"Trampoline" ejection of organic molecules from graphene and graphite via keV cluster ions impacts

SV Verkhoturov and M Golunski and DS Verkhoturov and S Geng and Z Postawa and EA Schweikert, JOURNAL OF CHEMICAL PHYSICS, 148, 144309 (2018).

DOI: 10.1063/1.5021352

We present the data on ejection of molecules and emission of molecular ions caused by single impacts of 50 keV C-60(2+) on a molecular layer of deuterated phenylalanine (D8Phe) 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 the transmission direction. To gain insight into the mechanism of ejection, Molecular Dynamic simulations were performed. It was found that the projectile penetrates the thin layer of graphene, partially depositing the projectile's kinetic energy, and molecules are ejected from the hot area around the hole that is made by the projectile. The yield, Y, of negative ions of deprotonated phenylalanine, (D8Phe-H)(-), emitted in the transmission direction is 0.1 ions per projectile impact. To characterize the ejection and ionization of molecules, we have performed the experiments on emission of (D8Phe-H)(-) from the surface of bulk D8Phe (Y = 0.13) and from the single molecular layer of D8Phe deposited on bulk pyrolytic graphite (Y = 0.15). We show that, despite the similar yields of molecular ions, the scenario of the energy deposition and ejection of molecules is different for the case of graphene due to the confined volume of projectile-analyte interaction. The projectile impact on the graphene-D8Phe sample stimulates the collective radial movement of analyte atoms, which compresses the D8Phe layer radially from the hole. At the same time, this compression bends and stretches the graphene membrane 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 ejected molecules are effectively ionized; the ionization probability is similar to 30X higher compared to that obtained for the bulk D8Phe target. The proposed mechanism of ionization involves tunneling of electrons from the vibrationally excited area around the hole to the molecules. Another proposed mechanism is a direct proton transfer exchange, which is suitable for a bulk target: 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 D8Phe molecule + CN-. Published by AIP Publishing.

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