The role of unsteady adiabatic expansion in laser sputtering

When the number of monolayers sputtered per particle or per pulse is sufficiently small the emitted particles fly freely from the target surface. For yields comparable to 0.5 monolayer a limited number of gas-phase interactions occurs, leading to a Knudsen layer. As a result the particles develop moderate forward peaking ~cos4 θ) and begin to flow (Mach number ≍ 1). It is common, however, for yields to exceed 0.5 monolayer. We show that the resulting gas-phase interactions cause the Knudsen layer to evolve into an unsteady, adiabatic expansion which is formally like a gun which fires a finite charge into an infinite, one-dimensional barrel. © 1989 SPIE.