Research Statement

In one series of experiments atoms in which one electron is excited to a state of very large principal quantum number n, termed Rydberg atoms, are being used to study the response of atoms to  perturbations in the form of pulsed electric fields whose characteristic times (duration, period, rise/fall times) are less than the classical electron orbital period (~10 ns at n ~ 400).   Application of such pulses leads to the formation of wavepackets that comprise a coherent superposition of adjacent Rydberg states. These wavepackets display novel dynamical behavior that can mimic the classical motion of an excited electron, thereby providing a bridge between quantum and classical physics. These studies show that, with careful choice of the amplitude and width of the pulses (or application of several pulses), it is possible to control and manipulate atomic wavefunctions with remarkable precision.  The long-term goal is to use carefully-tailored sequences of pulses to engineer atomic wavefunctions and produce “designer” atoms to study classical-quantum correspondence, to use periodic trains of pulses to create non-dispersive wavepackets, to study atom/field interactions in the ultra-fast ultra-intense regime, to explore information storage/retrieval in atoms, and to probe non-linear dynamics.

A protocol has been developed to create localized wavepackets that travel in near-circular Bohr-like orbits.  The opportunities afforded by the realization of such quasi-two-dimensional atoms are being explored.  These include the use of radio frequency driving fields to create non-dispersive Trojan wave packets and transport them to states of different n.  This work is being extended to include studies of planetary atoms containing two excited electrons, to explore Rydberg atom/Rydberg atom interactions, and to monitor the evolution of ultra-cold gases of Rydberg atoms.

The interactions of Rydberg atoms with surfaces are also being examined to probe electron tunneling at surfaces and to investigate how atoms respond to the presence of a nearby surface.  Our studies show that the effects of local stray fields present at the surface associated with surface inhomogeneities or surface charging can be sizeable and are being examined with the aid of a newly-developed model and direct measurements of the surface potential using AFM techniques.  Lithographically-patterned microscopic electrode arrays are being employed to generate controlled surface electric fields to better understand their effects.

Rydberg atoms are being used to examine electron attachment to molecules.  The creation of novel negative ion species in such collisions is observed including the formation of negative ions in which the electron is weakly bound by the electric dipole moment of the parent molecule.  The creation of heavy-Rydberg ion-pair states comprising a positive-negative ion pair that orbit at large radius weakly bound by their mutual electrostatic attraction is also seen.  The physical and chemical properties of such species are being examined.

Each of the above experimental programs makes use of advanced experimental techniques including ultrahigh vacuum technology, state-of-the-art dye, diode and fiber laser systems, high-speed analog and digital electronics, and computer-based data acquisition and control systems.  The work relies heavily on the efforts of several graduate students:

Yu Pu

Changhao Wang

Shuzhen Ye

Michael Kelley

Xinyue Zhang

Research Topics

Studies of the behavior of quasi-1D very-high-n (n ~ 300-350) Rydberg atoms subjected to one or more of short pulsed unidirectional electric fields, termed half-cycle pulses (HCPs), are underway. Such atoms, when subject to periodic alternating external electric field pulses exhibit chaotic behavior.  The ionization of the system is governed by a geometric structure of phase space called a homoclinic tangle and its turnstile.  The structure of the phase space turnstile is examined by creating time-dependent wave packets, applying a sequence of alternating field pulses, and measuring the survival prbability.  The results show that that portion of the wave packet inside the turnstile ionizes quickly, while that portion outside the turnstile ionizes only after multiple field cycles.  This provides a robust but simple way to control chaotic ionization of an atomic system and opens up new avenues for the control and manipulation of atomic wavefunctions which are being explored and for possible applications in information processing.

