The computer simulation of cluster induced desorption of molecules

摘要

Molecular dynamics computer simulation has been used to investigate the desorption phenomenon that takes place when an energetic molecule or cluster impacts a solid surface. The use of clusters and molecules in secondary ion mass spectrometry (SIMS) is growing as it is becoming evident that the yield of ionised particles is high by comparison with the more conventional single ion sputtering beam. It has also been noticed that there is a much higher desorption of intact molecular ions ejected from the surface. Computer simulations have demonstrated why this should happen. The cluster impact stimulates correlated motion in the surface of the target which encourages the gentle lift off of large molecular species from the target surface. The technique is finding particular uses in allowing SIMS techniques to be used on large organic molecules. What is less known at this point is the efficiency of the cluster beam in lifting off such molecules and the potential for the cluster to break the molecule if it hits it directly. The work presented here shows some early results from a systematic study of the desorption mechanism from a set of simulations of impact points close to and far from an adsorbed molecule. As expected the closer to the adsorbed molecule the cluster impacts the higher the probability for desorption. If the cluster impacts the adsorbed molecule it damages it as expected. However, there is a region such that the cluster does not strike the adsorbed molecule but within which the adsorbed molecule does not get desorbed, but remains intact and still bound substantially to the surface. At first sight this seems to be surprising, but the simulations show that in this region the surface of the target becomes substantially damaged and the broken surface bonds provide a strong capture site for the adsorbed molecule, bonding it more securely to the surface and "pulling" towards the impact site. An energetic particle exchange mechanism is shown in which a sputtered surface particle exchanges with an adsorbate atom in a dynamic collisional process. Descriptions of these processes and the behaviour of the size of this zone are given as a function of the initial cluster energy.