乙醇在TiO2(110)表面光催化解离的动力学和超快电子动态学 (cited: 2)

采用实时双光子光电子能谱和时间分辨双光子光电子能谱技术分别研究了乙醇在该表面光催化解离的动力学和超快电子动态学过程. 通过测量与乙醇光催化解离相关的电子激发态随时间的演化,发现这个反应满足分型动力学. 乙醇在还原性TiO₂(110)上的光催化解离比在氧化性表面快,这归结于缺陷的存在降低了反应能垒. 这样一个反应的加速过程很可能是与缺陷电子相关的. 通过干涉双脉冲相关的测量,得到了乙醇-TiO₂界面电子激发态的超快动态学. 与甲醇的情况类似,这个电子激发态的寿命为24 fs. 激发态的出现为TiO₂和它周围环境的电子转移提供了一个通道.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2(110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ singlecrystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.     

Theory of dynamics of electron wave packets in time-resolved two-photon photoemission via image states

Time-resolved two-photon photoemission (TR2PPE) from the Cu(100) surface is investigated by the Keldysh Green function method in order to analyze ultrafast dynamics of an electron wave packet in the image states of the surface. By numerical analysis, the quantum beats in the TR2PPE spectrum due to interference among the image states are reproduced, and the motion of the electron wave packet in front of the surface is demonstrated. It is discussed on the basis of the obtained results as to how the motion of the electron wave packet is evaluated from the TR2PPE spectrum.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO2 (110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the electronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ singlecrystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant information with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.      

Dynamics of electron scattering between bulk states and the C<Subscript>1</Subscript> surface state of InP(100)

Hot electron dynamics was investigated, with a focus on scattering between bulk states and the C₁ surface state that is formed on the (2×4)-reconstructed In-rich surface of InP(100). The latter surface was prepared via metal organic chemical vapor deposition (MOCVD) and monitored by reflectance anisotropy spectroscopy (RAS/RDS). Two-color twophoton photoemission (2PPE) was employed with laser pulses of about 50 fs duration. Hot electrons were generated in bulk states about 0.5 eV above the C₁ surface state, thereby avoiding any significant direct optical population of the surface state. A time constant of 35 fs was determined from the experimental data for electron scattering from isoenergetic bulk states to the C₁ surface state by analyzing the rise of a C₁-specific peak in the 2PPE spectrum. The decay of this C₁ peak was ascribed to energy relaxation of the photo-generated electrons to bulk states below the surface state. Analogous measurements were carried out with a (2×1/2×2)-reconstructed P-rich surface of InP(100) that was also grown via MOCVD. No sign of a surface statewas detected in the 2PPE spectra for the latter surface in the corresponding energy range of the conduction band. PACS 73.20.-r; 78.47.+p; 79.60.-i; 79.60.Bm     

Transition to fixed final states in two-photon photoemission from adsorbate–metal systems

A simple theory is presented to study the final state transition in the two-photon photoemission (2PPE) processes of the adsorbate on a metal surface, in the light of recent experimental observation for benzene/Cu(1 1 1) system. The 2PPE spectrum is calculated for a model system assuming that an unoccupied adsorbate level above the vacuum, populated by the two-photon excitation, couples to a continuum of free photoelectrons. An analysis of the 2PPE spectra as a function of the incident photon energy reveals the spectral features including the possibility of Fano-type modification and resonance enhancement of the final state transition in the 2PPE processes. Attempt is also made to explain why the d-band emission of the substrate is enhanced in the presence of adsorbates.     

氢负离子剥离电子通量的数值研究

本论文利用杜,赵等人的半经典轨道理论,推导了电场和金属面同时存在时,氢负离子的剥离电子通量分布,并对干涉图样进行了数值模拟。结果发现剥离电子通量分布不仅与激光光子能量和外场有关,并且敏感地依赖于金属面与氢负离子之间的距离。这些结果可以应用于许多重要的研究领域,包括超快激光表面催化,更准确的测量电子亲和力和控制低能的光电子和表面的相互作用等。     

氢负离子剥离电子通量的数值研究

本论文利用杜,赵等人的半经典轨道理论,推导了电场和金属面同时存在时,氢负离子的剥离电子通量分布,并对干涉图样进行了数值模拟。结果发现剥离电子通量分布不仅与激光光子能量和外场有关,并且敏感地依赖于金属面与氢负离子之间的距离。这些结果可以应用于许多重要的研究领域,包括超快激光表面催化,更准确的测量电子亲和力和控制低能的光电子和表面的相互作用等。     

