Molecular Dynamics for Surface Mass Spectrometry

 

A. Delcorte1*, P. Bertrand1 and B. J. Garrison2

 

1 PCPM, Université catholique de Louvain, Louvain-la-Neuve, Belgium.

2 Chemistry department, Penn State University, University Park, PA.

* Phone: 32-10-473582; fax: 32-10-473452; e-mail: delcorte@pcpm.ucl.ac.be

 

In the last decade, classical molecular dynamics (MD) has become the natural theoretical partner of surface mass spectrometries (SIMS, FABMS, LAMS, MALDI).1,2  MD simulations allow us to treat large number of atoms over a timescale of tens of picoseconds and provide a realistic description of the involved chemistry owing to the advent of reactive potentials such as Brenner’s for hydrocarbon molecules.

In this contribution, we present several case studies involving desorption induced by energetic particles, for which classical MD predicts the properties (nature, energy) of the desorbed species.  Beyond the experimental validation of the model, we provide examples of physically relevant results derived from MD simulations, e.g. the microscopic view of the operating mechanisms and processes they explain in many cases.  The case studies involve keV particle bombardment of organic molecules adsorbed on metal substrates or embedded in a purely organic matrix.  The physical processes explained by the MD model include: (i) the development of an atomic collision cascade in the metallic or the organic medium; (ii) the collisionally or vibrationally-induced fragmentation of organic molecules; (iii) the desorption of intact kilodalton molecules; (iv) the recombination reactions occurring upon molecular ejection; (v) the delayed decomposition of small fragments and non-covalent clusters.

All along the discussion, the simulation results are compared with experimental data. The conclusion highlights the successes of the classical MD approach as well as more challenging issues that have yet to be solved.  The outlook focuses on ongoing works, such as modeling the ejection of large molecules under polyatomic particle impact (C60), and long-run goals, including strategies for the implementation of ionization schemes in the MD model.

 

1. Garrison, B. J.; Delcorte, A.; Krantzman, K. D.; Acc. Chem. Res. 2000, 33, 69.

2. Delcorte, A.; Bertrand, P., Garrison, B.J. J. Phys. Chem. B 2001, 105, 9474.