Deep dips in MeV ion spectra are obtained from water droplet targets irradiated by intense [$(0.5–1.2)\times {10}^{19}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$] and ultrashort (35 fs) laser pulses. The existence of these dips is ascribed to the generation of a multielectron-temperature plasma, which is confirmed by our experiments. An existing fluid model based on hot-electron components with significantly different temperatures is consistent with the behavior we observe in the ion spectra of the femtosecond laser-driven interaction. The model provides a good simulation of the observed spectral dips and allows us to establish important parameters such as hot- and cold-electron temperatures and the respective electron density ratios. The result may be of interest for spectral tailoring of proton spectra in future applications of laser-generated proton beams.

• Received 22 September 2003

DOI:https://doi.org/10.1103/PhysRevLett.93.155006