We depict an experimental study of delayed fast, negatively charged particles from femtosecond laser-plasma interaction at an intensity of I ∼ 1017 W cm−2. Plates of 2 mm thickness made of 181Ta (∼100% abundance) and natural W were used as targets. We distinguished certain delayed events due to detection of negative H−, C− and O− ions. However, most events which were delayed by 0.5–5 μs with respect to the instantaneous plasma formation caused by the laser pulses, were identified as electrons with energies of 3–7 keV. A comparative analysis between the tantalum and tungsten spectra was undertaken. This revealed a close similarity between the measured spectrum for tantalum and the predicted spectrum for electrons arising from to the internal conversion decay of the 6.237 keV nuclear isomeric state in 181Ta.
Accelerated electrons for in situ peak intensity monitoring of tightly focused femtosecond laser radiation at high intensities