Ecole Normale Supérieure de Lyon, Laboratoire de Chimie, CNRS UMR 5182, 46 Allée d'Italie, 69364 Lyon Cedex 07, France.
J Chem Phys. 2012 Jun 28;136(24):244507. doi: 10.1063/1.4730168.
Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals at the atomic scale. However, direct IETS-based chemical analysis remains difficult due to the selection rules that await a definite understanding. We present IETS simulations of single formate and benzoate species adsorbed in the same upright bridge geometry on a (111)-cleaved Cu surface. In agreement with measurements on a related substrate, the simulated IET-spectra of formate/Cu(111) clearly resolve one intense C-H stretching mode whatever the tip position in the vicinity of the molecular fragment. At variance, benzoate/Cu(111) has no detectable IET signal. The dissimilar IETS responses of chemically related molecules--formate and benzoate adsorbates--permit us to unveil another factor that complements the selection rules, namely the degree of the vacuum extension of the tunneling active states perturbed by the vibrations. As a consequence, the lack of a topmost dangling bond orbital is entirely detrimental for STM-based inelastic spectroscopy but not for STM elastic imaging.
利用扫描隧道显微镜（STM）进行的非弹性电子隧道谱（IETS）被认为是在原子尺度上识别化学物质的终极工具。然而，由于等待明确理解的选择规则，直接基于 IETS 的化学分析仍然很困难。我们对在（111）- 切割 Cu 表面上以相同的直立桥构型吸附的单个甲酸盐和苯甲酸盐物种进行了 IETS 模拟。与在相关衬底上的测量结果一致，无论分子片段附近针尖的位置如何，模拟的甲酸盐/Cu（111）的 IET 谱都清晰地分辨出一个强烈的 C-H 伸缩模式。相比之下，苯甲酸盐/Cu（111）没有可检测到的 IET 信号。化学相关分子（甲酸盐和苯甲酸盐吸附物）的不同 IETS 响应使我们能够揭示另一个补充选择规则的因素，即由振动扰动的隧道活性态的真空扩展程度。因此，缺乏最顶层的悬空键轨道对基于 STM 的非弹性光谱完全不利，但对 STM 弹性成像没有影响。
