The need for femtosecond VUV pulses
Polyatomic molecules exhibit multi-configurational electronic, vibrational and rotational dynamics, as well as couplings between these. This molecular physics is driven by the nature and dynamics of the transition states. One way to investigate these processes, whose typical timescale is 100 fs, is to perform femtosecond pump probe studies of gas phase unimolecular reactions. In such a scheme a visible or UV femtosecond pump pulse excites the considered neutral molecule, e.g. initiating dissociation. A probe pulse then ionizes the reaction products which are detected by photoelectron or mass spectroscopy. Such gas phase experiments provide the standard against which high level ab-initio calculations are evaluated.
However there are many interesting dynamics that cannot presently be investigated in such experiments. In particular, many reactions involve photodissociations yielding to atomic fragments or light radicals. Such products have high ionization potentials and cannot be ionized in the visible-UV range. These experiments require a source of femtosecond pulses in the VUV.
Getting femtosecond VUV pulses from HHG
High-order harmonic generation produces a comb of odd harmonics of the fundamental frequency. Starting from a 25 fs laser pulse, each harmonic is emitted during about 10-15 fs. Using a low dispersion monochromator, it is possible to select a given harmonic while keeping a pulse duration below 100 fs. This source is naturally tunable by changing the order of the selected harmonic. It is thus possible to selectively ionize a given species.
Velocity Map Imaging
The method of choice to detect fragments, ions, electrons, and their angular distributions is to use a Velocity Map Imaging spectrometer. Such a spectrometer is extensively used at LCAR, and a new one is under development at CELIA. Other detectors (time-of-flight, magnetic bottle) are also available.