Theoretical study of the laser-induced femtosecond dynamics of small Si_n clusters

The ultrafast relaxation of Si_n clusters following laser excitation is studied as a function of energy and duration of the laser pulse. We perform molecular-dynamics simulations based on a tight-binding Hamiltonian and take the form and duration of the laser pulse explicitly into account. We show that the occurrence of the different relaxation channels is strongly dependent on the duration τ of the exciting pulse. Very short pulses (τ<50 fs) may induce melting or fragmentation of the cluster, depending on whether the intensity of the laser pulse is low or high, respectively. On the other hand, long pulses favor structural changes and an expansion of the cluster structure. By studying the response of the cluster as a function of the laser parameters we determine generalized “phase diagrams” for the occurrence of solidlike, liquidlike, and fragmented clusters as a product of the laser excitation. For certain values of τ and energy of the laser pulse structures are produced, which show a small energy gap. The laser induced “metallization” of small silicon clusters is also discussed.