My research activities have spanned a broad range of subjects.  I began my career examining problems in photoionization of molecules, many-body theory and electron scattering.  When I arrived at Los Alamos in 1972, I was struck by the incredible amount of computer power at the laboratory and how that power could be used to solve complex problems in atomic and molecular physics.  This was an opportunity that a young scientist could not resist and I began to concentrate on problems that required the development of methods and techniques that could be used in conjunction with these powerful computer resources to solve problems that did not yield to analytic solutions.  Some of this early work included studies of the electronic structure of rare gas eximer molecules (with James Cohen) and the scattering of electrons from simple molecules.  I developed a molecular R-matrix approach that enabled the calculation of elastic scattering from simple diatomic molecules.  The ideas for the work came from both the theoretical chemistry and atomic physics communities but with some novel twists.  The method was also employed very successfully to study the resonant vibrational excitation of N_2.  When vector computers became popular, a variant of this approach (with Lee Collins) was used to continue these studies and also to extend them to electronic excitation.  Eventually some of my colleagues (Tom Rescigno and Bill McCurdy) and I started to explore the Complex Kohn variational approach and applied it very successfully to a number of polyatomic molecules.  Both the R-matrix and Complex Kohn approaches are even today the methods of choice for these scattering problems.  To successfully carry out this work it became necessary to get much more deeply involved in numerical methods and I eventually published a few articles on the computation of certain special functions and the solution of large scale problems in linear algebra using iterative techniques.

 

After leaving Los Alamos and coming to the Physics Division of the National Science Foundation, I took the opportunity (courtesy of Charles Clark) to become a visiting scientist at the National Institute for Standards and Technology.  I became involved with ultra-cold atomic physics and along with colleagues at NIST (David Feder and Nicolai Nygaard) did some very timely work on elementary excitations in ultra-cold gases.  One of the papers, an examination of the creation of solitons and their propagation in the cold-atom media, which was carried out with the experimental group of Bill Phillips, has been cited close to 1,500 since its publication  in 2000.

 

My current research is focused on the interaction of short, intense laser fields with atoms and molecules.  Here it is critical to find very efficient approaches to the solution of the time dependent Schroedinger equation.  With colleagues at Drake University (Klaus Bartschat and Xiaoxu Guan) and the Vienna Institute of Technolgy ( Johannes Feist , Stefan Nagele, Renate Pazourek and Joachim Burgdoerfer), some very powerful approaches have been developed and applied to simple atoms and molecules.  This work has been incredibly fruitful and continues to be the central focus of my current research.  The problems are computationally extremely intensive and can only be solved using the largest and most powerful supercomputers in the world.  The group has been lucky to be able to obtain these resources from the NSF supported XSEDE project.  There is no doubt that without these resources we would have not been able to make the progress we have made in understanding how electron correlation influences and is itself influenced by the intense, short pulse wave packets.  A number of novel effects such as virtual sequential ionization, have been discovered and the numerical work has verified that it is possible to extract information concerning the double, multi-photon ionization of electrons by projecting onto simple coulomb waves, provided the computational region is sufficiently large and the propagation time sufficiently long so that the electrons are far enough away from the nuclei and one another to be so analyzed.  A number of well known theorists were skeptical that this was possible even though theory shows that to be the case.  Please see the XSEDE allocations page  (entry 84) for more information.