Two-photon Photoemission Spectra Research Articles

Ab Initio Linear and Pump-Probe Spectroscopy of Excitons in Molecular Crystals.

Linear and nonlinear spectroscopies are powerful tools used to investigate the energetics and dynamics of electronic excited states of both molecules and crystals. While highly accurate ab initio calculations of molecular spectra can be performed relatively routinely, extending these calculations to periodic systems is challenging. Here, we present calculations of the linear absorption spectrum and pump-probe two-photon photoemission spectra of the naphthalene crystal using equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD). Molecular acene crystals are of interest due to the low-energy multiexciton singlet states they exhibit, which have been studied extensively as intermediates involved in singlet fission. Our linear absorption spectrum is in good agreement with experiment, predicting a first exciton absorption peak at 4.4 eV, and our two-photon photoemission spectra capture the qualitative behavior of multiexciton states, whose double-excitation character cannot be captured by current methods. The simulated pump-probe spectra provide support for existing interpretations of two-photon photoemission experiments in closely related acene crystals such as tetracene and pentacene.

Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports

The interfacial electronic structure of various size-selected metal oxide nanoclusters (M3Ox; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu2O supports were investigated by a combination of experimental methods and density functional theory (DFT). These systems explore electron transfer at the metal–metal oxide interface which can modify surface structure, metal oxidation states, and catalytic activity. Electron transfer was probed by measurements of surface dipoles derived from coverage dependent work function measurements using two-photon photoemission (2PPE) and metal core level binding energy spectra from X-ray photoelectron spectroscopy (XPS). The measured surface dipoles are negative for all clusters on Cu(111) and Cu2O/Cu(111), but those on the Cu2O surface are much larger in magnitude. In addition, sub-stoichiometric or “reduced” clusters exhibit smaller surface dipoles on both the Cu(111) and Cu2O surfaces. Negative surface dipoles for clusters on Cu(111) suggest Cu → cluster electron transfer,...

Imaging and spectromicroscopy of photocarrier electron dynamics in C60 fullerene thin films

We have employed a two-photon photoelectron emission microscopy (2P-PEEM) to observe the photocarrier electron dynamics in an organic thin film of fullerene (C60) formed on a highly oriented pyrolytic graphite with a spatial resolution of ca. 135 nm. In this approach, photocarrier electrons in C60 single-layer islands generated by the first pump photon are detected by the second probe photon. These spectromicroscopic observations conducted over a 100 × 100 nm2 region of C60 islands consistently reproduced the macroscopic two-photon photoemission spectrum of fully covered C60 monolayer film, where the energy of photocarrier electron in the islands was +0.9 eV relative to the Fermi level. Time-resolved 2P-PEEM revealed that the photocarrier electron decayed from the monolayered C60 islands into the substrate with a time constant of 470 ± 30 fs.

Ultrafast spectroscopic study for singlet fission

Singlet fission is a spin-allowed process that creates two triplet excitons from one photo-excited singlet exciton in organic semiconductors. This process of carrier multiplication holds the great potential to break the theoretical efficiency limit in single-junction solar cells by making better use of high-energy photons, while capturing lower-energy photons in the usual style. Photovoltaic devices based on singlet fission have achieved external quantum efficiencies in excess of 100%. In this paper, we first introduce the basic concept about singlet fission and review the history of the field briefly. Then, we report some reflent advances in the reflearch of singlet fission progress with the combination of our group’s productions. Tetracene and pentacene are chosen as typical polyacene materials for discuss. We describe how scientists make progresses in understanding the underlying physics in singlet fission process. The experimental methods of transient absorption spectra, time-resolved fluorescence spectra and time-resolved two-photon photoemission spectra render numerous results for analysis. Moreover, a survey about the debate on the direct or indirect mechanism with transient optical study is provided. It has been verified that multiexciton state intermediates in singlet fission process and the factors of energy level alignments, intermolecular interaction as well as lattice vibrations play a role in it. Last, we briefly summarize the implications of singlet fission in organic solar devices by introducing several composite architectures for singlet-fission photovoltaics. Designing efficient and cheap solar cells is the ultimate goal for understanding the intrinsic photophysics of singlet fission. To obtain high efficiencies, it is important to adapt proper materials and new organic/inorganic architectures may become a promising direction. Also, finding a way for efficient triplet exciton dissociation should be considered seriously. It is believable that these guidelines can lead to the development of cheap and efficient fission-based devices.

