Aincident ^scattered in which Aincident and Ascattered are the wavelengths (in cm) of the incident and Raman scattered photons, respectively.

At room temperature, the thermal population of vibra-tionally (or rotationally) excited states is low, although not zero. Therefore, the initial state is the ground state, and the scattered photon will have lower energy (longer wavelength) than the exciting photon. This Stokes shifted scatter is what is usually observed in Raman spectroscopy. A small fraction of the molecules are in vibrationally (or rotationally) excited states. Raman scattering from vibrationally (or rota-tionally) excited molecules leaves the molecule in the ground state. The scattered photon appears at higher energy. This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-shifted spectrum, but at room temperature it is strong enough to be useful for frequencies less than about 1500 cm-1. The Stokes and anti-Stokes spectra contain the same frequency information. The ratio of anti-Stokes to Stokes intensity at any vibrational (or rotational) frequency is a measure of temperature. Anti-Stokes Raman scattering is used for contactless thermometry. The anti-Stokes spectrum is also used when the Stokes spectrum is not directly observable, for example because of poor detector response or spectrograph efficiency.

The frequency shifts in both the Stokes and the anti-Stokes spectra are characters for a bulk material. However, reducing the size of the material could change the frequency shifts in Raman scattering spectra. As discussed above, the limited systems could have different energy levels with those of the bulk materials. The energy levels for the vibrations and the rotations of the molecules can be different than those of the bulk materials. Thus the frequency shifts in Raman scattering spectra of the nanocapsules, which are responsible for the information of these energy levels, can be different with the bulk materials. Another reason for the change of the frequency shifts in Raman scattering spectra is the formation of the core/shell structures of the nano-capsules. The shape of the shell, the connection with the core materials, etc., could alter the vibration and the rotation of the molecules in the shell. Furthermore, usually, the peaks of the Raman scattering spectra become broad in the nanoparticles/nanocapsules. The broadening of the Raman peaks of the nanocapsules is ascribed to the decrease of the size of the material, the wide distribution of the size of the particles, the formation of the core/shell structure, etc. Figure 14 gives the Raman spectra of the [email protected] nanocapsules [125].

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