Formation of the ion current of a high-temperature femtosecond laser plasma on the target surface containing an impurity layer

The effect of an impurity film lying on the surface of a target in a vacuum of up to 10^-5 Torr on the ion acceleration in a plasma, which is formed by a 2 × 10¹⁶-W cm^-2 femtosecond laser pulse is studied using the time-of-flight and mass spectrometry techniques. It is shown that, under such conditions, the maximum mean energy per unit charge (8 keV) is gained by protons, whereas the ions constituting the target substance (Si, Ti) acquire an energy per unit charge of <1 keV. The use of a <10 J cm^-2 nanosecond laser pulse applied 0.1–100 ms before a femtosecond laser pulse allows the target surface to be efficiently cleaned due to the removal of H-, C-, and O-containing molecules from it. Unlike a continuous thermal heating of a surface, the pulse laser cleaning ensures higher heating temperatures and can be efficiently applied to any solid targets in both the thermal and plasma cleaning modes.