Inter-valley scattering

The second-order Raman spectra from 1,700 to 3,300 cm are relatively weak and broad compared to the first order Raman spectra. There are many papers devoted to the second-order Raman processes in graphite [49, 59-62] and carbon nanotubes [8, 34, 63]. The interpretation of the spectral behavior of the second-order bands in terms of DR have been done [3, 64, 65]. The corresponding phonon is involved in an inter-valley scattering process of in-plane modes (LO or TO) around the K-points in the two-dimensional (2D) Brillouine zone (BZ, Fig. 7.4) [66, 67]. 

In conclusion, the D and 2D peak are activated by an inter-valley resonant Raman scattering process. Consequently, these peaks are strongly sensitive to changes in the electronic structure, i.e. the phonon probed by Raman spectroscopy strongly depends on the electronic band structure. Thus, every change in the electronic structure will be reflected in a change of the D and 2D peaks. This effect allows identification of graphene. 

Fig. 10 (Colour online) a) Armchair and zigzag edges. The wavevector direction of electrons scattered by a zigzag, or armchair edge (q, is perpendicular to the edge direction, b) Schematic of the inter-valley process only the exchanged momentum from armchair edges qa can connect K and K. Adapted from Ref. 64.

The Andes reach their widest point around 17°S in Bolivia and occupy an extensive area in the southwest of the country. Much of this consists of a high plateau, or altiplano, lying at around 3700 m, most of which is extremely arid and consists of undulating plain separated by relatively low mountain ranges and covered in puna-type vegetation interspersed with lakes, salt flats and scattered areas of cultivation. By contrast, the eastern slopes of the Andes are still covered in extensive areas of moist forest of various types and enjoy lengthy cloud cover as well as relatively high levels of rainfall. Between these two areas lie the dry inter-Andean valleys with their distinctive flora. 

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