Crystal excitations

Observations of the decay time of luminescence from activator atoms in crystals excited by laser pulses have enabled lifetimes of... 

The probability per unit time for a single ion embedded in a crystal excited to state K to deexcite by spontaneous emission of electric-dipole radiation to a lower state M has been given by Axe (28) as... 

The theory of the model system for crystal excitation is analogous to molecular-orbital theory. The localized wave functions (5) and (6) can be written in alternative forms (7) and (8), which... 

At large distances compared with the size of the molecules, the dipolar transitions may be treated with point dipoles. This approximation is not valid for intermolecular distances met with in condensed phases. However, the point-dipole approximation allows one to discuss the various levels of interaction and proves very useful for the discussion of the general case, as illustrated in Section II. Historically, the point-dipole approximation was the first to be applied to molecular-crystal excitations.17-20... 

We must remark that the amplitude of these processes is generally weak compared to the direct exciton-photon amplitude, owing to the small libration amplitudes (of the order of 1 °) at low temperatures. It is still smaller when the incident light polarization is parallel to the molecular transition dipole. For instance, in anthracene-crystal excitation, we expect the exciton-photon-phonon contribution to be more important for the a than for the b polarization. On the contrary, these processes become much more important in nonresonant excitations, in Raman scattering for instance (cf. Section II.D). 

Figure 35 Relative fluorescence efficiency as a function of the quantal exciting intensity for a 200 pm-thick tetracene crystal excited with the 325 nm line of a He-Cd laser. The increasing segment shows the triplet—triplet fusion contribution to the fluorescence (delayed fluorescence) the decrease at high excitation levels is attributed to quenching of the singlets by singlet-triplet annihilation. Experimental data are represented by points, theoretical fits, as described in text, by the solid line. Adapted from Ref. 206.

The Raman spectrum of cobalt ferrite single crystals excited by 532 nm laser line, attained at room temperature, is reproduced in Fig. 14.5. By performing the fitting of the Raman spectra displayed in Fig. 14.5, it would be possible to observe Raman bands at 183, 304, 469, 573, 627, and 696, listed in Table 14.2. Apart from Fc304, y-Fe203, and CoFc204, others ferrites such as NiFc204 and Fc3 . Zn - O4 which... 

Fig. 20.30 Rock sample image recorded with a digital camera. In the red circle, the region wet with Ag colloid is highlighted. The micro-SERS spectrum is compared with the normal Raman spectrum of a pyroxene single crystal. Exciting line 785 nm...

Figure 9. Oscillations of phonon-polariton mode in lithium tantalate crystal excited through inverse electro-optic effect and impulsive stimulated polariton scattering. Time-dependent birefringence measured with probe pulse, which propagated parallel to but not collinear with excitation pulse. (Reprinted with permission from ref. 36.)...

Ryabchuk V.K. and Burukina G. V. (1991), Photophysical processes of ionic crystals excitation during photosorption and photocatalysis , Z. Fiziches. Khim. 65, 1621-1633. 

A thin disk of the reactant crystal, having a planar upper face, is mounted on a thermostat temperature control may be improved by contact through a metal eutectic. About 20 mm above the reactant surface is positioned a quartz crystal held at 78 K throughout the experiment, with the intervening gap maintained at a very low pressure (below 4 x 10 5 Pa) by continual pumping. The resonance frequency of the quartz crystal, excited through... 

The project Carl gave me was to build a sensitive instrument to search for luminescence from the permanganate ion, which had been the subject of a series of experimental single crystal absorption spectral studies and theoretical studies in the laboratory [6]. The spectrometer was built, but after repeated attempts using a range of crystals, excitation conditions and temperatures, no luminescence was detected. All subsequent efforts by others have confirmed this failure [7], under laser irradiation in iodide lattices some emission has been detected, but this is derived from the manganese ion MnO, 2 produced by a photoredox process [8]. This left me without many results to show for my year s work. I made some measurements on the intensely luminescent alkali metal platinocyanides but this did not lead to any new insights. 

