Reaction microscopes enable kinematically complete measurements of atomic and molecular fragmentation. An ultracold atomic target is usually provided by a supersonic gas jet. The apparatus developed in the course of this thesis for the first time combines the principle of the reaction microscope with a magneto-optical trap. This allows for the preparation of lithium atoms in the sub-mK range. Being a three-electron system, its simple atomic structure makes lithium a model system of great topical interest for all kinds of ionization reactions. In order not to deteriorate the determination of the momenta of the charged fragments by the magnetic field of the trap, a pulsed mode of operation is adopted, creating field-free conditions during data acquisition and making efficient use of the stored target. The novel apparatus provides a versatile target for investigations on fragmentation of lithium by electron, ion and photon impact. For the first time, momentum-resolved measurements on multiphoton ionization in intense laser fields with pulse durations of 25~fs and peak intensities in the range between 10
11~W/cm
2 and 10
16~W/cm
2 were performed. The acquired photoelectron spectra exhibit unexpected structures such as a preferred emission in the direction perpendicular to the laser polarization axis, which are not fully understood yet. Presently, corresponding calculations are being done in several theory groups.
Abstract This paper offers a toolbox for characterizing the initial conditions and predicting the evolution of the ultracold plasma that forms after resonant laser preparation of a Rydberg gas entrained in a differentially pumped supersonic molecular beam. The conditions afforded by a skimmed free-jet expansion combined with the geometry of laser excitation, determines the phase-space volume of the excited gas. A hydrodynamic shell model, that accounts for the ellipsoidal spatial distribution of this excitation volume in concert with the deforming effects of dissociative recombination, serves to simulate the ambipolar expansion of this molecular ultracold plasma.