The role of space charge in spin-resolved photoemission experiments

Abstract

Spin-resolved photoemission is one of the most direct ways of measuring the magnetization of a ferromagnet. If all valence band electrons contribute, the measured average spin polarization is proportional to the magnetization. This is even the case if electronic excitations are present, and thus is of particular interest for studying the response of the magnetization to a pump laser pulse. Here, we demonstrate the feasibility of ultrafast spin-resolved photoemission using free electron laser (FEL) radiation and investigate the effect of space charge on the detected spin polarization. The sample is exposed to the radiation of the FEL FLASH in Hamburg. Surprisingly, the measured spin polarization depends on the fluence of the FEL radiation: a higher FEL fluence reduces the Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. measured spin polarization. Space-charge simulations can explain this effect. These findings have consequences for future spin-polarized photoemission experiments using pulsed photon sources.Keywords: spin-polarized photoemission, free electron laser radiation, space charge New J. Phys. 16 (2014) 043031 A Fognini et al New J. Phys. 16 (2014) 043031 A Fognini et al 6 New J. Phys. 16 (2014) 043031 A Fognini et al 7

Space-, time- and spin-resolved photoemission

“…For normalization, the sample's magnetic orientation is reversed between bursts by magnetic field pulses. The 3000 X-ray pulses in one second may excite just a few electrons per pulse in order to avoid the influence of space charge in the electron analyzer optics [88]. The transmission of the electron analyzer was increased by a high pass energy of 100 eV and a larger entrance slit thus measuring an energy band of 5 eV.…”

“…Despite the poor statistics, this pilot study demonstrated that multi-hit spin analysis is possible in principle. However, due to the small number of photon pulses and due to severe space-charge problems [88] the noise level in the data is very large. M a n u s c r i p t M a n u s c r i p t M a n u s c r i p t 22 Fig.…”

Concept of a multichannel spin-resolving electron analyzer based on Mott scattering

“…We will now comment on possible saturation of the eCCDs with the very intense FEL pulses. Recent spin-polarized photoemission experiments at FLASH (Fognini et al, 2014) with a single-channel Mott detector have found that more than one electron per pulse reaches the detector of passivated implanted planar silicon (PIPS) which was prohibitive for single-pulse counting. However, these experiments were essentially angle-and energy-integrated, bringing to one detector the whole integral photoemission intensity.…”

“…However, given the very approximate character of this estimate, we cannot rule out that certain pulse energy attenuation may still be necessary. In any case, the problem of single-pulse counting in iMott is less restrictive compared with that of space charge (Fognini et al, 2014), the main encumbrance of photoelectron spectroscopy at FEL sources, which is relieved by an increase of the pulse repetition rate with simultaneous reduction of intensity of each pulse.…”

Correction of the deterministic part of space–charge interaction in momentum microscopy of charged particles

“…The large mean free path of electrons with kinetic energies above ∼5 keV opens access to bulk properties in hard X-ray PES (HAXPES) experiments. Due to the high pulse intensity of such sources, the Coulomb interaction of the particles released within a single short pulse becomes substantial and can result in prohibitively large energy broadenings [6] and a loss of angular-and even of spin information [7]. This is commonly referred to as the space-charge problem.…”