Vibrational Modes and Electron-Phonon Coupling

The modes excited by ultrafast laser pulses are usually not the same as those detected by spontaneous-Raman spectroscopy, mainly for symmetry reasons [3.53] (see Fig. 5.3-7). Moreover, the symmetry selection rules depend on the mechanism of excitation by the laser pulse. A thermal mechanism provokes 

In metal particles, the main consequence of changes in the electron-phonon couplings is a variation of the electron relaxation rates. This effect depends strongly on the intensity regime of the excitation, but experiments carried out at weak excitation on Ag and Au nanoparticles in various matrices show a strong increase of the relaxation rate, i. e. a decrease of the electron-lattice energy exchange time, with quantum confinement (see Fig. 5.3-8) [3.54]. In this case also, the increased 

3-6 Time-domain breathing-mode acoustic oscillations in 3 nm PbS quantum dots embedded in a polymer matrix, observed by transient saturated absorption. (After [3.42]) 

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