Research Projects, Jianyi Wang

Most of my research has been in the area of few-body and many-body processes in atoms, molecules, and solids. My main interest has been in the study of dynamics of charged particles and photons interacting with matter. I have used and developed various theoretical models ranging from exactly solvable systems to perturbative methods and statistical Monte Carlo methods. Recently, I have been studying the scattering of Rydberg atoms with large angular momenta by protons and positrons, which has received much international attention. I am also investigating non-linear multiphoton effects of Rydberg atoms in the presence of ultra-short intense laser fields. Currently, I am investigating the relationship between photons and charged particles for many-electron transitions such as double ionization and excitation plus ionization of helium.

Some of my more important contributions are:

• New ionization mechanism: A model three-body system was solved exactly and a new ``$v/2$'' ionization mechanism, generic in atomic collisions, was found.
• Wake-riding electrons: Simulations of ion transport in solids predicted the existence of electrons captured into the wake potential of antiprotons.
• Cluster impact fusion: An estimate was obtained for the upper limit of cluster-impact fusion rate at surfaces using multiple scattering approach.
• Ionization by structured particle impact: A much improved impulse approximation, first developed for ionization of atoms and molecules by partially-stripped ions (or atoms), was extended and applied to impact by highly charged ions. First results indicate qualitative agreement with experimental data in the on-going collaboration between experiment and theory.
• Mass transfer from circular Rydberg atoms: A mechanism known as the Thomas process for the transfer of an electron from an atom to a moving ion was predicted to dominate in collisions of circular Rydberg atoms by protons. This dominance, originally proposed by Thomas almost 70 years ago, had not been established until recently.
• Relationship between photons and charged particles: Both photons and charged particles interact with matter via electromagnetic fields. A relationship is derived in which the dipole and nondipole terms for charged particles are related to photoabsorption and Compton scattering, respectively. This relationship may be tested experimentally for double ionization (excitation) or excitation plus ionization of two-electron systems.

In several projects above, I have greatly enjoyed working with various experimental groups in US and Europe, including two on-going collaborations. I hope to continue the fruitful collaborations in future and to seek new opportunities in other areas as well.

RESEARCH PLAN

I am currently studying two-electron transitions of ions and atoms by photons and charged particles. These transitions include double ionization (or excitation) and excitation plus ionization of two-electron systems such as H$^-$, He, and Li$^+$. Photons and charged particles form a natural relationship as they both interact with matter via electromagnetic fields. This relationship is of fundamental and practical interest. Using our recently derived relationship, we calculated the asymptotic ratio of double to single ionization of He to be 0.272\% by protons using known data for photons.

Very recently, experimental data from different groups on the ratio of double to single photoionization of He show seemingly-irreconcilable differences of more than 40\% from threshold to 500 eV. By using our relationship, we can connect the photoionization data to the data for charged particle. The latter is believed to be well established and less unambiguous. Thus, the relationship bridging the data for charged particles and for photons will give us an independent way to identify inconsistencies in photoionization data.

I plan to continue my research on the interactions of circular Rydberg atoms with highly charged ions and photons. The field of high angular momentum states (to be referred to as high $l$ states below) of atoms is largely unexplored because experimental techniques for producing these states became available only recently. The intensive interest in these states is due to the increasing semi-classical character of high $l$ states. Besides its fundamental significance, the semi-classical character is also of practical importance.

In particular, the study of mass transfer from oriented Rydberg atoms may be of special interest to astrophysical simulations of encounters between hard binaries and intruders in globular clusters. It is believed that such encounters may lead to exchange of stars and transfer of energy, which in turn heats up the cluster. The frequency with which this happens requires simulations of binaries of various eccentricity and collision speeds. Since the forms of the basic forces are similar (Coulombic vs gravitational), electron capture from (the semiclassical) oriented Rydberg atoms may be viewed as a microscopic (``miniature'') astro-system. This system, accessible to atomic experiments, may prove useful in the studies of energy deposition as well as validating the theoretical models used.

The same semi-classical character of high $l$ states may be used in detecting nonlinear optical effects. Owing to the small position and momentum spread of these states, coherent superpositions of particle-like wave packets may easily be propagated along circular orbits. Effort is underway to investigate the coherent excitation and ionization by ultra short laser pulses, which may be turned on and off within a fraction of the period of the Rydberg state. An impulse method is being devised for laser-atom interactions in which the laser field is non-periodic.

I am also pursuing the interaction of many-electron atoms with photons. Recent studies show that ionization by lasers is a useful tool in probing electron-electron correlations. Correlation effects play a vital role in many-electron systems such as atoms, molecules (clusters) and solids, and are difficult to calculate. The goal in this study is two fold: (i) to understand how correlation may be treated, and (ii) to develop methods which will connect photon impact to charged particle impact. The underlying idea is that both photons and charged particles interact with matter via electromagnetic fields. A unified description is desirable from both fundamental and practical viewpoint.

I hope to study ways of high $l$ Rydberg states interacting with surfaces and solids using the information learned as outlined in the short-term plan. Scattering of oriented Rydberg atoms from surfaces is of interest because it may yield direction-sensitive effects otherwise undetectable without orientation. It is also known that high $l$ Rydberg states are created via resonant capture by an ion near surfaces. It is useful to know the properties of these states when they interact with image charges. Another interesting reason is that Rydberg states have large spatial dimension and may be sensitive to surface roughness or patterns on a nano- and even micro-scale on the surface.

There is evidence that an atom (or ion) traversing foils undergoes multiple encounters in which high $nlm$ states are populated by multistep excitation. Subsequent ionization of these states will result in electron distributions characteristic of electron emission from these atomic states, modified by solid effects. Combination of atomic ionization and ion transport in solids may yield useful information on this subject.

I also intend to improve and extend the applicability of the statistical quasi-classical method known as the classical trajectory Monte Carlo method, which is suitable for strong perturbations. But this method currently suffers a number of limitations, including its inability to model quantum interference and tunneling. There are indications that semiclassical propagators, derived from path integral formalism, may be cast into an initial-value form suitable for importance sampling. If implemented, this approach will vastly increase the range of problems this method may be applied to. A particularly good test case is ionization of atoms by impact of partially-stripped, highly-charged ions, where many experimental and theoretical research activities are directed at present. Electrons emitted in these collisions show strong interference effects caused by the field of the ions. Possible correlations effects may also be considered. Upon validation, this semiclassical Monte Carlo method may be applicable to systems of considerable complexity in physics and chemistry.

Also, the advent of new generation synchrotron light sources affords the opportunity to increase drastically the energy and intensity ranges of interaction of radiation with matter. Understanding of many-body transitions induced by such radiation is of considerably interest. As a compact system, many-electron atoms may be a an ideal test bed for studying electron correlation effects. I hope to investigate possible inner shell effects by high intensity, energetic photons.

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