Two-photon Photoemission Spectra Research Articles

Effective mass and momentum-resolved intrinsic linewidth of image-potential states on Ag(100)

The intrinsic linewidth of Ag(100) electronic image-potential states versus the momentum parallel to the surface has been obtained from line-shape analysis of high-resolution angle-resolved two-photon photoemission spectra. For the $n=1$ state, the measured intrinsic linewidths have been observed to increase from 14 meV at normal emission ${(k}_{\ensuremath{\Vert}}=0)$ to 70 meV at ${k}_{\ensuremath{\Vert}}=0.18{\mathrm{\AA{}}}^{\ensuremath{-}1}.$ This finding, when compared to the significantly lower inverse lifetime broadening measured on Cu(100), opens the question of the interplay between collective particle effects and nonlocal mechanisms associated with the image-potential electrons decay dynamics. In addition, the effective electron masses for the $n=1$ and $n=2$ states, observed in our experiments along the $\overline{\ensuremath{\Gamma}}\overline{M}$ and $\overline{\ensuremath{\Gamma}}\overline{X}$ directions of the surface Brillouin zone, improve the agreement between the measured values and the theoretical predictions.

Ultrafast hole dynamics in two-photon photoemission from metal surfaces

We investigate the effects of photoexcited-hole decay on time-resolved two-photon photoemission spectra of metal surfaces by non-equilibrium Green function method. We show that it is necessary for calculating the photoelectron intensity to introduce a Green function depending on the observation time of the photoelectron. The photoexcited-hole decay causes the loss of coherence of the system and hence accounts for acceleration of the decay of the photoexcited-electron density. Then the effective lifetimes estimated from the photoelectron intensity as a function of the pump–probe delay time can be the lifetimes of the photoexcited ‘(non-excitonic) electron–hole pairs’ even when direct interactions between photoexcited electrons and holes are negligible while it is usually considered that the effective lifetimes correspond to the electron lifetimes.

Time-Resolved Two-Photon Photoemission Study of Dodecanethiolate-Passivated Ag Nanoparticles

Metallic nanoparticles are attracting much interest from the viewpoints of both fundamental and device physics, since they show the distinctive optical, physical, and chemical properties found in neither bulk nor molecular/ atomic systems. Among the nanoparticle systems, the surface-passivated nanoparticles are very stable and exhibit the closed-packed nanoparticle self-assemblies on the singlecrystalline substrates. Therefore, it is considered that they could be important constituents of future nanostructured devices, such as single electron device, catalyst, and ultra highly density memory. On the other hand, the transient dynamics of hot-electrons created by photoexcitation or inelastic scattering of other energenic particles play an important role in the various physical and chemical phenomena. The excited hot-electrons relax through a number of decay processes, such as electron–electron scattering, electron-phonon scattering, electron-impurity/ defect scattering, and scattering with other elementary excitations. The detailed understanding of hot-electron dynamics will provide not only information about the fundamental interactions in the many-body systems but also knowledge of the various optical, physical and chemical processes. In order to elucidate the intriguing properties of metallic nanoparticles and to develop the future devices, it is indispensable to understand the excited-electron dynamics as well as their electronic structures. In this work, we have carried out a femtosecond time-resolved two-photon photoemission (TR-2PPE) study of dodecanethiolate (DT)-passivated Ag nanoparticles on the highly oriented pyrolytic graphite (HOPG) substrates. Among the time-resolved spectroscopic methods, TR-2PPE spectroscopy is a unique method to observe directly the temporal evolution of excited-electrons that may not be easily accessible through optical techniques. The DT-passivated Ag nanoparticles were synthesized by the two-phase (water–toluene) reduction method. The detailed procedure is described elsewhere. The size and shape of the synthesized nanoparticles were characterized by ex-situ observations with transmission electron microscope (TEM). Figure 1 shows the TEM micrograph and the size distribution in diameter of DT-passivated Ag nanoparticles used in this work. As shown in Fig. 1, the mean diameter and standard deviation are dm 1⁄4 5:3 nm and 1⁄4 0:47 nm, respectively. The present samples have an extremely narrow size distribution in diameter ( 3:0 eV). The total energy resolution was about 150meV, and time resolution was about 10 fs. All measurements were performed at room temperature. Figure 2 shows the two-photon photoemission (2PPE) spectrum of the DT-passivated Ag nanoparticles on the HOPG substrate with dm 1⁄4 5:3 nm measured with photon energy of 3.3 eV at zero pump-probe delay, compared with that of HOPG substrate. These spectra are plotted as a function of intermediate-state energy relative to the Fermilevel. In Fig. 2, the intermediate-state energy of 3.3 eV corresponds to the highest intermediate-state, with onephoton energy (h 1⁄4 3:3 eV) above the Fermi-level. As shown in Fig. 2, the 2PPE spectral features are definitely different among the HOPG substrate and DT-passivated Ag nanoparticles on HOPG substrate. The 2PPE spectrum of Fig. 1. TEM micrograph and size distribution in diameter of dodecanethiolate-passivated Ag nanoparticles with mean diameter of dm 1⁄4 5:3 nm and standard deviation of 1⁄4 0:47 nm.

