Superthermal Energy of Nascent Atomic Gold in Pulsed Laser Sputtering

In an earlier work it was reported that the nascent monatomic flux of gold, sputtered from the bulk with a KrF (λ=248 nm, τp=16 ns) excimer laser at fluences below plasma ignition, had a mean translational energy much too high to reconcile with the classical thermal model of evaporation. The present paper reports on a new study that explores further this paradoxical finding. The classical thermal evaporation model predicts that the mean energy of the nascent atomic flux is dictated by the surface temperature of the target. In the present study, energy is delivered to the target by steady-state electron beam heating of the back surface and pulsed laser heating of the front surface. For a peak surface temperature of ∼2700 K, the Au translational energy remains 2-3x higher than classical expectations and is insensitive to how energy is partitioned between steady-state and laser heating. Desorption energy appears linearly dependent on peak surface temperature, but having a slope of 15kB as opposed to the classical 2kB.