The central activities of the research group at the Laser Centre VU Amsterdam are the development of new laser tools, directed at opening of new wavelength regimes for sources of narrowband laser radiation, and the application of these sources in the spectroscopy and dynamics of molecules. The narrowband extreme ultraviolet laser source, delivering Fourier-transform limited pulses at 5 ns (bandwidth 300 MHz) in the range 80-100 nm is one example. Recently the development of a source delivering XUV pulses in the range 45-110 nm for pulses of 300 ps was successfully completed. This source can now be made available for spectroscopic studies to be carried out in the framework of atmospheric physics within the network. Furthermore efforts are under way to built a XUV source at a repetition rate of 10 kHz. Narrowband sources in the visible wavelength domain have been developed based on optical parametric oscillators. Continuous wave radiation has been made available in the ultraviolet range by building external cavities for frequency-doubling and mixing. A second activity entails the construction of cavity ring-down spectrometers based on pulsed laser radiation.

One scientific project focuses on the spectroscopy and dynamics of the excited states of molecular nitrogen. The N2 molecule is the major constituent of the Earth's atmosphere. N2 at high altitudes in the thermosphere shields the lower regions of the atmosphere and the earth surface from extreme ultraviolet radiation; the absorption of XUV occurs through a highly complex line spectrum, rather than via dissociative continua. Due to the fact that sophisticated narrowband laser sources until recently were not available in the XUV-domain many important molecular parameters are not accurately known: absorption cross sections, information on decay dynamics of excited states, particularly the competition between radiative and (pre)-dissociative decay, branching ratios and the production of activated atomic species. We have developed a narrowband and tunable XUV-laser source with unique properties that is now available for such detailed and accurate studies on molecular nitrogen. Also, by applying XUV-multiple resonance laser excitation we propose to investigate excited states of `gerade" symmetry, that play a role in collisionally induced emissions in the visible and near-uv spectral range. These states are virtually unexplored. Study of N2 also bears relevance for the Cassini mission, currently under way towards Titan, a moon of Saturn with an atmosphere mainly composed of molecular nitrogen. This project receives funding from the Netherlands Foundation for Fundamental Research of Matter (FOM) within the program of Molecular Atmospheric Physics (MAP).

A second atmospheric physics related activity delas with Collisions, Complexes and reactive internediates in the Earth's atmosphere. The issue of absorption of solar radiation by molecular complexes, either in the form of short-lived collisional complexes or in the form of stable VanderWaals clusters is an important one in view of the so-called missing part in the radiation budget of the Earth's atmosphere for about 25-30 W/m2, which is sometimes ascribed to the effect of complexes. Particularly water vapour dimers are held responsible for some missing fraction of the absorption budget. Furthermore, statistical thermodynamic calculations yield the result that at 2 degrees global warming there is a significant non-linear increase in absorption by water clusters, not only dimers, but also multi-component cyclic water-structures. On the other hand collisional-induced absorption resonances are identified as important actors in the atmosphere. Specific CIA-features are known, in which colliding oxygen molecules absorb a photon jointly, and take away part of the energy (as electronic and vibrational excitation) after the collision. For such resonances we are investigating pressure dependent cross-sections. The oxygen collision-induced resonances also have practical application in atmospheric physics. The intrinsic quadratic pressure dependence of the absorption feature makes it suitable for determining absorption path lengths, when compared with linearly absorbing features. In satellite data retrieval procedures the O4 resonances are being used for determining cloud top heights. The studies on collisonal complexes are funded by SRON and FOM.

Additionally we are initiating in molecular spectrosopic studies of reactives intermediates as occurring in the Earth's atmosphere. In detail we wish to investigate the dissociation properties of the hydroxyl (OH) radical at VUV wavelength. This project is carried out in collaboration with prof. D. Parker and Prof. H. ter Meulen (Nijmegen University) with support from the Molecular Atmospheric Physics program of FOM.