Laser Photoemission Spectroscopy Research Articles

Exploration of Gas-Liquid Interfaces for Liquid Water and Methanol Using Extreme Ultraviolet Laser Photoemission Spectroscopy.

We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas-liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas-liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a1 band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a1 splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na+ penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a1 splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.

Electronic structure of the iron-based superconductor (La,Eu)FeAsO1-xFx investigated by laser photoemission spectroscopy

We have implemented laser photoemission spectroscopy (PES) to investigate the electronic structure of the iron-based superconductor (La,Eu)FeAsO1-xFx (LaEu1111) which is an interesting compound in the “1111” family showing a high value of the superconducting (SC) transition temperature (Tc) due to Eu doping. At least two energy scales were observed from the PES data in the SC compound: One at ∼14 meV closing around Tc and thus corresponding to the SC gap. Another energy scale appears at ∼35 meV and survives at temperatures above Tc which represents the pseudogap (PG). The non-SC sample (La,Eu)FeAsO shows a PG at ∼ 41 meV. These observations in this new superconductor are consistent with the general trend followed by other compounds in the “1111” family.

Multiple-pseudogap phases in the hydrogen-doped LaFeAsO system

The low energy electronic structure of LaFeAsO1-xHx (0.0 < x < 0.60), the system which exhibits two superconducting domes in its phase diagram, is investigated by utilizing the laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find the suppression of the density of states with cooling, namely the pseudogap formation, for all doping range. The pseudogap in the low x range (i.e. the first superconducting dome regime) gets suppressed with increasing x, more or less similarly to the previous results in F-doped LaFeAsO system. On the other hand, the pseudogap behavior in the second superconducting dome regime at high-x becomes stronger with increasing the H-doping level. The systematic doping dependence shows that the pseudogap is enhanced toward the both ends of the phase diagram where the different types of antiferromagnetic order exist.

Hybridization gap formation in the Kondo insulatorYbB12observed using time-resolved photoemission spectroscopy

A detailed low-energy electronic structure of a Kondo insulator YbB$_{12}$ was revealed by a synergetic combination of ultrahigh-resolution laser photoemission spectroscopy (PES) and time-resolved PES. The former confirmed a 25-meV pseudogap corresponding to the Kondo temperature of this material, and more importantly, it revealed that a 15-meV gap and a Kondo-peak feature developed below a crossover temperature $T^\ast \sim 110$ K. In harmony with this, the latter discovered a very long recombination time exceeding 100 ps below $\sim$$T^\ast$. This is a clear manifestation of photoexcited carriers due to the bottleneck in the recovery dynamics, which is interpreted as a developing hybridization gap of a hard gap.

Low-Temperature and High-Energy-Resolution Laser Photoemission Spectroscopy

We present a review on the developments in the photoemission spectrometer with a vacuum ultraviolet laser at Institute for Solid State Physics at the University of Tokyo. The advantages of high energy resolution, high cooling ability, and bulk sensitivity enable applications with a wide range of materials. We introduce some examples of fine electronic structures detected by laser photoemission spectroscopy and discuss the prospects of research on low-transition-temperature superconductors exhibiting unconventional superconductivity.

Electron–phonon coupling in quantum-well states of the Pb/Si(1 1 1) system

The electron–phonon coupling parameters in the vicinity of the point, , for electronic quantum well states in epitaxial lead films on a Si(1 1 1) substrate are measured using 5, 7 and 12 ML films and femtosecond laser photoemission spectroscopy. The values in the range of 0.6–0.9 were obtained by temperature-dependent line width analysis of occupied quantum well states and found to be considerably smaller than the momentum averaged electron–phonon coupling at the Fermi level of bulk lead, (λ = 1.1–1.7). The results are compared to density functional theory calculations of the lead films with and without interfacial stress. It is shown that the discrepancy can not be explained by means of confinement effects or simple structural modifications of the Pb films and, thus, is attributed to the influence of the substrate on the Pb electronic and vibrational structures.

