The Physics Department houses one of the world's major centers for research in atomic, molecular, and optical (AMO) physics, and in the related field of laser physics, described in a later section. Several recent faculty appointments have augmented UConn's traditionally strong activity in this diverse field, both experimental and theoretical. Areas of particular emphasis include laser spectroscopy, cooling and trapping of atoms and molecules, collisional studies involving gases and surfaces, molecular physics, high intensity laser research, and quantum optics. The common goal of this work is to gain a fundamental understanding of the interactions of atoms, molecules, and electromagnetic radiation. The research spans energies from ultracold collisions, where T<1 mK, to high-temperature ionized gases and (with accelerated ion beams) into the kiloelectron volt and megaelectron volt regimes.
The Department's high activity level in AMO physics has fostered numerous collaborative projects both within the Department and outside it. This creates opportunities for interdisciplinary graduate research in newly emerging research areas. One example is the award of a major multi-investigator grant in 2000 that funds new research on collisions of trapped atoms and molecules, techniques for creating ultracold molecules and ultracold plasmas and related work on high-resolution spectroscopy. Active collaborations include colleagues in industry, in government laboratories, and at other major research universities. This diversity helps graduate students to examine options both for traditional and nontraditional careers in physics.
The remainder of this section outlines some of the current AMO activities in the Department. Though listed by subfield for convenience, many of these projects are closely interwoven with one another and with other Department activities such as laser physics and condensed matter research.
There is a substantial research effort in laser cooling and trapping of atoms, the field that garnered the 1997 Nobel Prize in Physics. These techniques take advantage of the fact that laser light tuned near an atomic resonance can exert significant forces on atoms. Such radiative forces are used to cool atoms to very low temperatures (< 1 mK) and confine them in a laser trap. Details of these forces and properties of the laser cooled samples are being investigated. In addition, collisions between these extremely cold atoms are being studied in order to shed light on atom-atom interactions in this novel regime. Since the atoms are moving so slowly (< 1 m/s), laser light can be used to control the collisions to a significant extent. These ultracold collisions are important to understand because they can be a density-limiting mechanism for laser cooled samples. Ultracold atoms are also an ideal starting point for photoassociation, a process in which two free atoms absorb a photon and form a long-range excited molecule. This has opened a whole new area in molecular spectroscopy since it allows very high-resolution probing of states which are otherwise inaccessible. Using these techniques, we are learning a great deal about exotic molecular states as well as the long-range interactions between atoms.
A significant multi-investigator effort is now underway to extend laser cooling and trapping techniques, which have been applied so fruitfully to atoms and ions, to molecules. This will allow the investigation of collisions and chemical reactions of molecules in the ultracold regime. Presently, ultracold molecules are being produced by binding together two ultracold atoms via the process of photoassociation. Concurrent with the experimental work, we are also surveying new theoretical concepts. For example, we are initiating both experimental and theoretical projects on potential uses of the STIRAP (Stimulated Raman Adiabatic Passage) process for the production of state-selected ultracold molecules.
Another new collaboration involves experimental and theoretical investigation of ultracold atoms excited near the ionization limit. This work includes both highly excited (Rydberg) atoms and dense plasmas formed at extremely low temperatures. The ultracold Rydberg atom work is focused on atomic interactions and collective behavior, while the plasma efforts include plasma dynamics and recombination. Related work involving interactions between ultracold atoms and ions is described in the section on atomic and ionic collisions.
As theoretical support for our experiments in laser cooling and trapping, we have constructed the ``numerical atom''. This is a system of computer programs that may be used to numerically simulate laser cooling, and laser spectroscopy in general, for an arbitrary atom in an arbitrary light field. Extension of this machinery to the case of molecules is in progress.
It is now possible to build laser systems that can produce focused intensities well over 1017 W/cm2 or electric fields exceeding 1 atomic unit ( 5×109 V/cm). In this regime, the interaction of light with matter is so strong that normal perturbative approaches break down, leading to new and unexpected phenomena. We are currently focusing on the behavior of small molecules in strong laser fields with the ultimate goal of controlling their dynamics.