Analysis of two-photon photoemission from Si(001)

We have applied our ab initio simulation approach for the photoemission process at solid surfaces to calculate two-photon photoemission spectra from the $p(2\ifmmode\times\else\texttimes\fi{}2)$-reconstructed Si(001) surface. In this approach, the ground-state electronic structure of the surface is obtained within density functional theory. The subsequent time-dependent simulation is carried through at frozen effective potential, while an optical potential is applied to account for inelastic scattering in the excited state. We have derived normal emission spectra for $s$- and $p$-polarized light with photon energies in the range $\ensuremath{\hbar}\ensuremath{\omega}=3.85--4.75\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The dependence of the theoretical spectra on photon energy and polarization is analyzed and compared to experimental spectra from the literature. To unravel the role of the unoccupied states between Fermi energy and the vacuum level which are acting as intermediate states in the excitation process, we investigate the expression for the two-photon photocurrent from perturbation theory. The scattering states, which serve as the final states of photoemission, are obtained from a time-dependent simulation of a LEED-type experiment. The evaluation of the dipole matrix elements allows us to identify the relevant bulk band transitions and to address the influence of surface states.

Two-Photon Photoemission Study of the Coverage-Dependent Electronic Structure of Chemisorbed Alkali Atoms on a Ag(111) Surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].

Novel Growth of Naphthalene Overlayer on Cu(111) Studied by STM, LEED, and 2PPE

Adsorbed structures of naphthalene on Cu(111) have been studied using low temperature scanning tunneling microscope (LT-STM) and low energy electron diffraction (LEED). Starting from single molecules, three kinds of long-range ordered superstructures, (5√3 × 5√3)R30°, (2√3 × 3)rect-1C10H8, and (−411−4) are observed depending on the molecular concentrations and the substrate temperatures during molecular adsorption. One of the self-assembled ordered phases with a (5√3 × 5√3) R30° periodicity is chiral in adsorption-induced arrangement though a single naphthalene molecule itself has no inherent chirality. In STM images, isolated single molecules appear as depressions whereas the molecules are seen as protrusions in self-assembled layers. Coverage dependent two-photon photoemission (2PPE) spectra show that the adsorption-induced occupied states is formed at around Cu 3d bands, and this results in the enhanced tunneling of occupied state images in assembled layers.

Two-photon photoemission spectra related to an ultrafast heterogeneous electron transfer from perylene toTiO2

Two-photon photoemission (2PPE) spectra related to sub-$100\text{\ensuremath{-}}\mathrm{fs}$ heterogeneous electron transfer from perylene to $\mathrm{Ti}{\mathrm{O}}_{2}$ are calculated. The approach accounts for the dominant intramolecular vibration of perylene as well as for the band structure of $\mathrm{Ti}{\mathrm{O}}_{2}$ described in a tight-binding model. The focus is on the influence of the pump and probe laser pulse duration, with the pump laser originating charge injection and the probe laser causing the photoemission process. The latter may proceed directly from the photoexcited molecule or, after charge injection, from the $\mathrm{Ti}{\mathrm{O}}_{2}$ conduction band. The time-dependent Schr\odinger equation which describes charge injection and accounts for the pump pulse is solved exactly within a time interval of about $250\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The action of the probe pulse is considered in linear response theory. While the vibrational structure in the 2PPE spectra broadens with decreasing pump pulse length, it is found that this structure is largely preserved when varying the probe pulse duration. In order to estimate dephasing caused by intramolecular vibrational energy redistribution in perylene and electron phonon coupling in $\mathrm{Ti}{\mathrm{O}}_{2}$, a density matrix scheme is also introduced describing heterogeneous electron transfer and the photoelectron emission processes. A finite escape depth for electrons at the $\mathrm{Ti}{\mathrm{O}}_{2}$ surface is finally taken into account to evaluate its influence on the spectra.

Photoinduced interface charging in multiphoton photoemission from ultrathin Ag films on Si(100)

The fluence dependence of two-photon photoemission and time resolved two-color pump–probe photoemission spectroscopy of a 25 ML thick Ag-film grown on n-doped Si(100) reveal a photoinduced work function reduction that is attributed to a reduction of the surface dipole. Time-resolved two-color pump–probe spectroscopy shows that this reduction persists for at least several microseconds. This and the pump-induced modification of the 4.65 eV two-photon photoemission spectrum indicate that the excitation of long-lived trap states at the Ag-Si interface affects the charge distribution in the Ag film and consequently is responsible for the reduction of the surface dipole.

