Atoms, clusters and photons: Energetic probes for mass spectrometry

doi:10.1016/j.apsusc.2006.02.085

Applied Surface Science

Volume 252, Issue 19, 30 July 2006, Pages 6409-6412

https://doi.org/10.1016/j.apsusc.2006.02.085 Get rights and content

Abstract

The physical mechanisms underlying the surface based mass spectrometry techniques of atomic SIMS, MALDI and cluster SIMS are discussed along with the relation of the physics to the measured quantities. In particular, there are at least two types of motion resulting from cluster bombardment in SIMS. One scenario involves the individual atoms in the cluster initiating collision cascades similar to atomic bombardment. The second mechanism involves a mesoscale motion of the cluster as a whole. This mesoscale motion can induce an organized flow of the ejected material in a plume.

Introduction

The surface based mass spectrometry techniques, secondary ion mass spectrometry (SIMS) and matrix assisted laser desorption/ionization (MALDI), involve fast energy deposition processes at surfaces. In the case of SIMS, the energy is deposited by a particle with several 1000s of eV of kinetic energy. For MALDI, the energy is deposited via a pulsed laser. Even though the two techniques may appear similar, the physics driving the ejection process in each technique is quite different. In the case of SIMS, the motion of the atoms and molecules can be described by a sophisticated billiards or pool game [1], [2]. As in a game of pool, the atomic motions following each ion event are variable. Laser ablation, i.e., MALDI, involves a phase transition of the material from a superheated liquid to a mixture of gaseous molecules and clusters of molecules [3], [4], [5]. The motion of particles following the phase transition is very organized and the material ablates in a plume.

The advent of cluster projectiles for use in SIMS and the corresponding computer simulations [6], [7], [8] open the question of whether cluster bombardment in SIMS should be discussed in the language of a collision cascade or whether the organized motion concepts from MALDI are more applicable. The motion giving rise to the crater formation [9] in cluster SIMS certainly implies a level of organized mesoscale motion. If cluster SIMS can have different basic physics than atomic SIMS, then one would expect ramifications on various measured quantities. Below we discuss atomic SIMS, MALDI and cluster SIMS and examine how the basic physics influences what is measured.

Section snippets

Atomic bombardment SIMS

The motion initiated in solids due to atomic ion bombardment is best described by a collision cascade although additional physics may be present for condensed rare gas solids [10], [11] or molecular solids [12]. The time scale of the ejection process is a handful to tens of ps. The diameter of action in the surface can range from very small to several tens of nm. The implantation depth of a 15 keV atomic projectile can be up to 30 nm in materials:

•
The number of particles ejected per incident ion

Conclusions

Cluster bombardment has introduced the potential of new physics to the SIMS process. By understanding the new physics and its consequences, the community will be better positioned to exploit the full potential of the technique.

Acknowledgements

The authors acknowledges the National Science Foundation through the Chemistry Division and the Information Technology Research Program for financial support of this work. Helpful discussions with a number of people including but not limited to Nick Winograd, Zbigniew Postawa, John Vickerman, Leonid Zhigilei, Andreas Wucher, Igor Wojciechowski, Arnaud Delcorte, Mike Russo and Kate Ryan are gratefully acknowledged. The Academic Services and Emerging Technologies group at Penn State University

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Atoms, clusters and photons: Energetic probes for mass spectrometry

Abstract

Introduction

Section snippets

Atomic bombardment SIMS

Conclusions

Acknowledgements

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