The growth mode of ultrathin films, deposited by laser ablation, crucially depends on the energy of the ablated species. Therefore, a time-of-flight (TOF) spectrometer has been constructed and measurements have been carried out in order to determine the energy distribution of laser ablated Fe and Pt atoms and ions in the plasma created by nanosecond pulses of a Nd:YAG laser. The experiments have been performed in UHV under restriction to low laser fluences (0.4 - 0.6 J/cm2) to avoid droplet formation and formation of multiply charged particles. In case of measuring the spectra of the neutrals, a cross-beam electron source for post-ionization and electric as well as magnetic fields for repelling the ions are employed. Nevertheless, measurements of neutral particles are restricted to low plasma densities due to electrostatic shielding within the plasma, leading to an inefficient deflection of a dense plasma by electrostatic fields. A deflection grid, arranged in a 45° angle with respect to the plasma propagation direction, directs the ions onto an off-axis microchannel plate. Test measurements have been performed by utilization of the TOF spectrometer as a pressure gauge and also by chopping the electron beam and thereby utilization of the TOF spectrometer as a residual gas mass spectrometer. The spectra of the laser ablated plasmas have shown plasma conditions with a Debye length of approximately 10-4 m, densities of 1015...1016 m-3 and ion energies up to 150 eV. Neutral spectra have shown a low fraction of neutrals with hyperthermal energies up to several 10 eV, most probably contributed by recombination of ions and electrons in the plasma. No thermally evaporated neutral atoms could be found, even though the residual gas spectra had demonstrated a proper function of the neutral TOF spectrometer.
We have analyzed the growth mode of ultrathin films deposited with different ion energies. For Fe grown on Cu(111), it was determined that bilayer growth occurs for kinetic ion energies below 40 eV and layer-by-layer growth for energies above 60 eV.
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