Comparison of soft-x-ray spectra from optical-field-ionized plasmas generated with linearly and elliptically polarized femtosecond laser pulses

We compare x-ray spectra from optical-field-ionized plasmas generated by linearly and elliptically polarized 250-fs Ti:sapphire laser pulses. The experiment demonstrates the difference in the spectra resulting from the different electron temperatures under these two conditions. To avoid high-density effects such as electron heating, collisional ionization, or ionization defocusing the plasmas are generated under well-controlled conditions in a low-pressure gas cell. For the gases investigated (He, N₂,{O}₂,$ and ${CH}₄)$ a significantly higher x-ray intensity is observed for elliptically than for linearly polarized light on the majority of lines. This finding is in accord with general theoretical expectations and with the result of simulations. However, for linear polarization, anomalies occur in the relative line intensities of several lines. Possible mechanisms responsible for this effect are discussed.