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SIMS21, Poland 2017 - Stanislav V Verkhoturov abstract

Stanislav V Verkhoturov oral presentation (PB1-Mon3-1-5)

“Trampoline” Ejection of Organic Molecules from Graphene via keV Cluster Ions Impacts

Stanislav V Verkhoturov1, Mikołaj Gołuński2, Dmitriy S Verkhoturov1, Sheng Geng1, Zbigniew Postawa2, Emile A Schweikert1

1 Texas A&M University, University Dr, TX 77843 College Station, United States
2 Jagiellonian University, ul. Lojasiewicza, 30-348 Kraków, Poland

We present data on the ejection of molecules and emission of molecular ions stimulated by single impacts of 50 keV C602+ on molecular layer of Phenylalanine deposited on free standing 2 layer graphene. The projectile impacts on the graphene side stimulate the abundant ejection of intact molecules and the emission of molecular ions in transmission direction. To gain insight into the mechanism of ejection, Molecular Dynamic (MD) simulations were performed.

The experiments were run with a custom-built Cluster ToF SIMS instrument [1]. The cluster ion source generates the 50 keV C602+ projectiles which impact the back side of the thin target (e.g. graphene) at the angle of incidence of 0° from the normal. This setup is used for detection of secondary ions which are emitted in transmission direction. The layer of deuterated phenylalanine (D8Phe) molecules was vapor deposited (~50 nm per minute) on the surface of graphene. The degree of coverage was ~90% for the molecular layer of ~1 nm. MD simulations use a Phe monolayer (thickness 1.1 nm) consisting of 5 013 molecules. The layer “is deposited” on graphene, and then re-equilibrated to achieve the configuration with a minimal potential energy. The ReaxFF potential is splined at a short distance with the ZBL potential to properly describe high energy collisions among all atoms in the system.

MD simulations show that the projectile impact stimulates the collective radial movement of analyte atoms, which compresses the Phe layer radially from the hole. At the same time, this compression pushes the graphene membrane down. There is bending and stretching around the hole thus accumulating potential energy. The accumulated potential energy is transformed into the kinetic energy of correlated movement upward for membrane atoms, thus the membrane acts as a trampoline for the molecules.

The measured mass spectra contain peaks of deprotonated molecular ions of Phe (D8Phe-H)- with a yield of 0.1 ions/impact. {(D8Phe)- not detected}. The yield of intact neutral molecules of Phe computed by MD is 9 molecules/impact. We infer ionization probability (PI) of ~0.01. A high PI=0.1 is observed for the molecular fragment CN.

The proposed mechanism of ionization involves tunneling of electrons from the vibrationally exited area around the hole to the molecules. The ionized Phe molecules experience a prompt fragmentation to (D8Phe-H)-. Another proposed mechanism is a direct proton transfer exchange: The ions of molecular fragments (i.e CN-) generated in the impact area, interact with intact molecules from the rim of this area. There is a direct proton exchange process for the system Phe molecule + CN- (energy of 15.2 eV for CN- protonation toward energy of 14.75 eV for molecule deprotonation).

This work was supported by NSF, Grant CHE-1308312 and NCN 2015/19/B/ST4/01892.

[1] S.V. Verkhoturov et al, J Chem Phys, 146 (8), 2017, p. 084308