The enhanced Raman scattering of phonons in CaF<sub>2</sub> and MgO samples containing Ca and Li colloids
M L Sanjuan; P B Oliete; V M Orera; M L Sanjuan; Inst. de Ciencia de Mater. de Aragon, Zaragoza Univ., Spain; P B Oliete; Inst. de Ciencia de Mater. de Aragon, Zaragoza Univ., Spain; V M Orera; Inst. de Ciencia de Mater. de Aragon, Zaragoza Univ., Spain
Журнал:
Journal of Physics: Condensed Matter
Дата:
1994-11-07
Аннотация:
We present Raman scattering spectra of CaF<sub>2</sub> samples containing Ca colloids and of MgO:Li with Li precipitates. The samples have been prepared by different thermal and irradiation treatments in order to achieve a range of colloid radii and density. The sizes of the colloids are determined by their optical absorption bands. Exciting in the surface-plasmon band of the metal colloid we observe surface-enhanced Raman scattering (SERS) of the whole density of states of CaF<sub>2</sub> or MgO phonons. The spectra are polarized and show a partial T<sub>2g</sub> character for CaF<sub>2</sub>. The intensity is enhanced as the laser energy approaches the colloid band maximum, where surface plasmons are excited. Comparing the results of different samples we find that the spectrum intensity is roughly proportional to the total amount of colloid material in the sample. In order to explain the appearance of the one-phonon density of states and its resonant behaviour we discuss the validity of the classical electromagnetic model and propose a mechanism consisting of surface-plasmon excitation followed by electron transfer from the metal to the host and de-excitation in the metal-host interface. In the host, electron-phonon interaction takes place, as in usual Stokes or antiStokes processes, giving rise to single-phonon modes. The lattice distortion and loss of periodicity in the vicinity of the colloid break the k=0 selection rule and account for the appearance of the whole density of states.
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