Spin-resolved two-photon photoemission study of the surface resonance state onCo∕Cu(001)

Bulk and surface states of a clean and Cs-doped surface of a Co film grown on $\mathrm{Cu}(001)$ have been studied by spin-resolved photoemission (SR-PE) and compared with band structure calculation results. One-photon (1PPE) and two-photon (2PPE) photoemission spectra from clean Co films are found to be dominated by a peak located at a binding energy of about $0.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with respect to ${\mathrm{E}}_{\mathrm{F}}$, which is assigned to the spin up $3d$ bulk state. Slight Cs-doping of a $\mathrm{Co}(001)$ surface shifts an image potential state in resonance with the sp-states of the conduction band. SR-2PPE study of the optically-induced electron population in such an image resonance reveals a strong dependence on the set polarization of the laser light. We are able to directly detect the spin polarization of electrons photoemitted from the image resonance state, which can be varied from highly-polarized (about bulk values) to almost unpolarized when tuning light polarization of the pump laser pulse from $s$ to $p$.

Electron dynamics in a heterogeneous system: thin Ag films on Si(1 0 0)

Photodesorption dynamics is strongly influenced by the electronic structure and electron dynamics of the substrate. Here we use time-resolved two-photon photoemission spectroscopy to investigate electron relaxation and transport in thin ultra-flat epitaxial Ag films on Si(1 0 0). Two-photon photoemission spectra obtained for excitation with 3.1 eV photons reveal that electron emission occurs preferentially from the Ag film. In contrast, the photoelectrons generated by two-photon excitation with 4.7 eV photons originate from the Si substrate. This conclusion is supported by intensity dependent spectral changes that can only be reconciled if both excitation steps occur in the Si substrate. In addition, this is corroborated by time-resolved experiments that reveal slow electron relaxation dynamics as it is expected for Si. Our results indicate that the excitation in the Si substrate and the transmission of electrons through the thin Ag film play an important role in electron dynamics in the heterogeneous system and can well be responsible for the film thickness and excitation wavelength dependence of photodesorption of NO 2.

Simulation of two-photon photoemission from the bulksp-bands of Ag(111)

Theoretical and experimental studies on the two-photon photoemission excited by femtosecond laser pulses from the $sp$-band of Ag(111) in normal direction are reported. At low temperatures, the two-photon photoemission spectrum is found to consist mainly of nonresonant direct transitions from the lower to the upper $sp$-band across the $L$-projected band gap involving the upper $sp$-band as virtual intermediate states. This direct photoemission signal is calculated from the Ag band structure using optical Bloch equations and dipole moments derived from the nearly free electron approximation. For finite temperatures, a minor additional temperature-dependent, continuous contribution to the two-photon photoemission spectrum is experimentally determined. This background due to the secondary electrons is theoretically modeled combining the Debye model and Fermi Liquid Theory to account for electron relaxation effects during the two-photon photoemission process.

The electronic structure of methanol covered TiO 2(1 1 0) surfaces

We report on the electronic structure rutile TiO 2(1 1 0) surfaces following the exposure to methanol. Two-photon photoemission (2PP) spectra reveal a strong resonance at 2.2–2.4 eV above Fermi level, which gains maximum intensity at ∼1 ML coverage of methanol. From the coverage, temperature and emission angle dependence of the methanol induced spectral features and similarities to a related state on water covered TiO 2(1 1 0) surfaces, we assign the resonance to charge transfer excitation from the reduced five-coordinate Ti 5 c + 4 - δ ions to hydroxyl groups on the bridging oxygen rows.

Two-photon photoemission spectromicroscopy of noble metal clusters on surfaces studiedusing time-of-flight photoemission electron microscopy

Time-of-flight two-photon photoemission spectromicroscopy was used to investigate Cusurface inhomogeneities and Ag cluster films deposited on a Si(111) substrate. Femtosecondlaser radiation with wavelength around 400 nm (photon energy 3.1 eV) was used forexcitation. The metal aggregates have linear dimensions in the range between 40 nm andseveral 100 nm. It is shown that the two-photon photoemission spectra of the Cu and Agnanoclusters reveal the same qualitative differences from the spectra of the correspondinghomogeneous and clean metal surfaces. In particular, they show an enhancedphotoemission yield (up to 70 times higher) and present a different overall shape,characterized by differences around the Fermi level onset and a steeper intensity increase atlower final state energies. These differences are discussed in terms of the excitation oflocalized surface plasmons in the clusters and a resulting modification of the near-zoneelectromagnetic field, which in turn influences the two-photon photoemission and itsdynamics. Moreover, it is shown that a positive unit charge (photohole) resides on theclusters during the timescale relevant for the two-photon photoemission process.

Femtosecond Transfer Dynamics of Photogenerated Electrons at a Surface Resonance of Reconstructed InP(100)

Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.

