苝四甲酸二酐薄膜电子结构的同步辐射共振光电子能谱研究

利用基于同步辐射的近边X射线吸收精细结构谱(NEXAFS)和共振光电子谱(RPES)研究了苝四甲酸二酐分子(PTCDA)薄膜的电子结构.碳K边NEXAFS谱中能量小于290 eV的四个峰对应于PTCDA分子不同化学环境碳原子1s电子到未占据分子轨道的共振跃迁.RPES谱中观察到共振光电子发射和共振俄歇电子发射导致的共振峰结构,以及二次谐波激发的碳1s信号.根据电子动能对入射光能量的依赖性分别对三类峰结构进行了归属.同时,发现PTCDA分子轨道共振光电子峰的强度具有光子能量依赖性.这种能量选择性共振增强效应是由于PTCDA分子轨道空间分布差异导致的.共振俄歇峰主要源于高结合能(4.1 eV)分子轨道能级电子参与的退激发过程.明确RPES实验谱图中各个峰结构的起源有助于准确利用基于RPES的芯能级空穴时钟谱技术定量估算有机分子/电极异质界面处电子从分子未占据轨道到电极导带的超快转移时间.

Electronic structure of a 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride thin film revealed by synchrotron-based resonant photoemission spectroscopy

The electronic structure of a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin film is investigated in situ using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant photoemission spectroscopy (RPES).The NEXAFS spectroscopy can monitor the electronic transitions from core level to unoccupied states.The C K-edge NEXAFS spectrum of the PTCDA thin film shows four distinct absorption peaks below 290 eV,which are attributed to the transitions from 1s core level of C-atoms in different chemical environments (perylene core C-atoms vs anhydride C-atoms) into lowest unoccupied molecular orbitals (LUMOs) with π^* symmetry. The RPES spectra are collected in the valence band region by sweeping photon energy across the C 1s → π^* absorption edge.Three typical features of the C 1s signals excited by second-order harmonic X-ray,resonant photoemission and resonant Auger features are observed in RPES spectra,and are identified,relying on the development of kinetic energy of the emitted photoelectrons upon the change of incident photons energy.It is found that the C 1s signals excited by second-order harmonic X-ray are present at high kinetic energy side of spectrum.The kinetic energy of this feature shows photon energy dependence,that is,this feature shifts to higher kinetic energy by photon energy increasing twice.Resonant Auger peaks in RPES spectra are located on the low kinetic energy side with constant kinetic energy regardless the change of photon energy.The resonant Auger may originate from deeper molecular orbitals with binding energy large than 4.1 eV,suggesting that the resonant Auger decay process involved in deeper molecular orbitals occurs on a time scale comparable to C 1s core hole lifetime of 6 femtoseconds.Resonant enhancement of highest occupied molecular orbitals (HOMOs) derived valence band features or HOMO-1 and HOMO-2 derived resonant photoemission features in our case are lying between the C 1s signals and the resonant Auger signals.The Kinetic energy increases as the photon energy sweeps across the absorption edge,whereas their binding energy remains constant.In addition, the enhancements of two resonances show photon energy dependence that enhancement of HOMO-1 related resonance dominates over HOMO-2 related resonance at energies corresponding to perylene core C 1s to LUMOs transitions, whereas HOMO-2 related resonance becomes dominant at transitions from anhydride C 1s to LUMOs.This behavior can be related to the wavefunction character and symmetry of the frontier molecular orbitals.Clarifying each resonant feature in RPES spectra and their origin will pave the way for accurately determining the ultrafast charge transfer time at organic/electrode interfaces using synchrotron-based core hole clock technique implementation of RPES.