Crystal surface excitation spectra

Figure 1 Schematic of eiectron energy-loss scattering process for electrons of energy striking a Rh single-crystal surface with adsorbed CO molecules present. The actual energy-loss spectrum, due to excitation of CO vibrations, is shown also.

Fig. 29. Comparison between the excitation spectrum of the sensitized current J and of the absorption coefficient of a 10-3 M rhodamine solution containing also 10-2 Fe(CN) at pH — 7. The absorption A is that of the dye molecules when the same solution is dried up at the crystal surface...

For large negative crystal field at low temperatures the stable stmcture is the ideal two-sublattice in-plane herringbone phase—that is, the 2-in phase in Fig. 4a. The two sublattices which can be oriented in three different ways relative to the triangular lattice lead to six equivalent ground states. The excitation spectrum of this phase in general has a gap. In this phase the molecular wave functions are localized in the substrate plane, and classically the molecular axes are parallel to the surface see Appendix A of Ref. 141 for an interpretation of the order parameters. Thus, the orientational degeneracy of the pararotational phase is broken by the quadrupolar interactions. A closely related structure was already proposed based on atomistic Lennard-... 

As in the case of metals and semi-conductors, there exist specific surface excitations in insulating oxides. Three types of surface phonon modes may be distinguished the Rayleigh mode, the Fuchs and Kliewer modes and the microscopic surface modes. The first two modes have a long penetration length into the crystal. They are located below the bulk acoustic branches and in the optical modes, respectively. The latter are generally found in the gap of the bulk phonon spectrum. 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

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