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Journal of Physics B Atomic Molecular and Optical Physics

Detection of electrons in the surface ionization of H Rydberg atoms and H2Rydberg molecules

Elin A. McCormack, E. So, Mark Dethlefsen, Mark S. Ford, T. P. Softley

Published Dec 15, 2011• 5 citations
Publisher

Abstract

The detection of electrons from the surface interaction of Rydberg hydrogen atoms and molecules at a metal surface is investigated experimentally for the first time. The experimental detection of an electron signal in a time gate corresponding to when the Rydberg atoms or molecules are expected to collide with the surface provides evidence for the previously proposed backscattered electron loss (So et al 2009 Phys. Rev. A 79 012901) as a route of surface ionization under electron-extraction fields. Comparison of the ion- and electron-extraction results shows that the dynamics as the Rydberg states approach the surface are very different in the two cases, leading to the possible loss of control of the electronic polarization in the initially prepared Rydberg state from the traversal of level crossings during the Rydberg trajectory. The difference in the ionization dynamics observed for the Rydberg H and H2 cases also illustrates some of the fundamental differences between atomic and molecular, and hydrogenic and non-hydrogenic systems.

DOI
10.1088/0953-4075/45/1/015204

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The behavior of Rydberg atoms near metal surfaces in the presence of an external electric field has been examined theoretically to evaluate their potential use as an experimental probe of electron tunneling processes at surfaces. These ab initio calculations reveal strong hybridization that leads to a shift and broadening of the Rydberg levels that depend sensitively on the atom-surface separation and the strength of the applied electric field. A range of external fields exists, however, for which the Rydberg atoms ionize at sufficiently large atom-surface separations that, even in the presence of image charge attraction, the resulting positive ions can escape the surface and provide an experimentally observable signal. \textcopyright{} 1996 The American Physical Society.