Dynamics of photoinduced electron transfer from adsorbed molecules into solids

Ultrafast interfacial electron transfer from the donor orbital of organic chromophores into empty electronic acceptor states of a semiconductor and of a metal was investigated by two-photon photoemission spectroscopy (2PPE). Experimental tools and procedures have been developed for carrying out wet-chemistry preparation of the molecule/solid interface. The organic chromophore perylene was investigated with several different bridge/anchor groups on TiO₂(110). One perylene compound was investigated for comparison on Ag(110). Angle and polarization dependent 2PPE measurements revealed the orientation of the perylene chromophore on the surface as controlled by the adsorption geometry of the respective anchor group on TiO₂. UPS measurements gave the position of the HOMO level of the chromophore with respect to the Fermi level of the solid. The donor level of each molecule was found high enough to fulfill the “wide band limit” of heterogeneous electron transfer dynamics. Time constants for heterogeneous electron transfer were extracted from 2PPE transients. A difference by a factor of four was found, 13 fs against 47 fs, when a conjugated bond was exchanged for a saturated bond in the otherwise identical bridge group. The two different contributions to the 2PPE transients arising firstly from the excited state of the chromophore and secondly from the injected electrons were separated by measuring the latter contribution separately in the case of instantaneous interfacial electron transfer realized with catechol as adsorbate. The time scales measured for the electron transfer step and for the subsequent electron escape process from the surface into the bulk of TiO₂ showed both good agreement with recent theoretical predictions of other groups for these systems. PACS 42.65.Ky; 79.60.BM; 78.47.+p; 73.20.-r; 79.60.Jv; 79.60.-i     

Theory of energy- and time-resolved two-photon photoemission from metal surfaces – influence of pulse duration and excitation condition

The theory of energy- and time-resolved two-photon photoemission (2PPE) spectra of metal surfaces is presented using density matrix formulation for a three-level system consisting of an initial occupied, intermediate unoccupied and final photoelectron states. A perturbation expansion method is employed to calculate the energy-resolved 2PPE spectrum for continuous light beams. We have obtained analytical expressions of the 2PPE spectrum corresponding to a step-by-step one-photon process through the intermediate state and a direct two-photon-ionization process via virtual transition. It is demonstrated that the intermediate state can also be populated via the nonresonant virtual process. This indicates an absolute importance of “pure dephasing” associated with the transition between the initial and intermediate states. Evolution of the 2PPE spectrum as a function of the pump photon energy is calculated to demonstrate the conditions under which the intrinsic linewidth (total dephasing time) can be deduced from the lineshape analysis. It is also found that the intensity ratio of the two peaks due to the initial and the intermediate states in 2PPE spectrum can be used to estimate the pure dephasing time. Transient behavior of the excited-state population following pulse excitation is calculated with a focus on how the ultrafast relaxation times of the excited states such as image-potential states of metal surfaces are deduced from the transient 2PPE response observed with a pulse laser with much longer duration. The time-resolved 2PPE spectra are calculated for varying detuning from the resonant excitation from the initial state to the intermediate state. Transient responses of the 2PPE signal due to direct ionization and step-by-step processes are also calculated to demonstrate that the nonresonant former process has an influence on the analysis of the cross-correlation trace of the intermediate state, by which the population relaxation time is estimated. Attempts are also made to apply the present theory to a recent time-resolved 2PPE study of the relaxation dynamics of the image-potential states as well as hot electrons in Cu(100) and Ag(100) surfaces. Received: 23 May 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

甲醇/TiO2(110)界面激发态的表征

用光电子能谱的方法研究了甲醇/TiO₂(110)界面的电子结构．在激发波长为400 nm的双光子光电子能谱(2PPE)中,探测到了一个末态能量在费米能级以上5.5 eV的共振信号．之前的研究[Chem. Sci. 1, 575 (2010)]表明,这个共振信号与甲醇在5配位的钛离子(Ti_5c)上的光催化解离相关．双光子光电子能谱同时携带初态和中间态的信息．为此设计了一个调谐激发光波长的2PPE实验以及一个单光子光电子能谱(1PPE)和2PPE对比的实验,结果一致表明这个共振信号来自于未占据的中间态,也就是激发态．能带色散关系测量表明这个激发态是局域的．时间分辨2PPE测得这个激发态的寿命是24 fs．     

Unoccupied band dispersion of Si(1 1 1):√3 × √3-Ag surface studied by time- and angle-resolved two-photon photoemission spectroscopy

Band dispersion and transient population of unoccupied electronic states on Si(1 1 1):√3 × √3-Ag surface have been studied by time-resolved (TR) and angle-resolved (AR) two-photon photoemission (2PPE) spectroscopy. The band dispersions originating from unoccupied electronic states have been identified from the comparison between AR-2PPE spectra and angle-resolved one-photon photoemission spectra with synchrotron radiation. A lifetime of unoccupied surface state has been determined from the TR-2PPE spectra.     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.     

Interferometric two-photon photoemission correlation technique and femtosecond wet-electron dynamics at the TiO (110) surface

The femtosecond time-resolved two-photon photoemission (TR-2PP) and the ultra high vacuum (UHV) surface science techniques are integrated to investigate the elec- tronic structures and the interfacial electron transfer dynamics at the atomically ordered adsorbate overlayers on TiO₂ single- crystalline surfaces. Our research into the CH₃OH/TiO₂ system exhibits complex dynamics, providing abundant informa- tion with regard to electron transport and solvation processes in the interfacial solvent structures. These represent the fundamentally physical, photochemical, and photocatalytic reactions of protic chemicals covered with metal-oxides.     

