Recombination lasers in short-pulse laser induced plasmas

Summary form only given. With intense short laser pulses highly ionized plasmas can be generated that allow the realization of lasers in the XUV spectral range. The needed population inversion in the studied systems is generated by the recombination of ions and electrons. Here we will present investigations on plasmas, starting from gaseous targets (gas jet) and targets consisting of particles and clusters produced by ablation mechanisms. For the ionization a 100 fs titanium sapphire laser is used, while the ablation from solid targets is performed by a ns Nd:YAG laser. It is shown that for atomic gas targets optimum conditions for possible lasing transitions are obtained with the shortest pump pulses (100 fs). In contrast, particle and cluster targets require longer pulses (ps-range). This indicates different involved ionization mechanisms like multiphoton- and optical field ionization (OFI) for the short pulses and cluster ionization with Coulomb explosion for the longer pump pulses. Specifically, short laser pulse ionization of oxygen gas and low ionized lithium ablation preplasmas are compared. For the O²⁺(2p3s → 2p²) transition at 37.4 nm, where gain was already observed, the line intensity strongly decreases with longer laser pulse duration (at a constant pulse energy), while for the Li²⁺(3d → 2p) transition at 72.9 nm the line intensity increases with longer laser pulses. This is attributed to cluster ionization effects in lithium plasmas. Experimental attempts to obtain gain at 72.9 nm will be presented.