Mode-selective electron-phonon coupling in laser photoemission on Cu(110)

By combining density functional perturbation theory (DFPT) calculations and laser photoemission spectroscopy (LPES) experiments, we have clarified the selective coupling between low-energy photoelectrons and subsurface phonon modes. A step structure resulting from the inelastic scattering of photoelectrons appeared at 14.7 meV below the Fermi level in the LPES of Cu(110). We found that the inelastic step originates from an indirect excitation in which the electron is excited by the low-energy photon near the $\overline{\mathrm{Y}}$ point and then scattered to the $\overline{\ensuremath{\Gamma}}$ point by phonon modes that are predominantly present in the subsurface region.

Femtosecond‐laser photoemission and photodesorption from magnesia supported gold clusters

AbstractNanosize gold clusters were grown by thermal evaporation on ultrathin magnesia films on Mo(100) and investigated by femtosecond (fs)‐laser photoemission spectroscopy. The surface work function in this system was found to increase with increasing nominal gold coverage. Methyl bromide (CH3Br) molecules adsorb molecularly on the magnesia surface as well as on the gold clusters. At sub‐monolayer coverage the CH3Br molecules were, however, considerably stronger bound to the gold particles than to the magnesia surface as revealed by temperature programmed desorption spectroscopy. To investigate the photoreaction dynamics of the adsorbate molecules fs‐laser pump–probe mass spectrometry was applied. In this new approach the methyl fragment appearance was monitored after photoexcitation of the adsorbed CH3Br with fs time resolution. The transient methyl data revealed that the CH3Br dissociation and subsequent methyl desorption proceeded in less than one picosecond. Most interestingly, a part of the transient signal could be unambiguously assigned to the dynamics of methyl desorption from the gold clusters. The appearance of this signal at about 270 fs after photoexcitation illustrates the massive distortion of the CH3Br electronic level structure due to the strong interaction with the gold clusters compared to the bare magnesia surface. magnified image

Effect of antiferromagnetic ordering on temperature dependent superconducting gap in ErNi 2B 2C: Laser-photoemission spectroscopy

We have performed temperature ( T)-dependent laser-photoemission spectroscopy of antiferromagnetic (AF) superconductor ErNi 2B 2C to study the electronic structure, especially the effect of AF ordering in T-dependent superconducting (SC) gap. To estimate the values of T-dependent SC gap, we fitted the experimental data by Dynes function having an anisotropic s-wave SC gap. From the fitting results, we find a sudden deviation from the BCS prediction just below T N. This observation can be well explained by the theoretical model, indicating that the origin of anomalous T-dependence is competition between rapid evolution of AF molecular field and SC condensation energy.

Bulk Electronic Structure of the Antiferromagnetic Superconducting Phase inErNi2B2C

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi2B2C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below TN. This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

Laser-photoemission study of antiferromagnetic superconductor ErNi2B2C

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (Tc=9.0K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (TN=6.0K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature.

Electronic structure and electron dynamics at the GaSb(0 0 1) surface studied by femtosecond pump-and-probe pulsed laser photoemission spectroscopy

Transiently excited electron states at the GaSb(0 0 1) surface have been studied by means of time- and angle-resolved photoemission spectroscopy based on a femtosecond laser system. A normally unpopulated surface electron state has been found at ∼250 meV above the valence band maximum with a strong confinement at the center of the surface Brillouin zone. The lifetime of transiently excited carriers at the intergap surface states has been found to be ∼11 ps, associated with rapid carrier diffusion.

Ultraviolet laser photoemission spectroscopy of FeSi: Observation of a gap opening in density of states

The temperature $(T)$ dependent gap formation in the density of states (DOS) of FeSi has been investigated by angle-integrated laser photoemission spectroscopy (PES). With decreasing $T$, the evolution of a small gap $(\ensuremath{\sim}60\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ at the Fermi level is observed in the DOS, indicating a $p$-type semiconducting character of this compound. The Fermi edge, which has been controversial in all past PES studies, is extremely small at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in accordance with transport and optical experiments. The $T$ dependence of the gap, which gets smeared out quickly at high $T$ as in optical conductivity spectrum, suggests the existence of a strong scattering mechanism beyond thermal excitations.

