Electron pair emission from surfaces: some general experimental considerations

We discuss some experimental facets of electron pair emission from surfaces using two different experimental approaches. In the first case the instrument consists of a pair of hemispherical analyzers which are operated with continuous primary beams of electrons or photons. The second instrument employs a pair of time-of-flight spectrometers which require a pulsed excitation source. A key experimental quantity is the ratio of ‘true’ to ‘random’ coincidences which can be determined in different ways. Regardless of the type of instrument the primary flux has to adopt a much smaller value than in single electron spectroscopy. We describe different approaches to obtain the relevant count rates, in particular the concept of operating with a delayed coincidence circuit. We also address the question on how to compare the two types of spectrometer in terms of their performance

Extended energy range analysis for angle-resolved time-of-flight photoelectron spectroscopy

An approximation method for electrostatic time-of-flight (ToF) spectroscopy on photoelectrons distributed over a wide energy range is presented. This method is an extension of conventional analysis and aims at specific energy and angular regions, where distinctly different emission angles and energies are mapped to the same ToF and detector position by the spectrometer. The general formulation and the systematic errors are presented, and a practical example is demonstrated for photoelectrons from Ag(001) with kinetic energies of 0.5–25 eV

Exchange-Striction Driven Ultrafast Nonthermal Lattice Dynamics in NiO

We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AFM) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce Q-resolved effective temperatures describing the transient lattice. We identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ∼0.3  ps and persists for 25 ps. We associate this with transient changes to the AFM exchange striction-induced lattice distortion, supported by the observation of a transient Q asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order

SiO₂/Si(001) studied by time-resolved valence band photoemission at MHz repetition rates: Linear and nonlinear excitation of surface photovoltage

The authors investigate the fluence and doping dependence of the surface photovoltage (SPV) shifts at SiO2/Si(001) interfaces by time-resolved photoelectron spectroscopy. Charge carriers are excited by pumping photon energies of hνpump = 1.2 and 2.4 eV and probed by high-order harmonics of hνprobe = 22.6 eV at 0.2 and 0.7 MHz repetition rates. The authors observe SPV shifts of the non-bonding O2p state by 240 meV for SiO2/p-Si and by -140 meV for SiO2/n-Si upon pumping with hνpump = 1.2 eV, and their decay rate is estimated from time-resolved measurements. Moreover, the authors observe a striking pumping fluence dependence of SPV at these interfaces, which indicates charge carrier generation by both linear and nonlinear optical excitations

Laser-based double photoemission spectroscopy at surfaces

The recent development of double photoemission (DPE) spectroscopy at surfaces using laser-based high-order harmonic generation in combination with time-of-flight electron spectroscopy is reviewed. Relevant experimental conditions including the solid angle for collecting photoelectron pairs, the energy and angular resolutions, as well as the repetition rate and the photon energy range of light sources are introduced. As examples, we provide an overview of laser-based DPE results on the noble metals Ag and Cu as well as transition metal oxides NiO and CoO. The DPE energy and angular distributions of photoelectron pairs are compared with emphasis on the possible indications of electron-electron interaction. Potential further developments including femtosecond time-resolved DPE experiments are outlined

Band-resolved double photoemission spectroscopy on correlated valence electron pairs in metals

Correlated valence electrons in Ag and Cu are investigated using double photoemission spectroscopy driven by a high-order harmonic light source. Electron pairs consisting of two d electrons as well as pairs with one sp and one d electron are resolved in the two-dimensional energy spectrum. Surprisingly, the intensity ratio of sp−d to d−d pairs from Ag is 3 times higher than in the self-convoluted density of states. Our results directly show the band-resolved configurations of electron pairs in solids and emphasize a band-dependent picture for electron correlation even in these paradigmatic metals

Exchange-Striction Driven Ultrafast Nonthermal Lattice Dynamics in NiO

We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AFM) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce Q-resolved effective temperatures describing the transient lattice. We identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ∼0.3  ps and persists for 25 ps. We associate this with transient changes to the AFM exchange striction-induced lattice distortion, supported by the observation of a transient Q asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order