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Research activities in basic atomic physics during the last two years have been strongly influenced by the establishment of the Lund High-Power Laser Facility [1]. This facility, which was brought into operation at the end of the previous two-year period, has opened doors to new fields of research. This is true in particular for basic atomic physics, but also applies to different areas of, e.g., photochemistry, medical diagnostics and industrial applications.
The High-Power Laser Facility consists of three major laser systems, all operating at 10 Hz repetition rate. The first is a narrow-bandwidth, tunable system with a pulse duration in the nanosecond range. This is designed to be used, in particular, for pulsed laser spectroscopy in the UV and VUV spectral ranges. The second system, being a mode-locked, Q-switched Nd:YAG laser, gives pulses in the range 30 to 300 picoseconds. It can be used either directly or after frequency upconversion in non-linear crystals. Most frequently, however, it is used to pump a short-pulse dye laser. The third, and most exciting of the three systems, is the terawatt laser. This system is based on chirped-pulse amplification in titanium-doped sapphire and provides 150 fs pulses of terawatt power. An upgrade of this system to still higher peak power is in progress. Continuous tunability over a large spectral range can be obtained with a specially designed optical parametric amplifier (OPA), pumped by the terawatt laser [2]. The laser radiation from each system can be sent through special beam ports connecting the different laser rooms and the target area rooms. The lasers can hence be combined in various ways to allow complex experiments to be performed. The layout of the facility is shown in Fig. 1.
Fig. 1. Layout of the nationally available high-power laser facility.
The Facility is nationally available and open for Swedish scientists and their international collaborators. However, it is part of the Division of Atomic Physics, and is operated entirely by the staff of this division. Sometimes, experiments initiated by external groups have also involved the participation of local scientists. Examples of this are the accurate determination of the ionisation potential of the CO molecule using the VUV laser system [3], chemical dynamics studied by means of white-light continuum generation and femtosecond absorption spectrometry [4] and the investigation of molecular fragmentation after multi-electron ionisation, using the terawatt laser [5].
Most of the work using the terawatt laser and part of the work with the picosecond laser has been devoted to the study of high-order harmonic generation in gases and plasmas, to X-ray laser related investigations and to the generation and applications of hard X-rays from laser-produced plasmas. This research is partly performed in international collaborations through our participation in three different European networks. It will be described in the following sections, together with a brief presentation of some experiments on emission spectroscopy of highly ionised species (Sect. A1, A2, A3, A4 ).
Our long tradition in time-resolved laser spectroscopy in the visible and UV spectral regions has been much extended during the last two years through work with the UV/ VUV laser system. A number of investigations of atomic and molecular excited states has been performed. In order to measure very short radiative lifetimes, and to perform spectroscopy in the XUV spectral range, new techniques using the picosecond laser system have been developed. In this way, the temporal and spectral ranges for spectroscopic investigations have been still further extended. This is all described in Section B. In Section C, we present our recent activities using continuous lasers in the visible region. They represent the continued development of high-contrast transmission spectroscopy, and the introduction of a new field of research into the group: the study of collision processes using high-resolution laser spectroscopy.
The activities in theoretical atomic physics, outlined in Section D, have continued along the same path as previously. Oscillator strengths, radiative lifetimes, hyperfine splittings and isotope shifts have been calculated with great accuracy from numerical atomic wavefunctions. The wavefunctions were obtained through large-scale calculations using the multiconfiguration Hartree-Fock approach.
During the past two-year period there has been a very international atmosphere in the basic atomic physics group. Senior scientists, post-doctoral researchers and PhD students from more than twelve different countries have visited the group and participated in experiments or in theoretical investigations. Much of the work in the group has been presented at international conferences on atomic physics, astrophysics, spectroscopy, strong-field interactions and quantum electronics [54]. The work is also described in a number of recent articles of review character [55,60]. During the period, two MSc projects have also been completed [61,62] and one of the students in the group has defended his PhD thesis [63].