Atomic clusters in intense laser fields

We have examined the energy distributions of ions and electrons ejected in the interaction of clusters of several thousand noble-gas atoms with high-intensity (1014-1016 Wcm-2) femtosecond laser pulses. The dependence of the ion energies on laser intensity and wavelength, cluster size and cluster medium has been studied. Ion energies of up to 1 MeV and electron energies of up to 2.5 keV have been measured. The interaction with clusters is considerably more energetic than that with single atoms or small molecules irradiated at the same intensity. A numerical model, which treats the cluster as a spherical nanoplasma, is used to analyse the explosion of a single cluster. The model suggests that the superheating 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. 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. The results of this new model support the idea that using two pulses of different frequencies will enhance the heating. We have performed experimental investigations of the dynamics of clusters of a few thousand Xe atoms when irradiated by two laser pulses of different frequencies in sequence. The dependence of the explosion temperature on the delay between the two pulses and on their relative polarisations is examined. It is shown that using two pulses with the optimum delay between them can indeed enhance the explosion temperature. The experimental results show good agreement with the modelling. (author)