Ultra-fast atomic dynamics in arbitrary light fields

State of the art and motivations Over the last decade, great experimental progress has been made in understanding the ultra-fast electron dynamics occurring inside an atom [1]. The typical time it takes an electron to go round a H nucleus is 150 attosecond (1.5 × 10−16 s). To capture motion on this timescale, it is necessary to have a time resolution much shorter. Recently, ultra-short light pulses have been generated which achieve this aim, with a record pulse duration of 67 as [2]. This type of light pulse has enabled researchers to explore new science, including, for example, establising a time delay of about 20 as between photoemission of a 2s or a 2p electron from Ne [3]. In-depth experimental understanding can only be obtained through comparison with accurate theoretical calculations. Over the last 10 years, at Queen’s University Belfast, we have developed time-dependent R-matrix theory and associated codes precisely for this purpose [4]. This theory is unique in the world in that it can be applied to any atomic system, and is capable of generating highly accurate data at a wide range of laser light of experimental interest, ranging from XUV to computationally demanding mid-IR light. Figure 1 demonstrates this capability by showing a harmonic spectrum of Kr, obtained by a QUB Ph.D. student [5]. In harmonic generation, a single high-energy photon is created from the energy of many (> 250) photons of mid-IR laser light. In order to maintain the QUB codes at the forefront of science, it is important that the capa-bility of the codes keeps up with the development in experiment. Up to now, most experiments have been performed with linearly polarized laser light, but interest is increasingly focused on laser fields with arbitrary polarisation. By using arbitrary polarisation, experiment can gain additional control over the ultra-fast electronic motion, and thereby exploit this dynamics to develop new technology. One recent example of this is the possibility to generate circularly po-larised harmonics by using two circularly polarised laser fields. This may lead to the development of XUV lasers with circularly polarised light, which are currently not available