Resonance processes excitation

The physical significance of Eq. (53) is clear. At an isolated resonance the excitation and dissociation processes decouple, all memory of the two excitation pathways is lost by the time the molecule falls apart, and the associated phase vanishes. The structure described by Eq. (53) was observed in the channel phase for the dissociation of HI in the vicinity of the (isolated) 5sg resonance. The simplest model depicting this class of problems is shown schematically in Fig. 5d, corresponding to an isolated predissociation resonance. Figures 5e and 5f extend the sketches of Figs. 5c and 5d, respectively, to account qualitatively for overlapping resonances. 

First, the solvent evaporates, leaving behind formula units of the formerly dissolved salt. Next, dissociation of the formula units of salt into atoms occurs—the metal ions atomize, or are transformed into atoms. Then, if the atoms are easily raised to excited states by the thermal energy of the flame, a resonance process occurs in which the atoms resonate back and forth between the ground state and the excited states. 

The most simple, but general, model to describe the interaction of optical radiation with solids is a classical model, due to Lorentz, in which it is assumed that the valence electrons are bound to specific atoms in the solid by harmonic forces. These harmonic forces are the Coulomb forces that tend to restore the valence electrons into specific orbits around the atomic nuclei. Therefore, the solid is considered as a collection of atomic oscillators, each one with its characteristic natural frequency. We presume that if we excite one of these atomic oscillators with its natural frequency (the resonance frequency), a resonant process will be produced. From the quantum viewpoint, these frequencies correspond to those needed to produce valence band to conduction band transitions. In the first approach we consider only a unique resonant frequency, >o in other words, the solid consists of a collection of equivalent atomic oscillators. In this approach, coq would correspond to the gap frequency. 

There are three possible mechanisms whereby an excited atom or ion can undergo an electronic transition near a metal surface (1) de-excitation involving the emission of radiation, (2) de-excitation involving a two electron Auger process, and (3) a resonance process whereby an electron is transferred from the metal to an equivalent energy level in the ion or a similar transition where the electron goes from the ion to the metal. However, Schekhter has shown that the probability... 

Type of process Resonant transition Excitation wavelength (nm)... 

Since the backup ions other than aluminum are of a size similar to the terbium ion, it is reasonable to assume that the structure of the glass matrix over the whole series is the same. Therefore, the concentration dependence of the lifetime is unequivocally due to terbium ions being packed closer and closer together. Pearson and Peterson postulate that, as the ions are situated closer and closer together, the quenching mechanism of Dexter and Schulman (45) becomes operative. That is, the excitation jumps from ion to ion by a resonance process until it reaches a sink. 

Near the resonances the cross section of vibrational excitation increases by several orders of magnitude. From the physical standpoint we can divide each resonance process into three stages. At the first stage the incident electron is captured by the electron shell of the molecule, forming an intermediate negative molecular ion. The second stage is the vibrational motion of the nuclei of the newly formed ion, which eventually leads to the third stage of the process—the decay of the intermediate ion. 

As resonant processes are avoided in the nonlinear optical experiments (except where two-photon absorption is optimized), no excited state molecules are created and triplet states are of no concern. For a more thorough investigation of the absorption process also the vibrations of the molecule have to be included. They are not considered here. 

Figure 6.1-24 Resonance CARS excitation of the u (= 850 cm ) vibration of the permanganate ion doped in a KCIO4 single crystal. The absorption spectrum at T = 15 K shows the vibronic structure of the Mn04-ion. CARS excitation with tar. = 18 050 cm and los = 17200 cm" gives rise to a twofold resonance since the intermediate states of the CARS process coincide with the sharp electronic transitions. The CARS excitation profile peaks particularly for this double resonance (see linear scale). Further resonances can be seen in the CARS excitation profile plotted on a logio scale (Adapted from Leuchs and Kiefer, 1993a, b).

In addition, CNTs exhibit several Raman features whose frequencies change with changing excitation wavelength. A prominent example for this unusual behavior is the disorder-induced D band which results from a defect-induced double-resonant process [46]. In the molecular picture, the D band originates from the breathing vibrations of aromatic rings in the honeycomb lattice. A quantitative description of the D band intensity in graphene was recently derived by Sato et al. 

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