Time-resolved two-photon photoemission spectroscopy of image potential states: A phenomenological approach

The dynamics of the image potential states n=3 and n=4 at the Cu(100) surface are studied. We solve the Liouville–von Neumann equation for an open system and treat the dissipation within the Lindblad formalism. We compute the two-photon photoemission spectrum (2PPE) using first order perturbation theory and study the populations of the image potential states and the two-photon photoemission spectrum as functions of the lifetimes and dephasings of the image potential states. The computed two-photon photoemission spectrum is compared with the measured spectrum obtained by Höfer et al. [Appl. Phys. B 68, 383 (1999); Science 277, 1480 (1997)].

Open-system density-matrix approach to image-potential dynamics of electrons at Cu(100): Energy- and time-resolved two-photon photoemission spectra

Open-system density-matrix approach to image-potential dynamics of electrons at Cu(100): Energy- and time-resolved two-photon photoemission spectra

Effect of adparticle motion on time-resolved two-photon photoemission spectra of metal surfaces

The effect of the adparticle motion on the time-resolved two-photon photoemission spectra of metal surfaces on which the adparticle is adsorbed is investigated theoretically. In the two-photon photoemission process, first a pump photon excites an electron from the bulk into the anti-bonding orbital formed between the adparticle and the metal, then the adparticle begins to move, and finally a probe photon excites the electron into a free electron state above the vacuum level. It is demonstrated by the Green function method that the peak position in the energy spectrum depends on the pump–probe delay time due to the adparticle motion and it represents the potential energy for the adparticle motion.

Dynamics of the image surface state in two-photon photoemission spectroscopy

The study of the dynamical evolution underlying the two-photon photoemission process has been quite active recently. It is now well understood that the cross-correlation curve, corresponding to the resonance n 51 of the two-photon photoemission spectrum, results from a simple sequential process where the laser pump pulse populates the image potential state, which is ultimately tested by the probe pulse. However, the case of the cross-correlation curve associated with the second resonance n50, observed at a different energy, is still the subject of controversy. In the present work, we demonstrate that the dynamical process underlying this photoemission corresponds to a purely coherent process where the electrons are photoemitted as a consequence of the coherence induced between the intrinsic surface state and the final photoelectron state through the intermediate image potential state. Both the spectral distribution of the photoemitted electrons and the analytical structure of the cross-correlation curve n50 confirm this model. Also, the influence of the relaxation and dephasing constants on the cross-correlation curves is clarified.

Theory of time-resolved two-photon photoemission from Cu(111): effect of Coulomb interactions among electrons

The effect of Coulomb interactions among electrons on the time-resolved two-photon photoemission spectra of Cu(111) is investigated by nonequilibrium perturbation theory. It is demonstrated that there are contributions to photoemission via an image state on the surface from processes involving scattering of photoexcited electrons and holes due to Coulomb interactions. As a result, the correlation trace (photoelectron intensity as a function of the pump-probe delay time) is affected by the lifetimes of the electron in the image state, the photoexcited electron and hole.

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.

