Intensity scan of NbxOy clusters through nearly 4 orders of laser intensity. The slopes according to MPI are shown for the various building blocks of the clusters. Clustered species were not observed in the neutral distribution indicating that the clusters fragment before ionization. A non-integer slope of 1.5 is indicative of more complex processes occurring (such as fragmentation), suggesting an energy requirement of 2.98 (1.5 × 1.99) eV.
The ion signal for the single charged metal species. The red and blue data is the same, only graphed according to different Y scales. The left y-axis is graphed logarithmically to show the slope measurement, which demonstrates the photon order according to MPI. The right axis (blue) shows the same data plotted linearly to determine the saturation intensity and ionization potential according to tunneling.
Intensity scan of NbxOy clusters demonstrating the high charge states past the valence shell of niobium, and high charge states of oxygen. Higher charge states are observed, but are not resolved for scanning measurements, as described in the text.
The sequential ionization probabilities according to the ADK model for the high charge states of atomic niobium and oxygen, assuming all electrons have ℓ = 0, for simplicity. This demonstrates that although Nb4+ and O2+ have similar sequential ionization potentials, ADK model suggests that they require different laser intensities for production. Additionally, the ADK prescription suggests that Nb5+ and O2+ should arrive at similar laser intensities. Neither of these results are observed experimentally.
The saturation intensities measured for the metal and oxygen species follow the sequential ionization potentials. This is in contrast with the expected results from the ADK model, thereby suggesting that the charge state plays a minor role compared to the ionization energy.
The measured saturation intensities and determined ionization energies for the ion peaks that are clearly resolved. The saturation intensities are given in units of 1 × 1013 W/cm2, and the corresponding ionization energies are given in eV. All charge states were treated as Z = 1 in determining the ionization potential through the ADK model. Not all ions provided useable ISS measurements as described in the text.
The sequential ionization potentials for reaching the ion state listed in the atoms studied in this experiment. The values for Ta beyond Ta+ are not yet available, and so are approximated here using Slater's rules. A better estimate for the Ta energies might be obtained by applying the energies for niobium.
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