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Summary form only given.Recent experimental results show that the interaction of atomic clusters with intense, femtosecond laser pulses is extremely energetic. The interaction with clusters is considerably more energetic than that with single atoms or small molecules irradiated at the same intensity. A numerical model that treats the laser-irradiated clusters as spherical micro-plasmas indicates that the super-heating of the clusters arises from a sharp increase in the laser-driven collisional heating. This increase occurs when the electron density in the expanding cluster drops to three times the critical electron density, at which point the electric field inside the cluster plasma is resonantly enhanced compared to the external field. This spike in the field leads to a sharp, high-energy peak in the electron energy distribution. The results from this model show good agreement with the electron energy distributions measured when the clusters are irradiated with single pulses. As the electron critical density depends on the laser frequency, we have investigated the possibility of using two pulses with different frequencies to excite the resonance in the heating twice during the expansion of the cluster, thereby enhancing the electron heating. The numerical model has been adapted to simulate the irradiation of a cluster with a sequence of laser pulses of the same or different frequencies. The results of this new model support the idea that using two pulses of different frequencies will strongly affect the measured electron energy distributions. An enhancement in hot electron production is predicted under some conditions.
Date of Conference: 8-8 May 1998