Protocols to create localized wavepackets in very-high-n (n ~ 300) states that travel in near circular orbits about the nucleus are being developed.  These wavepackets represent the closest analog yet achieved to the original Bohr model of the hydrogen atom, i.e., an electron in circular classical orbit about the nucleus.  Initially, quasi-one-dimensional Rydberg atoms strongly polarized along the x axis are created by photoexciting the lowest-lying states in the n ~ 306 Stark manifold in the presence of a weak dc field, directed along the x axis.  This is then turned off adiabatically and a strong transverse field is suddenly applied along the z axis.  This leads to the creation of a Stark wavepacket which undergoes periodic changes in its y component of angular momentum.  Turn-off of the transverse field at an appropriate time leaves a localized wavepacket traveling in a near-circular orbit (n ~ l ~ m) that undergoes transient localization forming a Bohr-like state that  remains localized for several orbits before dephasing and collapsing to a quasi-stationary near circular state.  The use of radio frequency fields to maintain strongly localized Bohr-like wavepackets for many hundreds of orbits is being explored.  With careful choice of the period, amplitude, and phase of the drive field wave packets have been generated, termed Trojan wave packets that execute near-circular motion and that remain localized indefinitely and whose motion mimics that of Jupiter's Trojan steroids.   Furthermore, the locking to the drive field is so strong that by slowly varying the drive frequency the wave packet can be transported to states of much higher n, furnishing a new technique for the control of atomic wavefunctions.  This work is being extended to explore the creation of quasi-stable planetary atoms containing two excited electrons.  In addition, the behavior of strongly interacting Rydberg atom pairs is being explored together with the evolution of ultracold gases of Rydberg atoms.

Recently, we have observed quantum revivals in very-high-n (n ~ 305) circular wavepackets.  Realizing mesoscopic systems that exhibit coherent quantum dynamics is a challenge because such systems typically have exceedingly small level spacings and are very susceptible to decoherence through energy exchange with the environment.  Nonetheless, with careful control of the experimental environment it is possible to see multiple quantum revivals (on time scales ~400 ns) demonstrating that quantum coherence of (near-circular) Rydberg packets is surprisingly robust and can be preserved on microsecond timescales.  The observed quantum revivals allow study of the transition from quantum to classical dynamics induced by interactions with the environment, i.e., by spurious electrical noise or by collisions.

The presence of revivals also allows high-resolution Fourier analysis of the data.  The extraction of detailed information on the density matrix of very-high-n near-circular wavepackets through such Fourier analysis is being explored, the remarkably long coherence times facilitating the preservation and read out of information contained in this matrix.  Measurements suggest that information might be encoded in near-circular wavepackets by controlling their production and retrieved from the phase and amplitude of their Fourier spectra.  The n resolution that can be achieved by such time-domain spectroscopy is very high, allowing protocols to be tested designed for very precise control of n distributions. 

The photoexcitation of Rydberg atoms in the presence of strong rf drive fields at, or near, the Kepler frequency of the product state is being explored to investigate the behavior of the electron in the combined nuclear and drive fields.  The drive field couples many Rydberg levels simutaneously and results in a coherent response that leads to a variety of multiphoton processes.  The data reveal signatures of quantum optical phenomena such as electromagnetically induced transparency and Autler-Townes splitting seen with few-level systems.

Collisions between high-n Rydberg atoms and molecular targets are being used to examine electron attachment to molecules.  The creation of novel negative ion species  is observed including the formation of ions in which the electron is weakly bound by the electric dipole moment of the parent molecule.  Such ions display many of the characteristics associated with Rydberg atoms which is, perhaps, not unexpected as they both contain a weakly-bound electron in a diffuse orbital.  Rydberg atom collisions can also lead to creation of so-called heavy-Rydberg ion-pair states comprising a positive-negative ion pair that orbit at large radius weakly bound by their mutual Coulomb attraction.  The lifetimes and decay modes of such species are being examined.  Ion-pair states involving molecular negative ions decay with lifetimes of a few microseconds through mutual charge transfer or the conversion of internal energy in the negative ion into translational energy of the ion pair.  Those states involving atomic negative ions have very long lifetimes, allowing studies of their chemical and collisional properties.

