We describe a laser system capable of producing tunable femtosecond VUV pulses around 100 nm by two-photon near-resonant four-wave difference-frequency mixing in argon. Two colour pump-probe ionization experiments allow the characterization of the VUV pulses as well as time-resolved studies of highly excited molecular systems (CS2 , toluene, HDO) in a supersonic jet.

The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump–probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350–450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive à state has been directly observed for the first time: τD⩽20 fs. For molecular oxygen highly excited in the Schumann–Runge band, a decay time of 40±20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times τD=(1.9±0.1) and τD=(90±20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states.

Summary form only given. It is well established, that the generation of higher harmonics, plasma sources or higher order wave-mixing schemes provide ultrashort VUV-pulses in a broad spectral range on ultrashort timescales. Our scheme, the two-photon near-resonant four-wave difference-frequency mixing (FWDFM) in a noble gas has the unique property to generate continuously tunable subpicosecond VUV pulses down to the 100 nm wavelength region on a high pulse energy level which is sufficient for nearly every common spectroscopic technique. The aim of the present work is to characterize the obtained sub-picosecond VUV-pulses especially at 155 nm, to discuss the actual limitations of our setup and to demonstrate the intrinsic potential of our source by investigating small molecular systems of interest (toluene, CS/sub 2/, NO, ...).

Alkaline-earth fluorides are widely used in optical transmission components in the deep UV as well as for optical coatings because of their wide-band-gap, low refractive indices and hardness. One essential drawback of these materials is the formation of defects (color centers e. g.) during intense irradiation. This limits the lifetime of such optical elements for high power propagation in the UV and reduces the transmission of ultrashort laserpulses nonlinearly. Details of this process, in particular the dynamics during the early stages (formation of self-trappped excitons STE [1]) are therefor of interest.

We report on the MBI User Facility at BESSY Il, presently under construction, which is dedicated to study the dynamics of photo-induced processes by combining laser and synchrotron pulses. In this paper we focus on the synchronization of a modelocked ultrafast Ti:sapphire laser to the Berlin electron storage ring for synchrotron radiation (BESSY). Two different techniques have been applied - one based on a digital phase comparator and the other based on analog high-harmonic mixing. Both schemes may be easily adjusted to either single, multi- or hybrid-bunch operation of the synchrotron. Moreover, the temporal accuracy of the synchronization unit suitably matches the widths of the synchrotron pulses (some ten picoseconds) to be expected at BESSY Il. Therefore, the currently performed test experiments at BESSY I provide the basis for time-resolved photon-induced experiments which combine laser and SR-undulator pulses in a pump-probe scheme at BESSY II. This facility will be available within the first half of 1999.