Quantum Control of Nuclear Motion at a Metal Surface

The possibility of quantum control of surface photochemical reactions is demonstrated for the system of Cs/Cu(111). Coherent nuclear wave packet motion is induced by resonant dipole excitation with 3.08 eV light from an occupied surface state intrinsic to the Cu(111) surface to an unoccupied Cs antibonding state for a Cs atom coverage of 0.1 monolayer. Interferometric time-resolved two-photon photoemission measurements show that the polarization induced between the two states decays in ∼25 fs. This makes it possible to control the position and momentum of the excited state wave packet through the phase of the excitation field. Quantum control is demonstrated for excitation with phase-related <20 fs pulse pairs or single chirped laser pulses. The phase dependence of two-photon photoemission spectra demonstrates the control of desorptive motion of Cs atoms.

Effect of Relaxation of Secondary Electrons and Holes on Time-Resolved Two-Photon Photoemission from Cu(111)

We investigate the time-resolved two-photon photoemission (time-resolved 2PPE, TR2PPE) spectrum of the image state on Cu(111) by nonequilibrium perturbation theory. We show that, in the 2PPE process, the scattering of photoexcited quasiparticles (electrons and holes) due to Coulomb interactions between electrons causes excitation of secondary quasiparticles in the vicinity of the Fermi level as well as electron transitions to the image state. We demonstrate quantitatively on the basis of numerical results how the effect of relaxation of secondary quasiparticles is manifested in the TR2PPE spectrum.

Theory of time-resolved two-photon photoemission spectroscopy from metal surfaces

We present a theory of energy and time-resolved two-photon photoemission (TR-2PPE) spectra from metal surfaces on the basis of the density matrix formulation. A system consists of an initial occupied (bulk and surface) states, intermediate unoccupied states such as an image potential state and a hot electron state in bulk, and a final photoelectron state. A focus is placed on how the pulse duration of the excitation laser influences the transient 2PPE signal and how the relaxation time of the intermediate states is determined using laser pulses with much longer pulse duration. The present analysis is successfully applied to the recent TR-2PPE experiments for Ag and Cu surfaces. The energy-resolved 2PPE spectrum is also calculated to demonstrate that when it is measured with a ultrashort laser pulses the width of the “one” photoemission peak from the intermediate state is a function of the delay time between pump and probe pulses, in addition to the relaxation time of the system and pulse duration.

Photo-induced electron transfer to molecular quantum structures on a metal surface

Electron transfer at organic–metal interfaces is important in molecule-based electronic and optoelectronic devices. We probe interfacial electron transfer using two-photon photoemission in two model systems: (i) hexafluorobenzene (C 6F 6) weakly adsorbed on Cu(111); and (ii) SAM of thiophenolate (C 6H 5S) on Cu(111). In the first system, the σ ∗ LUMO in C 6F 6 is found to be located around 3 eV above the Fermi level. With increasing overlayer thickness, the position of this molecular resonance decreases layer by layer, suggesting the formation of molecular quantum wells, each of which extends to the total thickness of the adsorbate layer. In the second system, which is a model interface for self-assembled molecular quantum wires, electron transfer to σ ∗ states is found to dominate the two-photon photoemission spectrum. The two σ ∗ states (LUMO and LUMO+1) are localized to the CSCu anchor and are strongly coupled to the metal substrate. These interfacial σ ∗ states may play important roles in electron transport.

The Effect of Interband Excitations on Time-Resolved Two-Photon Photoemission via a Localized State at a Metal Surface

The effect of interband excitations on time-resolved two-photon photoemission from an adsorbate-surface system is investigated. The system is assumed to be composed of two subbands in the bulk and a localized state of the adsorbate, which is in resonance with the upper energy subband. In the two-photon photoemission process, first, an electron in the lower energy subband is excited into the localized state by the pump photon, and subsequently excited above the vacuum level by the probe photon. The first excitation can be divided into two processes, i.e., the direct and the indirect one involving the interband excitation. The excitation probability due to the latter process reaches a maximum at a certain photon energy because of momentum conservation. A two-photon photoemission spectrum shows a peak due to the indirect process at an energy position where the initial state is fixed, as well as one due to the direct process where the intermediate state is fixed. The maximum of the cross-correlation trace of ...

Femtosecond dynamics of electrons on surfaces and at interfaces.