Because of their large physical size and weak binding, Rydberg atoms are strongly perturbed by nearby metal surfaces and thus provide a particularly sensitive probe of atom/surface interactions. Image charge effects shift the atomic energy levels and distort the electronic wavefunctions. Also, ionization can occur through resonant tunneling of the excited electron into a vacant level in the metal.  Sizeable discrepancies between theory and experiment, however, are seen that can be attributed to stray “patch” fields at the surface associated with surface charging (for silicon surfaces) or with contact potential differences resulting from surface inhomogeneities (for gold surfaces).  The effects of such patch fields are being explored both by measuring the potential variations directly using Kelvin force microscopy and by using lithographically-patterned microscale electrode structures.  Preliminary results suggest that, while stray fields are very important, the ionization of Rydberg atoms at a surface is well described by a simple “over-the-barrier” model of tunneling.  Lithographically-patterned microscopic electrode arrays are being used to generate better controlled surface electric fields and further the understanding of their effects.

Each of the above experimental programs makes use of advanced experimental techniques including ultrahigh vacuum technology, state-of-the-art dye, diode and fiber laser systems, high-speed analog and digital electronics, and computer-based data acquisition and control systems.  The work relies heavily on the efforts of several gradate students:

Yu Pu

Changhao Wang

Shuzhen Ye

Michael Kelley

Xinyue Zhang

C. O. Reinhold, S. Yoshida, J. Burgdorfer, J. J. Mestayer, B. Wyker, J. C. Lancaster and F. B. Dunning "Tailoring very-high-n circular wave packets" Phys. Rev. A 78, 063413 (2008)

D. D. Neufeld, H. R. Dunham, S. Wethekam, J. C. Lancaster, and F. B. Dunning "Ionization of xenon Rydberg atoms at Au(111) surfaces:  Effect of stray fields" Phys. Rev. B 78, 115423 (2008)

D. D. Neufeld, H. R. Dunham, S. Wethekam, J. C. Lancaster, and F. B. Dunning "Ionization of xenon Rydberg atoms at oxidized Si(100) surfaces" Surf. Sci. 602, 1306 (2008)

J. J. Mestayer, B. Wyker, F. B. Dunning, C. O. Reinhold, S. Yoshida and J. Burgdorfer "Generation of quasi-classical Bohr-like wavepackets using HCPs" Phys. Rev. A 78, 045401 (2008)

J. J. Mestayer, B. Wyker, J. C. Lancaster, F. B. Dunning, C. O. Reinhold, S. Yoshida and J. Burgdorfer "Realization of localized Bohr-like wave packets" Phys. Rev. Lett. 100, 243004 (2008)

M. Cannon, Y. Liu, L. Suess, and F. B. Dunning "Dipole-bound CH3CN- ions: Temperature dependence of ion production rates and lifetimes" J. Chem. Phys. 128, 244307 (2008)

M. Cannon, Y. Liu, and F. B. Dunning "Lifetime of K+-SF6- heavy Rydberg states formed by electron transfer in K(np)-SF6 collisions" Chem. Phys. Lett. 458, 35 (2008)

S. Yoshida, C. O. Reinhold, J. Burgdorfer, W. Zhao, J. J. Mestayer, and J. C. Lancaster "Extracting irreversible de-phasing rates from electric dipole echoes in Rydberg Stark wave packets" Phys. Rev A 78, 063414 (2008)

S. Yoshida, C. O. Reinhold, J. Burgdorfer. J. J. Mestayer, J. C. Lancaster and F. B. Dunning "Transferring Rydberg wave packets between islands across the chaotic sea" Phys. Rev. A 77, 013411 (2008)

Studies with near-circular high-n wavepackets -Cold Rydberg Gases and Ultracold Plasmas - MPIPKS-ITAMP Tandem Workshop, Dresden

Ionization of kicked Rydberg atoms via a turnstile mechanism. Chaos and Complexity Conference, Ann Arbor, Michigan. (May 2012) With K Burke, S Ye and B Wyker

The influence of stray fields on the ionization of Rydberg atoms at metallic surfaces - 2010 Annual meeting of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, Houston, TX (with Dennis Neufeld and Yu Pu)