Spectroscopic observation of highly-charged ions produced by cluster target

High-density nitrogen gas was irradiated with an ultrashort high-intensity pulse laser (pulse width: 1.5 ps, intensity: >1015 W/cm2). It was found that highly charged ions were produced effectively as the gas density became higher. From distribution of population densities of high-lying levels of lithium like ions, the electron temperature in a recombining phase was estimated. This electron temperature together with a calculation using a collisional-radiative model, in which doubly excited levels of beryllium like ions are included, indicated that the electron density was ne=3x1019 cm-3. Moreover, the spectra also showed that population inversions between some excited states of hydrogen-, helium-, lithium like nitrogen ions were generated under this plasma conditions. (author)