We develop a microscopic model for the interaction of small rare-gas clusters with soft x-ray radiation from a free electron laser. It is shown that, while the overall charging of the clusters is rather low, unexpectedly high atomic charge states can arise due to charge imbalances inside the cluster. These findings are explained by an increased absorption via inverse bremsstrahlung due to high intermediate charge states and by a nonhomogenous charge distribution inside the cluster.

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We demonstrate how the waveform of light can be used to control a molecular dissociation by steering and localization of electrons. Experimental results have been obtained for the dissociative ionization of the homonuclear and heteronuclear Hydrogen derivates D 2 and HD. Asymmetric ejection of the ionic fragments reveals that light-driven electronic motion prior to dissociation localizes the electron on one of the two ions in the diatomic molecular ions in a controlled way. Extension of these results to electron transfer in complex molecules suggests a new paradigm for controlling photochemistry.

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Calculating the energy absorption of atomic clusters as a function of the laser pulse length T we find a maximum for a critical T * . We show that T * can be linked to an optimal cluster radius R * . The existence of this radius can be attributed to the enhanced ionization mechanism originally discovered for diatomic molecules. Our findings indicate that enhanced ionization should be operative for a wide class of rare gas clusters. From a simple Coulomb explosion ansatz, we derive an analytical expression relating the maximum energy release to a suitably scaled expansion time which can be expressed with the pulse length T * .After a basic understanding of the mechanisms governing atoms and molecules subjected to an intense laser pulse [1,2], analogous studies on clusters pioneered by Rhodes [3] and Ditmire [4] have appeared over the last years with a recent spectacular culmination in the demonstration of deuterium fusion in clusters [5]. Most of these studies do focus on the situation after the laser pulse, namely on the abundance and kinetic energy spectra of electrons and ions. Some discussion has been devoted to the question if the expansion of the cluster is driven by hydrodynamics or by a Coulomb explosion. Only very little attention has been paid to this type of dynamics in the time domain [6,7]. This is even more surprising since the time scales involved show that the expansion of the nuclei occurs on the same time scale as the pulse lengths which can be chosen, namely some 10 to 1000 fs, or roughly 10 −3 atomic units (which we will use hereafter). Apart from the nuclear motion and the pulse length T energy absorption from a laser pulse and subsequent ionization and fragmentation of the cluster involve two additional time scales, the optical cycle 2π/ω = 0.055a.u. for the typically used Titan-Sapphire laser of 800 nm wavelength, and the period of the bound electrons, which is of the order (hydrogen) of 1 a.u.. We will work with peak intensities between 10 14 − 10 16 W/cm 2 . In the following we will demonstrate that the seemingly complicated process of energy absorption and fragmentation in the laser pulse can be split into three different phases, an 'atomic ' phase I, a 'molecular' phase II, and a relaxation phase III. Phase I lasts for a time T 0 after the pulse has begun and is characterized by boiling off electrons through multiphoton or tunneling ionization, hence we have termed it 'atomic' phase. We define it to last until every second atom in the cluster has lost one electron, or equivalently until the probability of loosing an electron in an atom has reached p = 1/2. This probability is calculated from a Krainov tunneling rate [8] where, however, the instant electric field is formed by the laser and eventually already existing charged particles in the cluster.Up to T 0 we may assume that the atoms/ions have not moved yet. The second, molecular phase is characterized by Coulomb explosion of the cluster. During this phase, as we will show below, the cluster expands to a critical radius R * which o...