Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Bulk exciton

The remaining results in Fig. 9 demonstrate that when a small amount of molecular oxygen is mixed in the Ar layer condensed on -hexane [Fig. 9(e)] or deposited onto an isolated Ar layer [Fig. 9(d)], the P hi resonance reappears in the Ar desorption yield function. Since the -hexane spacer inhibits Ar decay by electron transfer to Pt(l 11), the presence of Ar resonance in Fig. 9(c) and (d) was therefore interpreted [164] as due to electron transfer to O2 leading to the formation of O2 in its ground-state Og. With the electron affinity of O2 being of the order of the binding energy of the first electronically excited state of Ar, the decay of Ar P into lowest bulk excitons is possible by electron transfer to O2. [Pg.224]

Yanes et al. (2004) observed a very interesting size selective spectroscopy in 0.4 mol% Eu3+ SnC>2 nanocrystals ( 4 nm) embedded in SiC>2 glass prepared by thermal treatment of sol-gel glasses. The mean size of SnC>2 nanocrystals is comparable to the bulk exciton Bohr radius (4.8 nm). Thus the band-gap excitation energy depends on the nanocrystal size. [Pg.148]

II. COUPLING OF THE BULK EXCITONS TO THE MOLECULAR VIBRATIONS AND TO THE LATTICE PHONONS... [Pg.33]

One of the most obvious perturbations of the surface excitons by the bulk crystal is the short-range coulombic interactions causing surface-exciton transfer to the bulk. We discard these perturbations on the grounds both of theoretical calculations27 and of the experimental observations, which show the presence of a surface exciton (the second subsurface exciton S3 see Fig. 3.2) resolved at about 2cm-1 above the bulk-exciton resonance. [Pg.141]

When increasing temperature (T > 2 K), so introducing thermal disorder, the structure broadens rapidly. For each bulk reflectivity spectrum at a given temperature T, determining the radiative surface width by (3.26), we looked for the value of rle(T) allowing the best reproduction of the experimental spectrum. The various values obtained for re(T) vs T are plotted in Fig. (3.13) and compared with the width of the bulk exciton obtained by KK analysis (in Section II.C.3).70127... [Pg.145]

Figure 3.13. Temperature evolution of the parameter re T) of the first-surface exciton (hollow circles). The full circles indicate the values obtained for the bulk exciton (Fig. 2.14). The quasi-coincidence at T 0 is fortuitous, but the surface states appear less broadened than the bulk states in the region 30-50 K, which could correspond to a decay at the surface of the density of interplane phonons coupled to the exciton (see Section III.B). Figure 3.13. Temperature evolution of the parameter re T) of the first-surface exciton (hollow circles). The full circles indicate the values obtained for the bulk exciton (Fig. 2.14). The quasi-coincidence at T 0 is fortuitous, but the surface states appear less broadened than the bulk states in the region 30-50 K, which could correspond to a decay at the surface of the density of interplane phonons coupled to the exciton (see Section III.B).
From a comparison of the optical absorption and excitation data for the oxides (Table V), it is clear that the energy decreases with increasing cation size along the series Mg to Ba. The bulk exciton transitions of these oxides also decrease in a similar manner (Table VI). It is possible to make a semi-quantitative calculation of the intrinsic surface energy states using the approach of Levine and Mark (151) where the ions in an ideal surface are considered to be equivalent to bulk ions except for their reduced Madelung... [Pg.116]

Ignored in the above quantum confinement model is the fact that when an electron is excited into the conduction band and a positive hole is left behind in the valence band, the hole and electron are coulombically attracted. The pair can be treated as a well-defined qnasiparticle called an exciton, a hydrogen-like system for which a bulk exciton Bohr radius = tfKeJ nfj.f- can be defined. Here k is the dielectric constant (10.2 for bulk CdSe ) and bq is the permittivity of space (8.854 X 10 The quantity a is a... [Pg.495]

Surface excitons require less energy in their formahon than bulk excitons, owing to the reduced Madelung constant of the coordinahvely unsaturated ions at the surface [81]. [Pg.82]

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. [Pg.52]

After these remarks let us return again to the first monolayer of the anthracene crystal. Note, first of all, that the width of the exciton band in this crystal for wavevectors directed along the C -axis (i.e., along the normal to the (a, b) plane) is very small ( 5 cm-1) in comparison with bandwidth of excitons with wavevectors parallel to the (a, b) plane and it is very small in comparison with the blue-shift of the exciton level in the outermost monolayer. For this reason, first of all, the mobility of excitons located in monolayers in the direction towards the surface is rather small. On the other hand, as the energy of he exciton located at the outermost monolayer is larger than the energy of the exciton in the bulk the interaction of bulk excitons with the surface is repulsive. We have here a type of dead layer for the bulk exciton. Qualitatively this picture helps explain why the experimental observation of photoluminescence of excitons located at the outermost monolayer of the anthracene crystal was possible. [Pg.247]

Since the irreducible representations of the group of translations are onedimensional and are determined by the given wavevector, among the set of quantum numbers characterizing a surface exciton there is always a quasicontinuous quantum number - the wavevector. This vector differs from the corresponding one in the case of bulk excitons it may be directed only along the crystal surface and assumes values only within a two-dimensional Brillouin zone. In... [Pg.327]

If follows that with a reasonable accuracy we may ignore the terms containing the factors Cp(nz 1) in (12.25). In this approximation the exciton states, both bulk and surface, are formed as a result of the interaction of molecules localized in the same plane n3 = const. The latter result shows that even bulk excitons in anthracene are actually quasi-two-dimensional in the spectral region treated. [Pg.339]

Since T = T22 and T12 = T21, we find that the energy of a bulk exciton is given by... [Pg.339]

FlG. 12.5. The bulk exciton and the site shift surface exciton levels in anthracene-type... [Pg.340]


See other pages where Bulk exciton is mentioned: [Pg.2908]    [Pg.433]    [Pg.217]    [Pg.217]    [Pg.217]    [Pg.574]    [Pg.230]    [Pg.330]    [Pg.34]    [Pg.143]    [Pg.146]    [Pg.152]    [Pg.153]    [Pg.160]    [Pg.161]    [Pg.174]    [Pg.178]    [Pg.118]    [Pg.62]    [Pg.65]    [Pg.162]    [Pg.327]    [Pg.328]    [Pg.328]    [Pg.329]    [Pg.331]    [Pg.339]    [Pg.340]    [Pg.340]    [Pg.341]    [Pg.342]    [Pg.358]    [Pg.362]    [Pg.376]   
See also in sourсe #XX -- [ Pg.71 ]

See also in sourсe #XX -- [ Pg.1267 ]




SEARCH



Exciton

Exciton/excitonic

Excitons

© 2024 chempedia.info