Willingham D, Brenes D A, Wucher A, Winograd N
Chemistry Department, Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, USA.
J Phys Chem C Nanomater Interfaces. 2010 Apr 1;114(12):5391-5399. doi: 10.1021/jp9054632.
Molecular depth profiles of an organic thin film of guanine vapor deposited onto a Ag substrate are obtained using a 40 keV C(60) cluster ion beam in conjunction with time-of-flight secondary ion mass spectrometric (ToF-SIMS) detection. Strong-field, femtosecond photoionization of intact guanine molecules is used to probe the neutral component of the profile for direct comparison with the secondary ion component. The ability to simultaneously acquire secondary ions and photoionized neutral molecules reveals new fundamental information about the factors that influence the properties of the depth profile. Results show that there is an increased ionization probability for protonated molecular ions within the first 10 nm due to the generation of free protons within the sample. Moreover, there is a 50% increase in fragment ion signal relative to steady state values 25 nm before reaching the guanine/Ag interface as a result of interfacial chemical damage accumulation. An altered layer thickness of 20 nm is observed as a consequence of ion beam induced chemical mixing. In general, we show that the neutral component of a molecular depth profile using the strong-field photoionization technique can be used to elucidate the effects of variations in ionization probability on the yield of molecular ions as well as to aid in obtaining accurate information about depth dependent chemical composition that cannot be extracted from TOF-SIMS data alone.
使用40 keV的C(60)团簇离子束结合飞行时间二次离子质谱（ToF-SIMS）检测，获得了气相沉积在银衬底上的鸟嘌呤有机薄膜的分子深度分布。完整鸟嘌呤分子的强场飞秒光电离用于探测分布的中性成分，以便与二次离子成分进行直接比较。同时获取二次离子和光电离中性分子的能力揭示了有关影响深度分布特性的因素的新的基础信息。结果表明，由于样品中产生了自由质子，在前10 nm内质子化分子离子的电离概率增加。此外，由于界面化学损伤积累，在到达鸟嘌呤/银界面之前25 nm处，碎片离子信号相对于稳态值增加了50%。由于离子束诱导的化学混合，观察到改变后的层厚度为20 nm。总体而言，我们表明，使用强场光电离技术的分子深度分布的中性成分可用于阐明电离概率变化对分子离子产率的影响，以及有助于获取有关深度相关化学成分的准确信息，而这些信息无法仅从ToF-SIMS数据中提取。