Ultrafast electron dynamics at the Ge(111) surface investigated with subpicosecond laser photoemission

Abstract The technique of subpicosecond angle resolved laser photoemission spectroscopy is described. Investigations of the dynamics of electrons excited into normally unoccupied states at the cleaved Ge(111)2 × 1 surface are detailed. In particular, electrons which are excited into bulk states near the surface are observed to scatter into the normally unoccupied antibonding π ∗ band whose minimum is at J in the surface Brillouin zone. Cooling of the sample to near liquid nitrogen temperatures has permitted us to directly delineate the role played by optical phonons in the surface electron dynamics. In addition hole dynamics are reported and renormalization of the surface band gap is observed.

Ultrafast electron dynamics at the Ge(111) surface investigated with subpicosecond laser photoemission

The technique of subpicosecond angle resolved laser photoemission spectroscopy is described. Investigations of the dynamics of electrons excited into normally unoccupied states at the cleaved Ge(111)2 × 1 surface are detailed. In particular, electrons which are excited into bulk states near the surface are observed to scatter into the normally unoccupied antibonding π ∗ band whose minimum is at J̄ in the surface Brillouin zone. Cooling of the sample to near liquid nitrogen temperatures has permitted us to directly delineate the role played by optical phonons in the surface electron dynamics. In addition hole dynamics are reported and renormalization of the surface band gap is observed.

Ultrafast-electron dynamics and recombination on the Ge(111)(2 x 1) pi -bonded surface.

Angle-resolved laser-photoemission spectroscopy has been used to study the ultrafast-electron scattering and recombination processes on the Ge(111) \ensuremath{\pi}-bonded (2\ifmmode\times\else\texttimes\fi{}1) surface with subpicosecond time resolution. Electrons photoexcited into the bulk Ge conduction band scatter into the unoccupied surface antibonding ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band whose minimum is at the J\ifmmode\bar\else\textasciimacron\fi{} point in the surface Brillouin zone. Rapid relaxation to the surface-band minimum is followed by a unique phonon-assisted process in which electrons recombine with bulk holes at the valence-band maximum, which we find to be the primary mechanism responsible for the decay of the transient ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ population. Time-dependent measurements carried out at 300 and 120 K have been employed to determine the role of energetic phonons in the scattering processes. These processes are modeled with a set of rate equations, whose fits to the data yield scattering times used to determine a surface recombination velocity directly. Ultrafast surface-state hole dynamics are observed, and a renormalization of the surface band gap is studied as a function of electron density. The \ensuremath{\pi}-bonded states are fundamentally one dimensional in nature, and thus these results represent studies of band-gap renormalization in a one-dimensional system.

Ultrafast electron dynamics at semiconductor surfaces and interfaces studied with subpicosecond laser photoemission

AbstractThe technique of subpicosecond angle‐resolved laser photoemission spectroscopy is described. Investigations of the dynamics of electrons excited into normally unoccupied states at the cleaved GaAs (110) 1 × 1 and Ge (111) 2 × 1 surfaces are detailed. In addition, we discuss experiments carried out on heterostructures formed by the growth of Ge on the cleaved GaAs (110) surface as well as the molecular beam epitaxy (MBE) surface of GaAs (100) and (111).

Renormalization of the one-dimensional π-π* band gap on the Ge(111) 2×1 surface

Using angle resolved laser photoemission spectroscopy we have measured the transient narrowing of the surface state band gap on the cleaved Ge(111) 2×1 surface. The reconstruction of this surface results in the formation of one-dimensional π bonded chains of atoms which give rise to bonding and antibonding bands whose energy/wave vector dispersions are one-dimensional as well. Photoexcitation of this surface produces a transient π* population which, in turn, leads to an observed narrowing of the surface band gap. We have investigated this renormalization as a function of surface carrier density and laser excitation fluence.