Efficient electron heating in nitrogen clusters irradiated with intense femtosecond laser pulses

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Properties of nitrogen cluster plasmas produced by an intense, ultrashort laser pulse have been investigated numerically and experimentally. The classical dynamics simulations show that on increasing the cluster size a plasma with residual electron energy above $1\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ can be created due to collisional heating, which is considerably higher than the value obtained with a conventional low-density gas target. Experimentally, nitrogen gas jets created by two types of nozzles were irradiated with a laser pulse of $55\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, up to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. A seeded gas jet consisting of nitrogen and helium was also employed to promote the production of large clusters. The influences of the shape of nozzle, the seeded gas, and the gas jet stagnation pressure on the properties of plasmas were examined by spectroscopic observations. $K$-shell emissions showed that for the gas jet using the conical nozzle the electrons underwent intense collisional heating within the large clusters, resulting in the production of highly charged ions. In contrast, the emissions observed with the capillary nozzle exhibited the characteristics of a cold plasma without suffering substantial electron heating, indicating the absence of large clusters. That is, the differences between the two types of nozzles in the efficiency of electron heating and subsequent residual energies after the passage of the laser pulse, which are strongly dependent upon the cluster size, drastically changed the properties of the produced plasmas. The reason that for the capillary gas jet the plasma density deduced from the recombination spectra was significantly higher than the value obtained using the conical nozzle is also given by the difference in residual electron energy.