Direct detection of delayed high energy electrons from the ¹⁸¹Ta target irradiated by a moderate intensity femtosecond laser pulse

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.