Investigation of laser-irradiated Ar cluster dynamics from K -shell x-ray emission measurements

Intense (up to a few ${10}^{17}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$) femtosecond (down to $40\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$) laser pulses are focused onto a partially clusterized argon gas jet. The target was previously characterized and optimized in order to get a homogeneous and dense jet of clusters with a well controlled size. The interaction leads to x-ray emission that is absolutely calibrated and spectrally resolved using a high resolution time-integrated spectrometer in the $K$-shell range (from $2.9\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}4.3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$). X-ray spectra are investigated as a function of different laser temporal parameters such as the nanosecond prepulse contrast, the laser pulse duration, and the femtosecond delay between two different laser pulses. The cluster size ranges from $180\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}350\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ and irradiation by laser pulses with both linear and circular polarization is investigated. The experimental results are discussed in terms of the laser-cluster interaction dynamics. They are compared with the predictions of collision-dominated nanoplasma models. However, further interaction processes are required in order to explain the observed characteristic lines demonstrating highly charged ions up to ${\mathrm{Ar}}^{16+}$.