Exciton dynamics at interfaces of organic semiconductors

We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C₆₀ and pentacene. For C₆₀ thin films epitaxially grown on Au(1 1 1) and Cu(1 1 1) surfaces, we observe both charge transfer and exciton states. For excitons in C₆₀, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated.     

苯乙炔分子电子激发态超快动力学研究

采用飞秒时间分辨质谱技术结合飞秒时间分辨光电子影像技术研究了苯乙炔分子电子激发态超快非绝热弛豫动力学.用235 nm光作为泵浦光,将苯乙炔分子激发到第二激发态S2,用400 nm光探测激发态的演化过程.时间分辨的母体离子的变化曲线用指数和高斯函数卷积得到不同的两个组分,一个是超快衰减组分,时间常数为116 fs,一个是慢速组分,时间常数为106 ps.通过分析时间分辨的光电子影像得到光电子动能分布,结合时间分辨光电子能谱数据发现,时间常数为116 fs的快速组分反映了S2态向S1态的内转换过程.实验还表明S1态通过内转换被布局后向T1态的系间窜跃过程为重要的衰减通道.本工作为苯乙炔分子S2态非绝热弛豫动力学提供了较清晰的物理图像.     

Electron injection dynamics in dye-sensitized semiconductor nanocrystalline films

We have summarized recent ultrafast spectroscopic studies on phenomena associated with dye-sensitization of semiconductor metal oxide nanoparticles, especially TiO₂ nanocrystalline film from a surface science perspective with a strong relation to mechanism of electron injection in dye-sensitized solar cells, which are attracting much interest from both viewpoints of pure science and applied science.A lot of chemical and physical processes are involved in this solar cell, such as light harvesting by molecules and nanostructures, interfacial electron transfer, charge migration in solid and electrolyte, degradation of the materials, and so on. Among them, the very primary process initiated by photoabsorption by sensitizing dye molecules; that is, electron injection from excited adsorbates into the conduction band of semiconductor metal oxides is significantly important, because this process must be 100% efficient with a minimum driving force for high current and voltage generation.We have first focused on details of experimental methods used in this research area, and then in the following Sections, have organized this review by concentrating on each parameter that influences dynamics of electron injection in dye-sensitized semiconductors. Finally we have emphasized it is important to measure actual DSSCs for the precise comparison between electron injection dynamics and device performance.     

Theory of spectral hole burning for the study of ultrafast electron dynamics in metal nanoparticles

In order to study the ultrafast relaxation dynamics of surface plasmon excitation in metal nanoparticles in the presence of inhomogeneous line broadening and investigate the influence of the reduced dimensions on the dephasing time T₂ in the size regime below about 10 nm, we have recently demonstrated a novel technique based on persistent spectral hole burning [1]. Here, we describe a theoretical model that has been developed for evaluation of the experimental data and precise determination of T₂ for particles of different size and shape. Comparison of the model to experimental data for Ag nanoparticles on sapphire shows that the theoretical treatment does not only reproduce the shape of the generated holes but also the dependence of their widths on the applied laser fluence. As a result, we have a reliable and versatile tool at hand making possible systematic studies of the ultrafast electron dynamics in small metal particles, and the dependence of the femtosecond dephasing time on their size, shape and surrounding dielectric. Received: 12 September 2001 / Published online: 15 October 2001     

Diffusion and aggregation of Ag n -clusters ( n =2-9) on HOPG probed by fs-two-photon-photoemission

The diffusion and aggregation of preformed Ag_n-clusters ( n = 2-9) deposited onto a highly oriented pyrolytic graphite (HOPG) substrate is studied by two-photon-photoemission (2PPE). The sample is irradiated with ultrashort laser pulse pairs and the kinetic energy of the emitted photoelectrons is analyzed in a magnetic bottle type time-of-flight spectrometer. During annealing of the sample from 100 K up to room temperature, nanoparticles are formed on the surface by diffusion and aggregation of the silver clusters. A steep increase of the total photoelectron yield at a sample temperature of about 150 K is explained by the excitation of plasmons in the silver nanoparticles. From the kinetic energy distribution of the photoelectrons we deduce a strong variation of the work function of the sample during the formation of the nanoparticles, which is attributed to a quantum size effect.     

时间分辨偏振红外光谱及其应用

时间分辨偏振红外光谱已被广泛应用于研究光化学过程中的分子结构动力学. 通过测定瞬态物质跃迁偶极矩之间的角度等结构信息,可以提供光化学过程中伴随的电荷分布、分子结构和构象变化等动态信息. 包括简要介绍时间分辨偏振红外光谱技术的原理和应用：(i) 时间分辨偏振红外光谱概述;(ii) 时间分辨偏振红外光谱的原理及其优势;(iii) 利用时间分辨偏振红外光谱探测多种化学动力学过程,例如蛋白质构象动力学、激发态的电子局域化和光致异构化等;(iv) 时间分辨偏振红外光谱的局限和发展前景.