Department of Chemistry, Texas A&M University, College Station, Texas 77843-3144, USA.
Department of Physics, Jagiellonian University, Kraków, Poland.
J Chem Phys. 2018 Apr 14;148(14):144309. 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 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 ∼30× 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.
我们呈现了 50keV C 单颗粒子撞击沉积在自由支撑的双层石墨烯上的氘代苯丙氨酸（D8Phe）分子层时引起的分子喷射和分子离子发射的数据。在穿透方向上，来自石墨烯一侧的射弹冲击刺激了大量完整分子的喷射和分子离子的发射。为了深入了解喷射机制，我们进行了分子动力学模拟。结果发现，射弹穿透石墨烯薄层，部分沉积了射弹的动能，分子从射弹所造成的孔周围的热区中喷射出来。在穿透方向上发射的去质子化苯丙氨酸（D8Phe-H）的负离子的产额，Y，为每个射弹撞击 0.1 个离子。为了表征分子的喷射和离子化，我们还进行了从大块 D8Phe（Y=0.13）表面和沉积在大块热解石墨上的单个 D8Phe 分子层（Y=0.15）中发射（D8Phe-H）的实验。我们表明，尽管分子离子的产额相似，但由于射弹-分析物相互作用的受限体积，对于石墨烯情况，能量沉积和分子喷射的情景是不同的。射弹对石墨烯-D8Phe 样品的冲击刺激了分析物原子的集体径向运动，从而将 D8Phe 层从孔中径向压缩。同时，这种压缩使孔周围的石墨烯膜弯曲和拉伸，从而积累了势能。积累的势能转化为膜原子向上关联运动的动能，从而使膜成为分子的蹦床。喷射的分子被有效地离子化；与大块 D8Phe 靶相比，离子化概率提高了约 30 倍。所提出的电离机制涉及从孔周围的振动激发区到分子的电子隧穿。另一种提出的机制是直接质子转移交换，它适用于块状靶：在冲击区域中生成的分子碎片（即 CN）的离子与来自该区域边缘的完整分子相互作用。对于系统 D8Phe 分子+CN，存在直接的质子交换过程。