Modern spectroscopy is much more than the passive observation of energy levels. Taking a much more active approach, experimenters manipulate atoms and molecules into novel situations selected to yield clear answers to important physical questions. Spectroscopic studies at UConn take advantage of the extensive laser facilities available in several different laboratories, spanning the spectrum from the infrared to the far ultraviolet, with a variety of bandwidths and powers.
In atomic physics, projects include studies of highly excited (Rydberg) atoms, as well as a collaborative project with NIST to measure the far-UV 1S-2S interval in atomic helium, a fundamental system that tests the limits of our ability to calculate the effects of quantum electrodynamics in multi-electron systems. Frequency-stabilized diode laser systems have been developed and used for precision two-photon spectroscopy, excited-state atomic collisions, and laser cooling of atoms. Other projects seek to develop new spectroscopic methods that take full advantage of improved technology and ideas.
The study of molecules and interatomic interactions is a particular emphasis at UConn. Current projects include laser spectroscopy and photodynamics of diatomic molecules, photoassociative spectroscopy, laser ionization spectroscopy and laser produced plasmas, optically pumped molecular lasers, studies of transition state dynamics and cluster dynamics, precision measurements in small molecules, and investigations of molecular behavior in very highly excited vibrational and electronic states.
Recent accomplishments include measurements of some of the highest
vibrational levels in various states of both alkali dimers and
molecular hydrogen. This information complements theoretical and
experimental studies of long-range atomic interactions and
photoassociation spectroscopy in traps, completing our picture of the
interaction process all the way from atomic separations of a single
Bohr radius to many thousands of atomic diameters. Ionization
potentials and dissociation energies of basic molecular systems have
been determined at UConn with unprecedented accuracy. Investigations
are underway of optically pumped diatomic lasers and of neutral and
ionized excimer lasers, as well as the underlying basic physics of
dissociative recombination (for example,
Na3+ + e- 
Na2 * + Na) and its inverse,
associative ionization (
Na2 * + Na Na3+ + e-). Triatomic
associative ionization and its energy threshold in the reciprocal
process (
Na2 + Na * Na3+ + e-) are also being studied.
One of the newest efforts is an investigation of coherent optical interference in molecules and of the prospects for "coherent control" of photodissociation. Another new initiative is a coordinated effort to cool and trap molecules, already described.
A new state of matter, a weakly interacting Bose-Einstein condensate (BEC), has been produced in many laboratories worldwide by cooling a dilute gas of alkali atoms to nanoKelvin temperatures. These experiments have prompted an enormous surge of activity. Two distinct directions of theoretical studies have evolved at UConn. First, we have investigated the properties of a BEC. Our most important results to date bear on the nature of a peculiar phase coherence (and its diffusion), analogous to the phase coherence of a laser, which is thought to accompany Bose-Einstein condensation. The second line of research is the investigation of the optical properties and optical response of a BEC. As an example, we have discovered a method to detect the phase coherence of the condensate by driving transitions between two condensates with a laser. Our investigations integrate analytical and numerical methods. We are now in the process of building a powerful hardware and software facility, based on the C++ language, for numerical modeling of the Bose-Einstein condensate.
Another active research area in the department is the investigation of coherent population transfer and coherent control. We are studying variations of the STIRAP technique for coherently exciting an atomic or molecular state with nearly 100% efficiency, including multiphoton variations and a possible extension to ultracold molecule production by stimulated radiative recombination. Related techniques are under development for selective control of molecular ionization and dissociation processes.
A new field of research is Quantum Computing. New advances in the field of quantum information have shown that computational devices based on fundamental quantum principles, such as interference and entanglement, can perform certain tasks considerably more efficiently than any classical computers. In contrast to classical information, quantum states are present in the form of superpositions to create quantum bits, or qubits. The information contained in qubits and entangled states includes the phase, and coupling to the environment causes decoherence and errors. However, recent theoretical developments in quantum error correction have shown that quantum computing can be fault-tolerant. We are starting a project to study a novel approach for implementing quantum logic gates based on ultracold neutral Rydberg atoms. Atoms in an excited Rydberg state have long lifetimes and interact strongly, even at large distances. These properties can be exploited to entangle atoms, while minimizing decoherence due to spontaneous decay of the excited Rydberg atoms. The theoretical studies are closely linked to an experimental effort that will adapt well-developed techniques to realize quantum gates, e.g., optical lattices to control the separation of the atoms, and laser pulses to address individual atoms for the preparation and rotation of single qubits.
