Hot-electron generation in copper and photopumping of cobalt

Hot electrons generated upon interaction of p-polarized 130 fs laser pulses with copper and penetrating into the target material are characterized with respect to their energy distribution and directionality. “Experimental” data are obtained by comparing the rear-side x-ray emission from layered targets with Monte Carlo electron-photon transport simulations. Theoretical electron energy distributions are derived by means of a one and a half–dimensional particle-in-cell code. Both sets of data consist of a two-temperature distribution of electrons propagating in a direction almost perpendicular to the target surface. The “experimental” data contain a considerably higher population of the lower temperature electrons. The discrepancy is explained by the intensity distribution of the laser spot. The results are used to design an experiment for demonstrating photopumping of cobalt with copper $K\alpha$ radiation. A 10 μm copper foil is backed with 1 mm of polyethylene (PE) followed by 10 μm of cobalt, the rear-side $K\alpha$ emission of which is measured. The PE layer prevents fast electrons from reaching the cobalt. Comparing the cobalt $K\alpha$ emission with that of nickel, which is not photopumped by copper $K\alpha$ shows enhancement by almost a factor of 2.