Surface-plasma resonance in small rare-gas clusters by mixing ir and vuv laser pulses

The ionization dynamics of a xenon cluster with 40 atoms is analyzed under a pump-probe scenario of laser pulses where an infrared laser pulse of 50-fs length follows with a well-defined time delay a vuv pulse of the same length and peak intensity. The mechanism of resonant energy absorption due to the coincidence of the ir laser frequency with the frequency of collective motion of quasifree electrons in the cluster is mapped out by varying the time delay between the pulses.

Figure 1

Comparison of the inner ionization process for a single atom and a model cluster of three atoms. The effective barrier is lowered in a cluster due to the charged environment.

Figure 2

Single-ion charge spectra after the interaction of ${\mathrm{Xe}}_{40}$ with a vuv pump pulse followed by an ir probe pulse as a function of the delay $\mathrm{\Delta }t$ between the two pulses. The highest (maximum and average) charge states are created for $\mathrm{\Delta }t=3500\mathrm{a.u.}$ The scale of the ordinate axis is identical in all four cases.

Figure 3

Absorbed energy (a) and average charge per atom (b) as a function of time delay between the vuv and the ir pulse.

Figure 4

Phase lag between the center-of-mass oscillation of the electron cloud and the driving laser field as a function of time delay between the vuv and the ir pulse.