Queen's University Belfast
School of Mathematics and Physics
Department of Applied Mathematics and Theoretical Physics
Centre for Theoretical Atomic, Molecular and Optical Physics
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The development of the laser has had a profound impact on present-day society. An obvious example of the influence of lasers on day-to-day life is the CD as a storage medium for music, computer programs and videos. Nevertheless, many more uses of lasers can be envisaged. To make full use of laser technology, it is important, however, to understand how lasers interact with matter. While developing this understanding is relatively easy at low laser intensities, at (achievable) higher intensities the laser light becomes a strong perturbation of the regular dynamics. At these intensities, the interplay between experimental investigations and theoretical calculations becomes essential to understand, and eventually apply, the behaviour of matter in intense laser fields.
One of the main differences between low and high laser intensities is that at high laser intensities, matter will absorb more than just a single photon or even more than two photons. The figures above describe single photon absorption on the left and three-photon absorption on the right. In order to improve our understanding of light-matter interactions, we must develop an understanding of the absorption of many photons. In the Theoretical and Computational Physics Research Division of the Queen's University of Belfast, we are presently developing several computational approaches to describe such multiphoton processes occurring in atoms and molecules. One approach involves the direct determination of the behaviour of an atom or a molecule by solving the time-dependent equations governing their behaviour. A different approach, the R-matrix Floquet approach, involves transforming the time-dependent equations into a set of time-independent equations, the so-called Floquet-Fourier transformation, which then can be solved by the well-established R-matrix method, for which the Queen's University of Belfast has been known throughout the world.
The R-matrix approach was originally developed to describe electron scattering processes for atoms. Although the theory behind the theoretical determination of scattering cross sections is well known, the description of the interactions between all electrons requires sophisticated computer codes to describe the repulsion between the electrons in detail.
One of the most difficult tasks in the description of atoms and molecules in intense laser fields is to include the effect of the laser field and the repulsion between the electrons on the same footing. Since the description of an atom in the absence of a laser field already requires dedicated programs, it can be appreciated that adding a strong laser field makes the calculations even harder. Even for a simple atom containing no more than two electrons, such as He, this is a task demanding significant computational resources. For systems containing more atoms, time-dependent approaches become unfeasibly demanding. The main advantage that the R-matrix-Floquet approach has over other approaches is that it can be applied in principle to any atomic system.
Here, we hope to give a brief overview of the R-matrix-Floquet approach, and indicate to which types of investigations it has been applied. A literature database is also provided.
Any comments on this page can be sent to h.vanderhart@qub.ac.uk