Two-photon photoemission is a promising new technique that has been developed for the study of electron dynamics at interfaces. A femtosecond laser is used to both create an excited electronic distribution at the surface and eject the distribution for subsequent energy analysis. Time- and momentum-resolved two-photon photoemission spectra as a function of layer thickness fully determine the conduction band dynamics at the interface. Earlier clean surface studies showed how excited electron lifetimes are affected by the crystal band structure and vacuum image potential. Recent studies of various insulator/metal interfaces show that the dynamics of excess electrons are largely determined by the electron affinity of the adsorbate. In general, electron dynamics at the interface are influenced by the substrate and adlayer band structures, dielectric screening, and polaron formation in the two-dimensional overlayer lattice.

Two-photon photoemission spectroscopy at clean and oxidized Cu(110) and Cu(100) surfaces

Two-photon photoemission spectra of Cu(110) and Cu(100) are measured using ∼ 3.2 eV excitation laser with ∼ 20 fs pulse width. The spectra are measured as a function of laser polarization, excitation energy, bias voltage, and oxygen coverage. They are discussed in terms of the bulk and surface electronic structure of Cu and hot electron relaxation dynamics, and compared to previous nanosecond laser studies. In addition, three-photon photoemission through the n = 1 image potential state on Cu(100) is observed.

ISLAND-SIZE DISTRIBUTIONS OF Ag ON Pd(111)

Island-size distributions of submonolayer Ag films on Pd(111) adsorbed at 90 K and after annealing of the film are recovered from two-photon photoemission spectra of the first image state. The inversion of the ill-conditioned problem with the maximum-entropy method reveals magic numbers in the island-size distributions. Hypothesis testing within the framework of Bayesian probability theory indicates a critical nucleus size i=1. After annealing of the film large islands coexist with small clusters in a two-phase state.

Two-photon electron emission from smooth and rough metal films in the threshold region.

Two-photon photoemission spectra are reported for silver films deposited under ultrahigh vacuum onto a room-temperature (smooth film) or a cold substrate (rough film). A theory is developed which successfully accounts for the qualitative features of the observed spectra in the near-threshold region when the Ag is excited by the absorption of one or two photons. Photoemission from smooth films can be successfully described in terms of direct electron transitions. The traditional one-photon photoemission theory is generalized to include two-photon excitation. Photoemission from rough Ag surfaces is interpreted in terms of the excitation of localized surface plasmons in the roughness features which are assumed to be small polarizable elements of a self-affine fractal. The rapid spatial rate of change in the local electric field surrounding these elements breaks translational invariance, thereby allowing indirect electron transitions to take place, ultimately leading to a predicted (${10}^{3}$--${10}^{6}$)-fold enhancement of the two-photon photoemission yield. Approximate analytic expressions are derived for the total photoemission yields following one- or two-photon excitation of smooth silver and two-photon excitation of rough silver. \textcopyright{} 1996 The American Physical Society.

A two-photon photoemission study of GaAs(100)

Two broad spectral features are identified in the direct two-photon photoemission spectra for oxidized GaAs(100) for photon energies between 3.1 eV and 3.5 eV. The double structure is consistent with the optically allowed direct two-photon transitions from the valence bands near the Γ point. This interpretation includes intermediate-state relaxation within the time period of a single laser pulse, namely 180 fs. For annealed GaAs(100) surfaces the work function is approximately 0.6 eV larger, than for the oxidized sample and the double structure is no longer visible.

Characterization of the n=1 image state on Ni(111) with two-photon photoemission

The binding energy and E(k∥) dispersion of the n=1 image potential state have been measured on Ni(111) using angle-resolved two-photon photoemission. The binding energy, εn=1, is found to be 0.8±0.1 eV. The E(k∥) dispersion relation is well described by an effective mass m*/me, of 1.0±0.1, in disagreement with earlier momentum-resolved inverse photoemission results, but in agreement with theoretical predictions based on a phase analysis of the n=1 image state wave function. Two-photon photoemission spectra were also recorded for Ni(111)-p(2×2)-O, showing the disappearance of the image state and the emergence of a weak, broad, nondispersive feature attributable to two-photon photoemission from states originating near the Fermi level.