XPS evidence for negative ion formation in SIMS depth profiling of organic material with cesium

Low-energy cesium ions have been successfully used for depth profiling organic materials, owing to the high reactivity of implanted Cs atoms, acting as strong electron donors. X-ray photoelectron spectroscopy, or XPS, was used in this work on thin phenylalanine films deposited on silicon, before and after different times of sputtering with 250 and 500 eV Cs⁺ ions. First of all, the XPS data validate the ToF-SIMS conclusions that the organic molecules are barely affected by the Cs bombardment, as the chemical functionalities such as the aromatic ring, amine, and carboxylic groups of the phenylalanine molecule were preserved throughout the erosion with Cs. The Cs surface concentration measured at 500 eV was close to 9% at steady state. Moreover, the XPS data demonstrate that the Cs implantation effectively promotes the formation of negative ions via electron transfer, as observed by the decrease of the N 1s NH₃ ⁺ contribution along with a fairly constant O 1s COO^- contribution in the zwitterionic form of phenylalanine. The charge balance of the molecule becomes largely negative following Cs implantation, and it correlates very well with the surface Cs content. The crucial implication for SIMS is that Cs implanted into organics effectively creates negatively ionized species at the surface which can be ejected as negative ions without any further charge transfer mechanisms. (Graph Presented).