EFFECT OF GAS-PHASE COLLISIONS IN PULSED-LASER DESORPTION - A 3-DIMENSIONAL MONTE-CARLO SIMULATION STUDY

The gas flow of particles laser desorbed from an elemental target into a vacuum is studied by Monte Carlo simulation. Pulsed desorption off a finite area is modeled; this is possible by using a three-dimensional simulation algorithm. We monitor the temporal evolution of the desorption cloud and global features of the flow, such as the number of collisions occurring in the gas, and the fraction of particles backscattered to the surface. The angle and energy distribution of the desorbed particles is investigated as a function of the number of monolayers desorbed, and the laser spot width. Our results show the formation of a desorption jet, in which fast particles are focused towards the jet axis, while slow particles leave the jet at oblique angles. Many features of the particle flux may be fitted by so-called elliptical distributions. However, these represent the velocity distribution of particles at oblique angles only poorly. Finally, we demonstrate the differences which exist between our three-dimensional simulation and previous, one-dimensional treatments. These studies are of fundamental interest for the characteristics of thin films deposited from the desorbed material.