Introduction

Our computational research program is aimed at understanding the complex energy transfer processes that occur in the high-energy bombardment of organic solids with atomic and polyatomic projectiles. In the last twenty-five years, molecular dynamics simulations have contributed immensely to our understanding of the processes underlying the bombardment of atomic solids and molecular overlayers on atomic solids, which has been applied to the keV bombardment of solids in secondary ion mass spectrometry (SIMS), fast atom bombardment mass spectrometry and sputtering. Recently, there has been considerable interest in the use of polyatomic projectiles for organic and biological SIMS because they have been shown to increase the secondary ion yield by an order of magnitude or more. Our simulations have provided mechanistic insights into how polyatomics projectiles affect both the yield of desorbed molecules and the damage to the sample as compared to atomic projectiles.

Barbara J. Garrison, Arnaud Delcorte and Kristin D. Krantzman, "Molecule Liftoff from Surfaces", Accts. Chem. Res. 2000, 33, 69-97. [PDF]

Molecular Dynamics Simulations

In our group, we perform experiments on the computer using the method of molecular dynamics simulation. Molecular dynamics is a sophisticated game of billiards where the atoms are the 'balls' and move according to classical equations of motion. The key to building a viable model is to use forces that describe the appropriate chemistry. Animations of the motions of the atoms as they move in time can be created from the simulations. We use the method to study how processes between gaseous atoms and solids occur on a microscopic level.

Barbara J. Garrison, "Molecular Dynamics Simulations of Surface Chemical Reactions", Chem. Soc. Reviews 1992, 121, 155-162. [PDF]

Secondary Ion Mass Spectrometry

Secondary ion mass spectrometry (SIMS) is a technique used to analyze surfaces of solid samples. A high energy primary ion beam, such as Ar+ or Cs+, impacts the surface. The resulting sputtered species are collected and analyzed by a mass spectrometer. The primary ion dose is very low so that each ion essentially impacts a fresh area of the surface. There is considerable interest in the use of polyatomc projectiles (Agn+, SF5+, C60+) because they may increase the secondary ion yield without a comparable increase in the damage to the surface, thereby improving sensitivity.

Collaborators

Arnaud Delcorte, Dept of Materials Science, UCL, Belgium

Professor Barbara J. Garrison, Shapiro Professor of Chemistry, Department of Chemistry, Penn State

Professor Michael L. Myrick, Professor , Department of Chemistry and Associate Director, USC Nanocenter, University of South Carolina

Professor Zbigniew Postawa, Instytut Fizyki, Uniwersytet Jagiellonski, Kraków, Poland

Professor Tracy A. Schoolcraft, Professor of Chemistry, Department of Chemistry, Shippensburg University

Professor Steven J. Stuart, Associate Professor of Chemistry, Department of Chemistry, Clemson University

Professor Nicholas Winograd, Professor of Chemistry, Department of Chemistry, Materials Science Institute, Penn State

Mechanisms for ejection of molecules and molecular fragments

An illustration of the bombardment process is shown in the animation, which can be viewed by clicking on the figure. Here, an SF5 projectile impacts a monolayer of biphenyl molecules on a silicon substrate. The energetic particle strikes the surface and initiates a high degree of action in the solid. Collision cascades develop and atoms are displaced from their initial positions. Although the incident energy is much higher than the bond strength between individual atoms in the molecules, intact molecules are ejected from the surface.

Molecules that are hit directly by the bombarding projectile are fragmented. In order for molecules to be ejected intact, they must be gently hit from underlying substrate atoms that lift the molecule off the surface. For large molecules with multiple contact points to the surface, cooperative lifting in which several substrate atoms hit different parts of the molecule is necessary for the ejection of stable, whole molecules. It is the upward motion of the substrate atoms in the top few layers of the substrate that is responsible for the production of ejected intact molecules. Consequently, the downward momentum of the bombarding projectile must be redirected into the upward momentum of the top few layers of substrate atoms.

Enhancement with Polyatomic Projectiles

Our simulations have identified three factors that are important for the enhancement with polyatomic projectiles.
Collaborating collision cascades.

Polyatomic projectiles enhance the emission yield by increasing the probability of collaborating collision cascades. Molecules that have multiple contact points to the surface are ejected intact when several substrate atoms hit different parts of the molecule. The adjacent figures illustrates how a copper dimer initiates adjacent collision cascades in the substrate, resulting in the cooperative uplifting of the biphenyl molecule. Collaborating collision cascades and cooperative uplifting will be more essential to eject intact larger molecules.

Radomir Zaric,*; Brenda Pearson,*; Kristin D. Krantzman and Barbara J. Garrison, "Molecular Dynamics Simulations to Explore the Effect of Projectile Size on the Ejection of Organic Targets from Metal Surfaces", Int. J. Mass Spectrom. Ion Processes 1998, 174, 155-166. [PDF]

Open lattice structure of substrate
0.60 keV SF5 bombardment of a monolayer of biphenyl molecules on Cu(001) Click on the figure below to view an animation.	0.60 keV SF5 bombardment of a monolayer of biphenyl molecules on Si(100) Click on the figure below to view an animation.
The nature of the substrate is also a critical factor for the effectiveness of polyatomic projectiles. The enhancement in yield is greater on a substrate with a more open lattice structure such as silicon than on a more closely packed substrate such as copper. For the close-packed copper substrate, the SF5 breaks apart as it hits the surface and is reflected toward the vacuum. Collision cascades can be generated in the top layer of the surface that result in the ejection of molecules and fragments. With the silicon substrate, on the other hand, the entire SF5 projectile is able to penetrate the surface and break apart within the substrate. The breakup of the cluster within the lattice initiates collision cascades that lead to substrate atoms hitting the biphenyl molecules from below, which results in upward movement of the molecules towards the vacuum. Consequently, a greater number of intact molecules are ejected. Jennifer A. Townes, Anna K. White, Elizabeth N. Wiggins, Kristin D. Krantzman, Barbara J. Garrison, Nicholas Winograd, "Mechanism for Increased Yield with the SF5+ Projectile in Organic SIMS: The Substrate Effect", J. Phys. Chem. A 1999, 24, 4587-4589. [PDF]