Imaging self-sputtering and backscattering from the substrate during pulsed laser deposition of gold

The expansion dynamics of the plasma generated during pulsed laser deposition of gold in vacuum has been investigated at the laser fluences of 2.5, 6.0, and 9.0 J cm−2. A severe distortion of the expansion is observed in the presence of a substrate that is accompanied at 9.0 J cm−2 by the appearance of a secondary plasma front expanding from the substrate surface. Langmuir probe analysis at 9.0 J cm−2 shows that the substrate surface is bombarded by a high transient flux of energetic Au+ ions 3.01019 ions cm−2 s−1 having very large kinetic energies 400 eV. Analysis of the plasma dynamics shows that these observations are consistent with self-sputtering of Au neutrals from the substrate induced by incident Au ions while a fraction of them are backscattered. Self-sputtering is found to be 2 orders of magnitude larger than backscattering. The comparison with experimental data allows concluding that the apparent recoil of the plasma front is caused by collision with self-sputtered neutrals, while the secondary emission is originated by backscattered ions.