We now consider the lowest crystal excited state, when the molecules remain neutral. In some cases by computing the crystal energy and the properties of corresponding excitonic states it is enough to consider only one molecular excited state. In such cases the summation in (3.16) reduces to the terms where / and g are equal 0 or /. 

Molecular eigenfunctions (f° and tpf can always be chosen as real. In this case the matrix elements in the operator (3.16) are real and symmetric. We first consider such crystal excited states, which correspond to the excitation of only one crystal molecule. According to this assumptions the operator (3.16) must... 

In the previous section we have obtained the crystal excitation energy in the Heitler-London approximation, i.e. neglecting the term H3 in the crystal energy operator (3.19). We now wish to determine the crystal energy without using this simplification. 

If we determine the crystal excitation energy referred to the renormalized vacuum state 0), i.e. the state without excitons, then the excitation energy operator (3.58) takes the form... 

The wavefunctions corresponding to crystal excited states have the form... 

In particular, Rashba (23) has considered such crystal excited states, where the excitation is not centered at one molecule, as was assumed in (3.162), but is smeared out about a certain finite crystal region. In this case, when the time of the resonant excitation transfer from one molecule to another is small compared to the time needed by the molecule to achieve a new equilibrium position21 a local deformation can occur within some excitation region similar behavior is observed in the case of polarons (20). The shape of the deformation is consistent with the shape of the excitation distribution inside the same crystal region. If, in particular, the resonant interaction tends to zero, the states, obtained in Ref. (23), are identical with that given by formula (3.162). 

In previous chapters we considered elementary crystal excitation taking into account only the Coulomb interaction between carriers. From the point of view of quantum electrodynamics (see, for example, (1)) such an interaction is conditioned by an exchange of virtual scalar and longitudinal photons, so that the potential energy, corresponding to this interaction, depends on the carrier positions and not on their velocity distribution. As is well-known, the exchange of virtual transverse photons leads to the so-called retarded interaction between charges. 

The energy needed for transferring valence electrons to the conduction band can be supplied by nuclear radiation. The average energy needed to produce an electron-hole pair in silicon at room temperature is not 1.115 eV but 3.62 eV because some ergy is lost as crystal excitation (3.72 eV in Si and 2.95 eV in Ge at 80 K). The electron removed from the valence band leaves a vacancy or "hole". The ionization is said to give an electron-hole pair. Just as the electrons move towards the anode, the holes move towards the anode. 

Fu L, Xia HP, Dong YM, Li SS, Gu XM, Jiang HC, Chen BJ (2014) White light emission from Tb YSm codoped IiYF4 single crystal excited by UV light. IEEE Photonics Technol Lett 26 1485... 

According to the hand theory for elements, the valence electrons are found in a valence hand. If this is only partly filled with electrons, there are occupied and unoccupied levels (with small energy differences) within the valence band. A small electrical po-tenhal difference applied across the crystal excites electrons from the highest (regarding energy) filled levels to unfilled levels immediately above. The unfilled valence band is also a conduction hand, according to Figure 40.11a. This explains the ability of metals to conduct electricity. 

Vacek K (1971) Some remarks on photoluminescence of AgCl crystals excited by laser or after deformation. Czech J Phys B2L303-308... 

PTS single crystals show below the excitations of the polymer backbone weak absorption which is probably related to defects (7). Different from absorption tails in other PDA crystals excitation of PTS in this range contributes to photoconductivity (8) and gives rise to an electroabsorption signal, whose evolution during the polymerization process has been studied. 

The IR beam is focused onto the single crystal at a grazing angle and, during the reflection, of the />-component of the IR light on the single crystal excites those vibrations of the adsorbed molecule for which the component of the dipole moment perpendicular to the surface changes (this is an additional surface selection rule for IRRAS).29... 

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