Investigation of characteristic x-rays and hot-electron temperature in the interaction of normally incident femtosecond laser pulses with nanostructured foils

Enhancement of K_α emission from a copper foil covered with micron-sized nanorods with a high aspect ratio is experimentally obtained for perpendicularly incident femtosecond laser pulses. Hot-electron temperature was determined simultaneously with characteristic x-ray measurements. A model of the nanorods' transformation by an amplified spontaneous emission prepulse is suggested. Modeling of the K_α yield and hot-electron temperature demonstrates good agreement with the measured values under the assumption that the nanorods are transformed into hemispherical clusters by the prepulse. It is revealed that an increase in K_α yield is caused by an increased number of hot electrons at the clustered surface. To increase the conversion efficiencies of the laser pulse energy into hot electrons and x-rays, the foil covered with tightly arranged spherical clusters of optimal size should be used. (papers)