Primary and secondary mechanisms in laser-pulse sputtering

Primary sputtering mechanisms are conventionally grouped in terms of the categories collisional, thermal, and electronic. With pulsed photons one must, in addition, consider the emission of droplets and fragments in thermomechanical processes. Pulsed photons also lead to the situation that the density of emitted particles is sufficiently high for gas-dynamic effects to enter and for the system therefore to lose memory of the primary mechanism. One then distinguishes secondary mechanisms which include outflow, as when a finite reservoir expands, effusion, effusive release from the outer surface without recondensation, and recondensation, effusive release with recondensation. If the photon pulse interacts with the emitted particles then still further secondary mechanisms are relevant due to energy deposition in the plume of emitted particles as well as due to ionization. Finally, the laser-pulse sputtering of the polymer PMMA (polymethylmethacrylate) and the superconductor YBCO (YBa2Cu3O7-x) is discussed on the basis of explicit photographs of the sputtered particles. In the case of PMMA there are two groups of particles, the first group having primary and secondary mechanisms which are presently unestablishable but the second group being reasonably attributed to thermal primary release and to secondary behavior of the effusion (or recondensation) type. In the case of YBCO there is only one group of particles having a primary mechanism which is almost certainly electronic and a secondary mechanism which is tentatively identified with outflow.