We present an alternative explanation of the high-order-harmonic-generation experimental results published recently by Seres et al. [Nature Phys. 6, 455 (2010)]. We show that the physical interpretation can be comprehensively done in the frame of classical theory of high-order harmonic generation without referring to a parametric effect in the XUV domain. The experimental conditions explored by Seres et al. indeed correspond to the case of long-pulse, low-infrared-energy laser beams for which tight focusing is necessary to reach the minimum intensity required for high atomic response. The positive atomic dispersion can compensate for the Gouy phase and explains the behavior of the experimental variation of the harmonic signal presented. We will show that our interpretation explains not only the global behavior of the curves but also the second-order variation of the signal as a function of experimental parameters.

We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond X-ray free electron laser is used to heat a solid, which separates the electron and ion heating timescales. The changes in absorption are measured with an independent probe pulse created through high harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. We compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data, suggestive of a temperature dependant electronic structure in warm dense matter.