Thermally Enhanced Neutralization in Hyperthermal Energy Ion Scattering

Neutralization probabilities are presented for hyperthermal energy $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ ions scattered from a Cu(001) crystal as a function of surface temperature and scattered velocity. A large enhancement in neutralization is observed as the temperature is increased. Velocity-dependent charge transfer regimes are probed by varying the incident energy, with the most prominent surface temperature effects occurring at the lowest energies. The data agree well with results obtained from a model based on the Newns-Anderson Hamiltonian, where the effects of both temperature and velocity are incorporated.

Figure 1

Time-of-arrival spectra obtained with the NPD for 250 eV $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ ions scattered from a Cu(001) crystal. The surface temperature was held at $328\pm 5\mathrm{K}$ during this measurement. The three peaks present in these spectra are due to trajectory types that involve scattering from one or more individual atoms at the surface [1].

Figure 2

Temperature-dependent neutralization results for 22, 152, and 630 eV $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ ions scattered from a Cu(001) crystal. The lines are drawn to guide the eye, and a typical error bar is shown.

Figure 3

The neutralization probability, $P_{\mathrm{0}}$, for $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ ions scattered from a Cu(001) crystal at surface temperatures of 350, 773, and 923 K. $P_{\mathrm{0}}$ is plotted on a logarithmic scale versus the inverse scattered perpendicular velocity to show the exponential dependence present at low energies and high temperatures. The velocity is shown in atomic units (a.u.), where 1 a.u. is approximately $2.2\times {10}^8\mathrm{c}\mathrm{m}/\sec $. The corresponding incident energy scale is noted along the top axis.

Figure 4

An ionization level diagram for Na outside of a Cu(001) surface. The Fermi level lies at an energy $\Phi$ below the vacuum level, where $\Phi =4.59\mathrm{e}\mathrm{V}$ for Cu(001). Filled electron levels in the metal are indicated by the lines drawn on the far left. The distance, $z$, is plotted in atomic units (a.u.), where 1 a.u. is $0.529\AA$.

Figure 5

Comparison of the experimental neutralization probability (symbols) to the results of the independent-particle calculation (lines) for $\mathrm{N}{\mathrm{a}}^{\mathrm{+}}$ scattering from Cu(001). A typical error bar is shown.