Physical review. B, Condensed matter•Mar 15, 1996•32 citations
The detection of electrons from the surface interaction of Rydberg hydrogen atoms and molecules at a metal surface is investigated experimentally for the first time. The experimental detection of an electron signal in a time gate corresponding to when the Rydberg atoms or molecules are expected to collide with the surface provides evidence for the previously proposed backscattered electron loss (So et al 2009 Phys. Rev. A 79 012901) as a route of surface ionization under electron-extraction fields. Comparison of the ion- and electron-extraction results shows that the dynamics as the Rydberg states approach the surface are very different in the two cases, leading to the possible loss of control of the electronic polarization in the initially prepared Rydberg state from the traversal of level crossings during the Rydberg trajectory. The difference in the ionization dynamics observed for the Rydberg H and H2 cases also illustrates some of the fundamental differences between atomic and molecular, and hydrogenic and non-hydrogenic systems.
The charge transfer of Rydberg hydrogen atoms at a metal surface is investigated for the first time. The surface ionization of Stark states with various electron density distributions with respect to the surface is examined. Unlike the nonhydrogenic species studied previously, genuine control over the orientation of the electronic wave function in the surface-ionization process is demonstrated. A comparison of the results for a range of collisional velocities for the most redshifted Stark state with principal quantum numbers n=20-36 with the classical over-the-barrier approach shows a good agreement for the onset of the ion signal, but the shallow rise in signal is not accounted for. An excellent fit of the experimental results can be achieved using a simple semiempirical model.
The work of this thesis focuses on the interaction of high Rydberg states of hydrogen molecules and atoms with various doped Si semiconductor surfaces with the results compared with those obtained with an atomically flat gold surface. The major part of the thesis was carried out using para-H₂ molecular Rydberg states with principal quantum number n = 17 - 21 and core rotational quantum number N⁺ = 2. Subsequently, this study was continued using H atomic Rydberg states with principal quantum number n = 29 - 34. The high Rydberg states have been produced using two-step laser excitation. For close Rydberg surface separation (< 6 n² a.u.), the Rydberg states may be ionized due to an attractive surface potential experienced by the Rydberg electron, and the remaining ion core may be detected by applying an external electric field.An efficient ion detectability method is introduced to compare the many surface ionization profiles quantitatively. The p-type doped Si surfaces enhance the detected ion-signal more than the n-type doped Si surfaces due to the presence of widely distributed positive dopant charge fields in the p-type doped Si surfaces. As the dopant density increases, the area sampled by the resultant ions becomes effectively more neutral, and the decay rate of the potential from the surface dopant charge with distance from the surface becomes more rapid. Therefore, the minimum ionization distance is also reduced with increasing dopant density. It is found that the detected ion-signal decreases with increasing dopant density of both p- and n- type doped Si surfaces. The higher-n Rydberg states have shown higher ion detectability than that of lower-n Rydberg states and this variation also becomes smaller when increasing the dopant density.Experiments involving H2 Rydberg molecules incident on various doped Si surfaces in the presence of a Stark field at the point of excitation are also presented here. The surface ionization profiles produced via both electron and ion detection schemes are measured by changing the Stark polarization. Positive surface dopant charges oppose production of backscattered electrons and negative surface dopant charges enhance the electron-signal. For the electron detection scheme, lightly doped n-type Si surfaces show higher detectability but in the case of p-type Si surfaces the more heavily doped Si surfaces give a higher detected signal. This different behaviour of the detected ion or electron signal implies a different production mechanism.Theoretical trajectory simulations were also carried out based on a new 2D surface potential model. The results qualitatively agree with the experimental results and explain the changes of the surface ionization profiles between the various dopant types and dopant densities of the Si surfaces.
This thesis reports the first controlled study of the interactions of Rydberg atoms with a metal surface. In these experiments, a collimated beam of potassium Rydberg atoms is directed at a plane surface at near grazing incidence. Positive ions formed by surface ionization are attracted to the surface by their image charge, which is counterbalanced by an external electric field applied perpendicular to the surface. The ions are detected by a position-sensitive detector (PSD). At some critical value of the external field, the ion trajectories just miss the surface, suggesting that analysis of the dependence of the ion signals on external electric field can be used to determine the distance from the surface at which ionization occurs. This distance, and thus the corresponding critical electric field, is expected to be n-dependent. Experimentally, however, it was observed that the ion signal had a sudden n-independent onset when only a small positive perpendicular electric field was applied at the surface. This observation requires, surprisingly, that the ions produced by surface ionization can readily escape from the surface. The data do, however, show that Rydberg atoms are efficiently ionized in collisions with the surface. This process may provide a useful new detection technique for Rydberg atoms.
The technique of electric field ionization has been used to study molecular Rydberg states of H2{\mathrm{H}}_{2}H2​ and D2{\mathrm{D}}_{2}D2​ formed by electron capture of H2+{\mathrm{H}}_{2}^{+}H2+​ or D2+{\mathrm{D}}_{2}^{+}D2+​ beams in H2{\mathrm{H}}_{2}H2​. The neutral molecules were passed through two intense electric field ionization regions in tandem, separated by an \ensuremath{\sim} ${10}^{\ensuremath{-}7}$-sec drift region. Up to 15% of the Rydberg population removed by the first ionization region was repopulated during transit through the drift region. This result has been interpreted in terms of nonadiabatic state mixing of nearly degenerate adiabatic molecular states. A difference in the state repopulation between Rydberg states of H2{\mathrm{H}}_{2}H2​ and D2{\mathrm{D}}_{2}D2​ was observed, indicating the importance of the rovibrational spectrum on the repopulation mechanism. By comparison, similar studies on atomic Rydberg states showed no difference between H and D. Moreover, the atoms exhibited only a small repopulation effect, consistent with the contribution due to $\stackrel{\ensuremath{\rightarrow}}{1}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{s}}$ coupling.
The behavior of Rydberg atoms near metal surfaces in the presence of an external electric field has been examined theoretically to evaluate their potential use as an experimental probe of electron tunneling processes at surfaces. These ab initio calculations reveal strong hybridization that leads to a shift and broadening of the Rydberg levels that depend sensitively on the atom-surface separation and the strength of the applied electric field. A range of external fields exists, however, for which the Rydberg atoms ionize at sufficiently large atom-surface separations that, even in the presence of image charge attraction, the resulting positive ions can escape the surface and provide an experimentally observable signal. \textcopyright{} 1996 The American Physical Society.
The behavior of Rydberg atoms near metal surfaces in the presence of an external electric field has been examined theoretically to evaluate their potential use as an experimental probe of electron tunneling processes at surfaces. These ab initio calculations reveal strong hybridization that leads to a shift and broadening of the Rydberg levels that depend sensitively on the atom-surface separation and the strength of the applied electric field. A range of external fields exists, however, for which the Rydberg atoms ionize at sufficiently large atom-surface separations that, even in the presence of image charge attraction, the resulting positive ions can escape the surface and provide an experimentally observable signal. \textcopyright{} 1996 The American Physical Society.