Measurement of the energy penetration depth into solid targets irradiated by ultrashort laser pulses

The energy penetration depth of a short (100 fs) Ti-sapphire laser pulse (0 .8 mu m) of intensity 3x10(16) W/cm(2) in solid density materials has been measured. High-Z (BaF2) and low-Z (MgF2) solid layers targets were used. Th e penetration depth was determined from the measurement of the x-ray emissi on spectra, as a function of the target thickness. The investigation of the se spectra showed that in the low-Z case, solid density material to a depth of 50 nm was heated to a peak electron temperature of similar to 150 eV. F or the high-Z material, the penetration depth corresponding to this tempera ture exceeded 100 nm. This is evidence of a larger heat penetration depth i n a high-Z material in comparison to a low-Z material. A model based on ele ctron heat conduction is used to estimate the energy penetration depth. It is suggested that the larger heat penetration in high-Z material is due to heating of the material, caused by the radiation flux, generated by the ele ctron heat conduction.