably the least used of those described here, because of experimental difficulties, but it is also one of the physically most interesting. Ions of He" of a chosen low energy in the range 5-10 eV approach a metal surface and within an interaction distance of a fraction of a nanometer form ion-atom pairs with the nearest surface atoms. The excited quasi molecule so formed can de-excite by Auger neutralization. If unfilled levels in the ion fall outside the range of filled levels of the solid, as for He", an Auger process can occur in which an electron from the va-... 

It is useful to view optical absorption and emission processes in such a system in terms of transitions between distinct vibrational levels of the ground and excited electronic states of a metal atom-rare gas complex or quasi-molecule. Since the vibrational motions of the complex are coupled with the bulk lattice vibrations, a complicated pattern of closely spaced vibrational levels is involved and this results in the appearance of a smooth, structureless absorption profile (25). Thus the homogeneous width of the absorption band arises from a coupling between the electronic states of the metal atom and the host lattice vibrations, which is induced by the differences between the guest-host... 

Figure 2c corresponds to a strong donor bond of a Na atom which is formed from a weak bond (see Fig. 2a) as the result of ionization of the adsorbed Na atom, i.e., as the result of its valence electron going over into the free state (the free electron population in the crystal is thus increased by one electron), or, what amounts to the same thing, as the result of the capture of a free hole by an adsorbed Na atom (4 S). In this case, the adsorption bond is of a purely ionic nature we obtain a quasi-molecule NaCl. 

Figures 2c and 2b depict, respectively, a strong donor and a strong acceptor bond of a Cl atom formed from a weak bond (see Fig. 2a ), when a free hole or, respectively, a free electron, is drawn into the bond. In the first case (Fig. 2c ) we have a quasi-molecule CI2 with typical homopolar bond, in the second case (Fig. 2b ) a quasi-molecule NaCl with its characteristic ionic bond.

The ionizing reaction occudrs over a certain range of intermolecular distance R, where He(2 S) and M can be regarded as intermediate quasi-molecules. 

According to the theoretical investigations by Nakamura [126-128] and Miller [129], the collisional energy dependence of these processes can be calculated if the interaction potential V(R) for the system He(2 S)-M and the autoionization rate r K)jh from the intermediate quasi-molecule [He(2 S)M] to the resulting quasi-molecular ion [HeM] are known. For example, by the classical formula of Miller [129], we have theoretical cross sections for Penning ionization as ... 

As it is shown in Fig. 3.1(b) in Chapter 3, typical hole centres in alkali halides - H and centres being XJ - quasi-molecules oriented along the (110) axis are rather anisotropic which is observed experimentally, e.g., via polarized recombination luminescence [70, 71]. Their analog is a dumb-bell interstitial in many metals. 

For an anisotropic defect, like crowdions or di-atomic quasi-molecules (H and Vic centres), the problem becomes much more complicated and often permits only a numerical solution (e.g., [72]). For example, an estimate of the interaction energy of a crowdion with a vacancy in Cu in the direction perpendicular to the crowdion axis is 0.1 eV at the relative distance /2ao (ao is a lattice constant) if both are in the same plane, but this energy becomes 0.02 eV only for a distance twice as large (ip = 0 in Fig. 4.8(a)). Increase of the angle

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Jerome Faist, Federico Capasso, Carlo Sirtori, Deborah L. Sivco, and Alfred Y. Cho, Quantum Cascade Lasers Federico Capasso, Carlo Sirtori, D. L. Sivco, and A. Y. Cho, Nonlinear Optics in Coupled-Quantum- Well Quasi-Molecules... 

When our present group became interested in quasi-molecules, which are generated in atomic collision systems, we started to use the relativistic DV Xa-method. With the help of Arne Rosen we were the first to calculate correlation diagrams of relatively heavy systems. ... 

For azomethane at a reaction temperature of T = 576 °K Forst found = 10 and AS = 17.4 eu. This suggests that the activated molecule is much closer to the final state than the initial state. Arguments based on thermochemical data (vide infra) indicate that bond fission in azomethane cannot lead to a radical which contains a divalent nitrogen the activated complex must therefore be a kind of a quasi-molecule composed of two methyls and a nearly zero-valent Nj molecule, or an ionic structure as suggested by Benson. 

In moderate density systems we can terminate the series (2.3) after a finite number of terms. For example, if we take only the first two terms into account, we obtain an expression for the inelastic cross section of two-atom quasi-molecules. The regular part is equal to... 

The quasi-molecule complexes consist of two atoms of the same element, one of which is in an excited state. The electronic states are divided into two groups, even (g) and odd (u), in accordance with the property of wavefunctions. Even states conserve sign under inversion in the plane of symmetry odd states change sign. In Eqn. (2.4) a may be equal to g or u. Using zero-order perturbation theory and neglecting overlap interactions, the wavefunctions of the ground state Fo( r, R) and the excited states Pi,j( r, R) may be written ... 

TABLE 7. The ion-atom exchange interaction potential for the quasi-molecule S- /2) — 0 Pj) when its quantum numbers arc J and M (the total atom moment and the projection of the atom orbital momentum onto the molecular axis). The partial cross sections of resonant charge exchange at indicated quantum numbers and collision energies in the laboratory frame of reference arc expressed in. ... 

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

D. Kolb. Solution of the one-dectron Dirac equation for the heavy diatomic quasi-molecule NiPb(109+) by the finite dement method. Chem. Phys. Lett., 229 (1994) 667-670. 

The MPL spectrum exhibits two broad peaks, one at 467 nm (2.64 eV) and the other one at 422 nm (2.94 eV). These peaks appear to be related also to defects in the Si02 structure. Several such defect models have been discussed in the literature (47,48). The emission at 2.65 eV has been assigned to a new intrinsic defect in amorphous Si02 for which a two-fold coordinated Si is proposed, i.e., a Si(II) (neutral) center (48). Chemically, this is equivalent to the quasi-molecule Si02 with a 1Ai ground state, first excited singlet state, and a Bi triplet state. The... 

In contrast to this, the potential curves of an oxygen-argon quasi-molecule show a crossing which is responsible for the high probability of the 0( D) quenching. 

The stabilization of a quasi-molecule can occur in two ways by radiation of light (radiative stabilization) and as a result of collision (collisional stabilization). 

Radiative recombination is a very slow process. It has been deduced from experimental data that the probability P,. of a bromine-atom radiative recombination per collision is of the order of 10" [235]. However, despite this value, radiative recombination is manifested in many low-density systems and sometimes plays the crucial role in chemical transformations occurring in such systems. The continuous luminescence spectra observed for various flames are undoubtedly due mostly to recombination processes, i.e. to radiative stabilization of quasi-molecules formed by collisions of atoms and radicals with each other or with molecules present in the combustion zone (see e.g. [153, 359]). Radiative recombination determines the rate of the formation of molecules in interstellar clouds [102, 514] and the spectra of gas discharge in molecular gases [89, 472]. 

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