Understanding Gold-Thiolate Cluster Emission from Self-assembled Monolayers upon Kiloelectronvolt Ion Bombardment B. Arezki, † A. Delcorte,* ,† B. J. Garrison, ‡ and P. Bertrand † UniVersite ´ Catholique de LouVain, PCPM, Croix du Sud, 1-B1348 LouVain-la-NeuVe, Belgium, and Department of Chemistry, 104 Chemistry Building, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: September 14, 2005; In Final Form: December 23, 2005 This article focuses on the emission of organometallic clusters upon kiloelectronvolt ion bombardment of self-assembled monolayers. It is particularly relevant for the elucidation of the physical processes underlying secondary ion mass spectrometry (SIMS). The experimental system, an overlayer of octanethiols on gold, was modeled by classical molecular dynamics, using a hydrocarbon potential involving bonding and nonbonding interactions (AIREBO). To validate the model, the calculated mass and energy distributions of sputtered atoms and molecules were compared to experimental data. Our key finding concerns the emission mechanism of large clusters of the form M x Au y up to M 6 Au 5 (where M is the thiolate molecule), which were not observed under sub-kiloelectronvolt projectile bombardment. Statistically, they are predominantly formed in high- yield events, where many atoms, fragments, and (supra)molecular species are desorbed from the surface. From the microscopic viewpoint, these high-yield events mostly stem from the confinement of the projectile and recoil atom energies in a finite microvolume of the sample surface. As a result of the high local energy density, molecular aggregates desorb from an overheated liquidlike region surrounding the impact point of the projectile. 1. Introduction Static secondary ion mass spectrometry (SIMS) is a surface analysis technique with a very large spectrum of application fields, including, for example, geology, materials science, biology, and medecine. 1 To pursue the performance improve- ment (smarter projectiles, enhanced sample preparation proce- dures) and refine the quality of the data interpretation in organic SIMS, a good understanding of molecular desorption is neces- sary. 2 This, however, is a challenging task, because the mechanisms of polyatomic particle sputtering are complex and many-body in nature. For instance, the desorption of fragile (bio)molecules following the impact of particles with energies exceeding largely those of chemical bonds is, at first sight, a surprising and even counter-intuitive observation. Unraveling such, and other, effects is the key to controlling the relevant parameters of (supra)molecular emission from organic samples. In practice, analytical models can explain relatively “simple” sputtering results, 3 but they cannot treat the sequence of many- body interactions leading to the emission of complex polyatomic ensembles. For this purpose, classical molecular dynamics (MD) has now proved to be the method of choice. Thin organic overlayers have been widely investigated using MD simulations. Several studies explain desorption from physisorbed layers including small molecules and polymers on metals. 4-9 However, only a few reports have been devoted to the understanding of particle-induced desorption from chemi- sorbed systems, where molecules are strongly bound to the substrate. 10-12 Self-assembled monolayers (SAMs) constitute good models of those systems because their structure and properties have been characterized by a variety of analytical techniques over the years. 13 One specific effect observed upon energetic ion bombardment of strongly bound molecules is the emission of large numbers of organometallic clusters, M x Me y , where M represents the organic molecule and Me the metal atom. The explanation of this effect remains elusive, especially for large clusters with more than 3-4 constituents. Among the experimental studies carried out to explain sputtering from SAMs, 14-19 little is said concerning the origin of the M x Me y cluster ions observed in the SIMS mass spectra. The first hypotheses were proposed by Tarlov et al. in their pioneering study of various alkanethiols (CH 3 (CH 2 ) n SH) on gold, under Ar + kiloelectronvolt bombardment. 15 The authors suggested that a recombination reaction could explain the formation of M 2 Au - (M ) CH 3 (CH 2 ) n S), the largest cluster ion they observed at that time. A similar aggregation process has been proposed recently in another ToF-SIMS study to explain the emission of higher mass gold-thiolate cluster ions. 18 In our previous experimental contributions, the kinetic energy distribu- tions (KEDs) of gold-thiolate cluster ions have shown that collisional processes are involved in their emission. 20-22 More- over, the metastable decay of M x Au y clusters in the acceleration section of the spectrometer has been demonstrated, indicating that other processes besides just recombination influence the measured mass distributions of clusters. Concerning the specific processes leading to cluster formation in the surface region, however, experimental methods such as SIMS and SNMS may provide some pieces of the puzzle but not the complete explanation. From the theoretical viewpoint, Liu et al. have reported the results of a detailed MD study involving alkane- thiols on gold under 700 eV Ar bombardment. 10-12 In their * To whom correspondence should be addressed. Tel: 3210473582. Fax: 3210473452. E-mail: delcorte@pcpm.ucl.ac.be. † Universite ´ Catholique de Louvain. ‡ The Pennsylvania State University. 6832 J. Phys. Chem. B 2006, 110, 6832-6840 10.1021/jp058252f CCC: $33.50 © 2006 American Chemical Society Published on Web 03/10/2006