Effects In Photoelectron Spectroscopy Research Articles

High photon flux table-top coherent extreme-ultraviolet source

High harmonic generation (HHG) enables extreme ultraviolet radiation with table-top setups. Its exceptional properties, such as coherence and (sub)-femtosecond pulse durations, have led to a diversity of applications. Some of these require a high photon flux and megahertz repetition rates, e.g. to avoid space charge effects in photoelectron spectroscopy. To date this has only been achieved with enhancement cavities. Here, we establish a novel route towards powerful HHG sources. By achieving phase-matched HHG of a megahertz fibre laser we generate a broad plateau (25 eV - 40 eV) of strong harmonics, each containing more than $10^{12}$ photons/s, which constitutes an increase by more than one order of magnitude in that wavelength range. The strongest harmonic (H25, 30 eV) has an average power of 143 $\mu$W ($3\cdot10^{13}$ photons/s). This concept will greatly advance and facilitate applications in photoelectron or coincidence spectroscopy, coherent diffractive imaging or (multidimensional) surface science.

Quantum well states and film structure

Pronounced quantum size effects in photo-electron spectroscopy from 5 to 26 monolayer thick Au films deposited on Nb(1 0 0) occur only when the deposition is made at low temperatures (150 K). The low deposition temperature causes the long-range order of the frozen Au surface to be quite poor, making the film structure determination with standard methods such as RHEED or LEED impossible. Also techniques which do not need long-range order, such as X-ray photo-electron diffraction, cannot be used in this case to determine the structure of the Au films. The XPD measurements only show that the hexagonal dense-packed Au layers are situated perpendicularly to the substrate surface, but cannot reveal their stacking sequence. Using an ARUPS electron spectrometer and the properties of the quantum well states of the film we can determine, even in this case, the periodicity of the film states in the reciprocal space, and, consequently, the structure of the Au films. The structure turns out to be hexagonal (hcp and/or dhcp) and not fcc.

Quantum well states in thin(112¯0)oriented Au films

We have measured pronounced d-type quantum size effects in photoelectron spectroscopy from 5--26 monolayer thick $(112\ifmmode\bar\else\textasciimacron\fi{}0)$ oriented Au films whose surface normal direction is not perpendicular to any face of the bulk Brillouin zone. Also in this case, the quantum well states can be explained by the discretization of the bulk band structure if it is given in a BZ which is consistent with the symmetry of the films, but different from the bulk one.

Final-state effects in photoemission from metal-semiconductor interfaces.

In this Letter we stress the importance of final-state effects in photoelectron spectroscopy. In particullar, we address the problem of Schottky-barrier formation, as studied via core-level shifts in photoemission. We have calculated the shift of the core-level distribution when a semiconductor surface is covered with a metal, using a wave-vector-dependent image-screening model. We conclude that final-state effects, which are generally neglected in this context, are in fact quite important. This conclusion is supported by experimental observations reported in the literature

On the separation of initial and final state effects in photoelectron spectroscopy using an extension of the auger-parameter concept

First, we show that the quantity Δβ( i) = Δ E A( kii) + 2Δ E B ( i) — Δ E B ( k) is directly related to the final state relaxation contribution Δ E R ( i) of the binding energy shift Δ E B ( i). Δ E A ( kii) is the kinetic energy shift of the Auger transition which corresponds to the decay of a hole state with a hole in level k into a final state with two holes in level i. The shift parameter Δβ( i), which is based on information on two binding energies, is conceptually similar to Wagner's Auger parameter. To establish the relation between Δβ( i) and Δ R( i) one needs, however, less drastic approximations than in the case of Auger parameter shifts. The only approximation necessary is the assumption that Δ R( i) is determined by coulomb contributions. Secondly, we use Δβ ( i) to analyse the experimental data of eighteen gaseous phosphorus-containing compounds obtained by Sodhi and Cavell 1 1 See ref. 1. . It is shown that Δ R(P 2 p ) is strongly related to changes in the polarizability of the ligands. The initial state effects derived from our study deviate from those expected on the basis of simple electronegativity considerations.

Anomalous effects in photoelectron spectroscopy. The third electronic state of HCN+

The potential energy surface of the ? 2Σ+ state of HCN+ has been calculated ab initio. Due to an avoided crossing process, it exhibits two minima, separated by a saddle point. This entails the following surprising conclusions concerning the corresponding photoelectron band. The band exhibits a vibrational structure in an energy range where the potential energy surface is dissociative. The observed vibrational structure should be assigned to a composite motion involving the in-phase stretching of CH and CN bonds. Since the electronic transition moment strongly depends upon the nuclear geometry, the observed vibrational intensities cannot be related to Franck–Condon factors. The band cannot be described in terms of simple electron removal from a particular molecular orbital. Finally, the dissociation mechanism of HCN+ in photoionization experiments is also briefly discussed. Production of H+ fragments is shown to take place via several conical intersections between potential energy surfaces. Some general comments are added concerning the validity of the orbital picture in photoelectron spectroscopy.

Photoionization of4felectrons from rare-earth atoms at a surface

Measurement of photocurrent versus frequency of $4f$ electrons from surface-rare-earth atoms is proposed as a means for identifying refraction effects in photoelectron spectroscopy. The idea of using surface-rare-earth-atom $4f$ electrons takes advantage of the fact that since their wave functions are atomiclike (even though their binding energies are typically only a few eV) any unusual behavior of the photoyield versus frequency will not be ascribable to poorly known surface-electron wave functions, but will have to be associated with dielectric interface effects. Jellium-model calculations are presented which show qualitatively how surface dielectric phenomena should appear in the frequency variation of the ratio of photoyields with $p$-polarized and $s$-polarized light.

Conformational effects through temperature variation of photoelectron spectra

Temperature effects in photoelectron spectroscopy can be used to study electronic structure variation between conformational isomers; this is illustrated by reference to tetramethyl-diphosphine and some disubstituted ethanes.