| For ZEKE spectroscopy the experimental conditions are usually set to produce an ensemble of only weakly interacting, very high-n Rydberg states at low density (typically < 105/cm3), very far away from plasma conditions. In contrast, the formation of a Rydberg plasma by ZEKE threshold excitation can be expected when increasing the ion/electron density. We have produced such an ultra-cold molecular Rydberg plasma with ion densities around 1015/cm3 for two molecules, para-difluorobenzene (pDFB) and nitric oxide. For NO (in Ne as carrier gas) we produce such an ultra-cold, very long lifetime Rydberg plasma by two-photon excitation into the threshold region, resonant via the A-state (N=0), in the expansion region of a supersonic jet close to the nozzle. Collisions in the jet expansion cool the ions in the plasma to ca. 0.1K thus avoiding disorder heating as in MOT Rydberg plasma experiments. After 320 s, when the plasma hits a 4mm diam. aperture, a dramatic increase in density is observed experimentally just behind this aperture. These observations can be explained by a compression of the plasma, which exhibits the properties of a sponge like material, i.e. a plasma crystal. When the residual plasma after passing this aperture interacts with a grid at a distance of 30 mm it is observed that this residual plasma is decelerated, stopped and reflected. This can be explained by a polarization of this plasma with the negatively charged part being trapped on the grid. This non-moving negatively charged cloud then decelerates the positively charged cloud. These observations substantiate the evidence that this experimental route creates a strongly-coupled plasma with condensed phase properties. |
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