Two-photon Photoemission Process Research Articles

Relativistic theory of two-photon photoemission from ferromagnetic materials

A fully relativistic theory of angle-resolved two-photon photoemission for ferromagnetic materials is derived from the general theory of pump-probe photoemission that is based on the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study nonequilibrium phenomena in nonmagnetic as well as in magnetic solids, as for example ultrafast demagnetization processes. Our theoretical approach aims at a quantitative description of the time-dependent spectroscopic properties of specific solid systems under consideration, and it allows for the inclusion of static correlation effects via the local spin-density and dynamical mean-field theory electronic structure approach. To this end we follow Pendry's one-step theory of the photoemission process as closely as possible and make extensive use of concepts of relativistic multiple - scattering theory within the layer-Korringa-Kohn-Rostoker method in order to guarantee angular resolution in the spectroscopic calculations. As usual the final state of the two-photon photoemission process is represented by a time-reversed low-energy electron diffraction state. The formalism allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple ferromagnetic metals. A first application to the Fe(100) surface is discussed in detail.

Two-photon photoemission from a copper cathode in anX-band photoinjector

This paper presents two-photon photoemission from a copper cathode in an $X$-band photoinjector. We experimentally verified that the electron bunch charge from photoemission out of a copper cathode scales with laser intensity (I) square for 400 nm wavelength photons. We compare this two-photon photoemission process with the single photon process at 266 nm. Despite the high reflectivity ($R$) of the copper surface for 400 nm photons ($R=0.48$) and higher thermal energy of photoelectrons (two-photon at 200 nm) compared to 266 nm photoelectrons, the quantum efficiency of the two-photon photoemission process (400 nm) exceeds the single-photon process (266 nm) when the incident laser intensity is above $300\text{ }\text{ }\mathrm{GW}/{\mathrm{cm}}^{2}$. At the same laser pulse energy ($E$) and other experimental conditions, emitted charge scales inversely with the laser pulse duration. A thermal emittance of 2.7 mm-mrad per mm root mean square (rms) was measured on our cathode which exceeds by sixty percent larger compared to the theoretical predictions, but this discrepancy is similar to previous experimental thermal emittance on copper cathodes with 266 nm photons. The damage of the cathode surface of our first-generation $X$-band gun from both rf breakdowns and laser impacts mostly explains this result. Using a 400 nm laser can substantially simplify the photoinjector system, and make it an alternative solution for compact pulsed electron sources.

Nonadiabatic dynamics of electron scattering from adsorbates in surface bands

We present a comparative study of nonadiabatic dynamics of electron scattering in quasi-two-dimensional surface band which is induced by the long-range component of the interactions with a random array of adsorbates. Using three complementary model descriptions of intraband spatiotemporal propagation of quasiparticles that go beyond the single-adsorbate scattering approach we are able to identify distinct subsequent regimes of evolution of an electron following its promotion into an unoccupied band state: i early quadratic or ballistic decay of the initial-state survival probability within the Heisenberg uncertainty window, ii preasymptotic exponential decay governed by the self-consistent Fermi golden rule scattering rate, and iii asymptotic decay described by a combined inverse power-law and logarithmic behavior. The developed models are applied to discuss the dynamics of intraband adsorbate-induced scattering of hot electrons excited into the n = 1 image-potential band on Cu100 surface during the first stage of a two-photon photoemission process. Estimates of crossovers between the distinct evolution regimes enable assessments of the lifespan of a standard quasiparticle behavior and thereby of the range of applicability of the widely used Fermi golden rule and optical Bloch equations approach for description of adsorbate-induced quasiparticle decay and dephasing in ultrafast experiments.

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.

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.

Electronic structure of SiC(0001) surfaces studied by two-photon photoemission

The electronic structure of the valence and conduction bands at the Si(100) surface has been studied by two-photon photoemission over a wide photon-energy range. The ionization energy was determined to 5.40 ′ 0.03 eV. Theoccupied surface state at r is placed 0.15 ′ 0.06 below the valence-band maximum. Several other spectral features are assigned to transitions involving surface states and between bulk bands including backfolded bands due to the surface reconstruction. The moderate agreement between experimental data and band-structure calculations calls for an improved theoretical description of the two-photon photoemission process at semiconductor surfaces incorporating, e.g., a one-step model and excitonic effects.

Electronic structure of Si(100) surfaces studied by two-photon photoemission

The electronic structure of the valence and conduction bands at the Si(100) surface has been studied by two-photon photoemission over a wide photon-energy range. The ionization energy was determined to $5.40\ifmmode\pm\else\textpm\fi{}0.03\mathrm{eV}.$ The occupied surface state at $\overline{\ensuremath{\Gamma}}$ is placed $0.15\ifmmode\pm\else\textpm\fi{}0.06$ below the valence-band maximum. Several other spectral features are assigned to transitions involving surface states and between bulk bands including backfolded bands due to the surface reconstruction. The moderate agreement between experimental data and band-structure calculations calls for an improved theoretical description of the two-photon photoemission process at semiconductor surfaces incorporating, e.g., a one-step model and excitonic effects.

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.

Cluster-surface interaction studied by time-resolved two-photon photoemission

+ clusters (n=2-9)deposited onto highly oriented pyrolytic graphite (HOPG) substrates at liquid nitrogen temperatures. The deposition was carried out with variable kinetic energies of the clusters. Clusters deposited with high kinetic energy (up to 60 eV/cluster) become fragmented upon impact. For low deposition energies (1–4 eV/cluster) the size dependence of the photoelectron spectra reveals a pronounced odd/even effect, which is well known for gas phase silver clusters. This indicates that the soft deposited clusters retain their size and identity on the sample. The phase of the odd/even effect suggests that transient negatively charged cluster ions serve as an intermediate step in the two-photon photoemission process. The lifetime of the anions rises with cluster size. This is attributed to an increasing electronic density of states for larger clusters.

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 ...