Invited Paper AS-TuA1
Collective Action, the Key to Soft Molecule Desorption under Particle Bombardment

Tuesday, October 23, 2018, 2:20 pm, Room 204

The focus of this contribution is on desorption of large non-volatile organic molecules induced by ion beams, for 2D and 3D analysis by secondary ion mass spectrometry (SIMS). It is also the story of my long-term collaboration with Pr. Barbara Garrison since 1999, when I started my post-doc under her guidance. Our first molecular dynamics (MD) simulations of polystyrene emission from silver induced by atomic projectiles gave us direction: in order to emit large molecules with minimal internal energy, collective action of the substrate atoms was required, which only occurred in so-called high (sputter) yield events where most of the projectile energy was deposited in the extreme surface [1]. These high yield events, rare with atomic ions, happened to be the rule with cluster projectiles such as C₆₀, where the collision cascades fully overlapped [2]. The use of C₆₀ therefore led to remarkable sputtering yield enhancements for organic samples, and much improved emission of molecular ions in SIMS [3]. Clusters were the solution. We then naturally moved to larger clusters, where the physics shifted again, to macroscopic-like impacts phenomena [4]. Our simulations using large organic and argon clusters showed similar trends and, in particular, they quantitatively predicted the experimental ‘universal’ sputtering curves for the Ar cluster bombardment of organics [5]. The simulations also uncovered a region of energy, scaled by the number of atoms in the projectile (E/n), where fragmentation was minimized and intact molecular emission, maximized. All of this could not have been achieved without the great collective spirit and action that were maintained by Pr. Garrison in her ‘extended team’ over the years.

[1] A. Delcorte and B. J. Garrison, High Yield Events of Molecular Emission Induced by Kiloelectronvolt Particle Bombardment, J. Phys. Chem. B2000, 104, 6785.

[2] A. Delcorte and B. J. Garrison, Sputtering Polymers with Buckminsterfullerene Projectiles: A Coarse-Grain Molecular Dynamics Study, J. Phys. Chem. C2007, 111, 15312.

[3] D. Weibel, S. Wong, N. Lockyer, P. Blenkinsopp, R. Hill, J. C. Vickerman, A C60 Primary Ion Beam System for Time of Flight Secondary Ion Mass Spectrometry: Its Development and Secondary Ion Yield Characteristics, Anal. Chem. 2003, 75, 1754.

[4] A. Delcorte, B. J. Garrison, K. Hamraoui, Dynamics of Molecular Impacts on Soft Materials: From Fullerenes to Organic Nanodrops, Anal. Chem.2009, 81, 6676.

[5] A. Delcorte and M. Debongnie, Macromolecular Sample Sputtering by Large Ar and CH4 Clusters: Elucidating Chain Size and Projectile Effects with Molecular Dynamics, J. Phys. Chem. C 2015, 119, 25868.