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Femtosecond pump-probe spectroscopy of surface bound alkali nanoparticles

Second harmonic generation at the surface of Na and K clusters was studied using femtosecond laser pulses. The observed size dependence of the second harmonic intensity can be explained by resonant enhancement of surface plasmon polariton excitation. At different cluster sizes, the second order interferometric autocorrelation of a 25 fs pulse was recorded using the clusters as nonlinear medium. There is no broadening of the cluster autocorrelation with respect to a non resonant reference autocorrelation within the experimental error of 1 fs. This experimental finding is independent of particle size, resonance of the fundamental (potassium) or the second harmonic (sodium) or surface modification by chemisorption of oxygen.