Microscopic analysis of relaxation mechanisms from energy- and time-resolved two-photonphotoemission spectra of metal surfaces

Microscopic theory of energy- and time-resolved (ER- and TR-) two-photon photoemission(2PPE) from metal surfaces is presented from the viewpoint of many-body dynamics of theelectron system. The effects of hole dynamics in the occupied states at the surfaces areinvestigated by the nonequilibrium Green function method. In the macroscopic theorybased on the density matrix method, it is usually considered that dephasing is due toelastic scattering and energy relaxation is due to inelastic scattering; however, it isdemonstrated that inelastic electron–electron scattering of the photoexcited holes from theoccupied states to the bulk can account for both dephasing and acceleration of energyrelaxation. On the basis of analytical and numerical analyses for ER- and TR-2PPE onCu(111), the relations of the macroscopic relaxation times with the electron and holelifetimes are clarified.

Two-photon photoemission spectroscopy ofTiO2(110)surfaces modified by defects andO2orH2Oadsorbates

Two-photon photoemission (2PP) spectra of $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surfaces are measured for the nearly perfect surface, and surfaces modified by introduction of defects and adsorbed molecules. Defects are generated on nearly perfect surfaces by three methods: electron irradiation, annealing in vacuum, and ${\mathrm{Ar}}^{+}$ sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of ${\mathrm{O}}_{2}$ or ${\mathrm{H}}_{2}\mathrm{O}$ molecules. 2PP spectroscopy is used to systematically investigate the work function change due to the presence of defects or adsorbates. 2PP spectroscopy detects both surface and bulk oxygen vacancy defects. We find from the results on oxygen adsorption that oxygen vacancies created by electron irradiation are localized on the surface and may be removed by ${\mathrm{O}}_{2}$ adsorption at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The surface defects are substantially different from those created by annealing or by ion sputtering where vacancies in the subsurface region are proposed. We find that ${\mathrm{O}}_{2}$ acts as an acceptor molecule on surface defect states whereas ${\mathrm{H}}_{2}\mathrm{O}$ acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of ${\mathrm{H}}_{2}\mathrm{O}$ adsorbed on $\mathrm{Ti}{\mathrm{O}}_{2}$ surface is derived to be $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{D}$ positive outward. We also find an unoccupied electronic state $2.45\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the Fermi level that appears at submonolayer coverage of ${\mathrm{H}}_{2}\mathrm{O}$, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed ${\mathrm{H}}_{2}\mathrm{O}$ molecules or OH ligands.

Valence electron orbitals of an oligo(p-phenylene-ethynylene)thiol on gold.

This communication reports measurement of one-photon (21.2 eV) and one-color, two-photon (3.2-4.5 eV) photoemission spectra of 4,4'-bis-(phenylethynyl)benzenethiol chemisorbed on gold. Four features are observed in these spectra: two occupied, predominantly molecular levels below the Fermi level and two unoccupied, predominantly molecular levels above the Fermi level. The occupied and unoccupied bands closest to the Fermi level are assigned to delocalized pi-bands, and the other occupied and unoccupied bands, to localized pi-bands. With this assignment, the hole- and electron-injection barriers and the transport gap for those levels are deduced.

Effects of hole scattering on two-photon photoemission from metal surfaces

By the nonequilibrium Green function method, we investigate the effects of the inelastic scattering of photoexcited holes at metal surfaces on the two-photon photoemission (2PPE) spectra. The 2PPE spectra of a metal surface we study show two peaks attributed to an occupied localized state $(2\ensuremath{\omega}$ peak) and an unoccupied localized state $(1\ensuremath{\omega}$ peak). Energy transfer between the surface and the bulk due to the hole scattering accounts for the occurrence of the $1\ensuremath{\omega}$ peak, while it is usually considered that this peak occurs due to dephasing in macroscopic theories. As well as the $1\ensuremath{\omega}$ peak, the peak height and width of the $2\ensuremath{\omega}$ peak, which is usually considered to occur due to a direct two-photon excitation process, are affected by the hole scattering. By comparing with a macroscopic theory based on the density matrix method, we discuss the relation of the macroscopic energy relaxation time and the dephasing time with intrinsic lifetimes of the electrons and holes in detail.

Time-resolved two-photon photoemission spectroscopy of semiconductor bulk states

Time-resolved two-photon photoemission spectra (2PPE) are modeled using the Heisenberg equations of motion, wherein electron relaxation (thermalization) is treated phenomenologically. Energy dispersion is incorporated for initial, intermediate, and final states in the model calculations. The question of whether time-resolved 2PPE spectra can help to determine the electron distribution in intermediate states (such as conduction band states), and thereby enable the extraction of their temperature, is the focus of this work. By exploring different types of energy dispersion for the final states and by varying the pump and probe energies, the study indicates that 2PPE spectra provide qualitative insight into the process of hot-electron thermalization and cooling, but do not directly yield quantitative information, such as the actual distribution or temperature.