Resonant Charge Transfer of Hydrogen Rydberg Atoms Incident on a Cu(100) Projected Band-Gap Surface

The charge transfer (ionization) of hydrogen Rydberg atoms ($n=25–34$) incident on a Cu(100) surface is investigated. Unlike fully metallic surfaces, where the Rydberg electron energy is degenerate with the conduction band of the metal, the Cu(100) surface has a projected band gap at these energies, and only discrete image states are available through which charge transfer can take place. Resonant enhancement of charge transfer is observed for Rydberg states whose energy matches one of the image states, and the integrated surface ionization signals (signal versus applied field) show clear periodicity as a function of $n$ as the energies come in and out of resonance with the image states. The surface ionization dynamics show a velocity dependence; decreased velocity of the incident H atom leads to a greater mean distance of ionization and a lower field required to extract the ion. The surface ionization profiles for “on resonance” $n$ values show a changing shape as the velocity is changed, reflecting the finite field range over which resonance occurs.

Figure 1

The energies of the $n=25–36$ $k=0$ H atom states, and the image states ($n_{\mathrm{img}}=6$ to $n_{\mathrm{img}}=8$) of the Cu(100) surface. The widths of the H atom lines represent the range of surface perturbation of the $k=0$ states for distances from the surface of $3n^2{a}_0$ to $6n^2{a}_0$. The lengths of the lines indicate the range of applied fields over which surface ionization can be measured, before the onset of field ionization.

Figure 2

(a) Two-color excitation of H atoms produces a beam of Rydberg atoms to probe a Cu(100) surface. Field and surface ionization signals are spatially and temporally separated due to the different ionization positions with respect to the surface. (b) Schematic of the surface analysis chamber. (c) The $xyz$ manipulator allows the surface to be moved between the surface analysis chamber and the Rydberg-surface experiment under vacuum.

Figure 3

H atom surface ionization profiles for $n=26$, 27, 33 at various perpendicular velocities: black lines, $850\mathrm{m}\hspace{0.17em}{\mathrm{s}}^{-1}$; red lines, $700\mathrm{m}\hspace{0.17em}{\mathrm{s}}^{-1}$; green lines, $600\mathrm{m}{\hspace{0.17em}\mathrm{s}}^{-1}$; blue lines, $500\mathrm{m}\hspace{0.17em}{\mathrm{s}}^{-1}$.

Figure 4

The integrated surface ionization signal for H atoms incident on a Cu(100) surface as a function of $n$. The yellow line shows the corresponding behavior for a gold surface at a velocity of $660{\mathrm{m}\hspace{0.17em}\mathrm{s}}^{-1}$.