Emission of Thermally Activated Electrons from Rare Gas Clusters Irradiated with Intense VUV Light Pulses from a Free Electron Laser

The ionization dynamics of Ar and Xe clusters irradiated with intense vacuum ultraviolet light from a free-electron laser is investigated using photoelectron spectroscopy. Clusters comprising between 70 and 900 atoms were irradiated with femtosecond pulses at 95 nm wavelength ($\sim 13\mathrm{eV}$ photon energy) and a peak intensity of $\sim 4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. A broad thermal distribution of emitted electrons from clusters with a maximum kinetic energy up to 30–40 eV is observed. The observation of relatively low-energy photoelectrons is in good agreement with calculations using a time-dependent Thomas-Fermi model and gives experimental evidence of an outer ionization process of the clusters, due to delayed thermoelectronic emission.

Figure 1

Photoelectron spectra from ${\mathrm{Ar}}_{300}$ clusters (a), Ar atoms (b), ${\mathrm{Xe}}_{70}$ clusters (c), and Xe atoms (d) irradiated at $4.4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. Electrons originating from clusters (light gray shaded) and atoms (dark gray shaded) are indicated.

Figure 2

Photoelectron spectra from Ar clusters comprising 300 (open circles) and 900 atoms (closed circles) plotted in log-linear scale. Contributions from the ionization of atoms, which are always present in the cluster beam, are indicated.

Figure 3

Simulation of the photoemission spectra from ${\mathrm{Ar}}_{55}$ clusters irradiated with laser pulses at a peak intensity of (a) ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and (b) ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ using a time-dependent Thomas-Fermi model. Exponential fits of the kinetic energy distributions are indicated (gray shaded).

Figure 4

Calculated number of emitted electrons as a function of the irradiation time. The inset shows a 2D projection of the pseudoparticles that model the electron density distribution, in a “snapshot” at the end of the pulse.