Ion-beam particle accelerators spanning the kiloelectron volt (keV) to megaelectron volt (MeV) range are used in ion-atom, ion-molecule and ion-surface collision studies for a variety of basic and applied projects, providing broad experimental experience for graduate students. Collisions of atom and ion beams with atomic and molecular gas targets probe the fundamental interaction mechanisms between individual atoms and molecules. Energy-loss, x-ray and Auger-electron emission spectroscopies probe the quantum states excited in such collisions, and differential cross sections are measured where possible. One project studies common single and double-electron transfer collisions between atomic ions and small-molecule gas targets. In another, Rutherford backscattering of low-Z MeV energy ions is used as a tool for depth-profiling of ion-implanted and other specially prepared surfaces. These are often carried on in collaboration with local industrial laboratories.
In a new project, highly charged ion beams collide with both gases and with clean surfaces under ultrahigh vacuum. This work includes explorations of ion beam etching of diamond and other superhard materials. Slow, highly charged ions have exotic properties compared with neutral atoms. They are normally observed only in the outer atmospheres of stars, in interplanetary space or in energetic plasma fusion devices, but they can now be studied in the laboratory. For example, at the Lawrence Livermore National Laboratory "EBIT" facility, we have measured the x-ray emission spectrum of Bi50 - 71 + ions impacting on a gold surface, and we are studying the nanoscale damage caused by highly charged ion impact on various insulating surfaces.
Another major effort studies ionization and other reactive scattering processes that can occur in collisions of laser-excited atoms and molecules in an atomic beam at thermal energies. Tunable lasers are used to excite the atoms into known quantum states of relatively high internal energy. Diatomic processes investigated include excitation transfer and "laser switched" associative ionization processes (to produce molecular ions plus electrons). In this work new types of collisions with large rate coefficients have been observed, such as triatomic associative ionization between laser-excited sodium atoms and ground-state sodium molecules and selective collisional dissociation of diatomic molecules by laser-excited atoms. A current project is the study of the (highly efficient) formation of molecular ions by associative collisions of sodium atoms in two different laser-excited states. We are now looking at the surprisingly complex low energy (0-4eV) electron spectra associated with the collisional reaction of Na(3p) + Na(3d) atoms (producing Na2+ + e-) along with other accompanying processes such as Penning ionization, photoionization and superelastic (energy gain) scattering of secondary electrons.
A new project involves the simultaneous trapping of ultracold atoms and cold ions in a hybrid trap under high vacuum. The object is to study the efficiency of cooling of the dilute ion cloud by the cold atoms due to polarization forces and to study quantum mechanical mechanisms that may be applicable to ion molecule reactions in space or to collisional perturbations of trapped ions in the context of trapped ion frequency standards or quantum computing.
A collaboration has been started with the NASA/Caltech Jet Propulsion Laboratory in Pasadena, involving measurements of the lifetimes of singly and multiply-charged ions that contribute to optical absorption, emission and energy balance in the interstellar medium, stellar atmospheres, etc. Related possible future work involves measurements of charge exchange and x-ray emission cross sections for solar-wind interactions with comets.
Finally, theoretical studies of transport properties of ions in ultracold gases are underway. The charge transfer cross sections become extremely large at temperatures of a few microKelvin, and the charge mobilities increase sharply in this regime. Intriguing behaviors may result from these phenomena: e.g., a cold gas doped with ions could undergo a transition from a poorly conducting medium to a very good conductor as T is lowered. More exotic behaviors may occur when degenerate ultracold gases like BEC or Fermi gases are considered. Theoretical investigations of their transport properties are being pursued to guide future experiments.
AMO physics has many diverse connections to other fields including astrophysics, plasma physics, chemistry, materials science, medicine, and engineering. At one end of the spectrum, it contributes to very fundamental issues in physics, such as quantum electrodynamics and parity non-conservation. At the other end, it sheds light on real-world issues such as electric lighting and semiconductor processing. The phenomena investigated span a tremendous range of both energies and time scales, and call for the use a variety of techniques, both experimental and theoretical.