Recoil effects of photoelectrons in a solid

Abstract

High energy resolution C 1s photoelectron spectra of graphite were measured at the excitation energy of 340, 870, 5950 and 7940eV using synchrotron radiation. On increasing the excitation energy, i.e., increasing kinetic energy of the photoelectron, the bulk origin C 1s peak position shifts to higher binding energies. This systematic shift is due to the kinetic energy loss of the high-energy photoelectron by kicking the atom, and is clear evidence of the recoil effect in photoelectron emission. It is also observed that the asymmetric broadening increases for the higher energy photoelectrons. All these recoil effects can be quantified in the same manner as the Mössbauer effect for γ-ray emission from nuclei embedded in crystals.PACS numbers: 79.60.-i, 79.20.-m, 79.20.-m Photoelectron spectroscopy is widely used for the study of electronic structure of solids [1]. The binding energy E B of the electron is often calculated from the photoelectron kinetic energy using the following equation:where E kin is the measured electron kinetic energy, hν is the photon energy for excitation, and φ is the work function. This procedure overlooked recoil effects, which lead to part of the kinetic energy being imparted to the emitting atom. As a result, the binding energy determined in this way is greater than the true binding energy. This effect is very small for vacuum ultraviolet and soft x-ray photoelectron spectra, so that recoil effects can be safely neglected. For photoelectrons with 1000 eV kinetic energy emitted from carbon atom, the recoil energy is estimated to be only ∼ 45 meV, although Kukk et al. succeeded recently to observe small deviation in spectral shape of vibronic lines in gaseous methane, which have been attributed to a recoil effect [2].The momentum transfer at recoil is a fundamental process observed in experiments of neutron and x-ray scattering [3], high-energy electron backscattering [4,5], etc. For photoelectron emission, Domcke and Cederbaum [6] predicted that the recoil effect can be observed as a spectral modification for gaseous molecules with light atoms. Quite recently, Fujikawa et al. [7] evaluated the amount of shift and broadening of core-level photoelectron spectra, as well as for electron backscattering, due to recoil effect in solids. It is noted that at keV energies, since the momentum of an electron is much larger than that of a photon of the same energy, and the transferred momentum is largely that of the emitted electron, it should be possible to detect recoil effects in photoelectron emission with keV energies.In the last few years, hard x-ray photoelectron spectroscopy with the excitation energy of 6-8 keV has been realized using high brilliance synchrotron radiation [8,9,10], resulting in useful studies on semiconductors and correlated materials [11,12]. Since the achieved energy resolution is quite good (∆E < 80 meV), it gives us an opportunity to investigate recoil effects in a solid.In this study, we measured the C 1s core level photoelectron spectra of highly oriented p...

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

“…Recoil is present in all photoemission processes and the corresponding loss of photoelectron kinetic energy (ΔE K ) is given by ΔE K = E K (m e /M), where E K is the photoelectron kinetic energy, m e is the electron rest mass, and M the emitting atom mass (49). Therefore, recoil effects are not negligible for high photoelectron kinetic energy and light elements (49,57) and the energy difference observed here is a consequence of the larger photon energy used in the present work (4 keV) compared to typically used soft X-rays (< 2 keV). A detailed analysis of the VB photoemission spectra from the biphasic coating is reported in Figure 2d for the cases of measurement under pristine, hydrated, and electrochemical conditions.…”

Atomic Auger Doppler effects upon emission of fast photoelectrons

“…Some years later rotational recoil was also observed 13 . Using hard X-rays of several keV in energy, the recoil effect was also observed in the valence 14 and core ionization 15 of solids. Another reported observation of Doppler phenomenon in electron emission is the rotational Doppler effect, where the rotational motion of an object affects the energy of emitted radiation.…”

Phonon effects on X-ray absorption and X-ray photoemission spectra

“…Recent experimental work by Takata et al clearly shows that C 1s peak shift to high binding energy side with increase of photon energy [13]. The energy shift is well explained by…”

“…Of course the core displacement should have some influence on the diffraction terms B,C,D,..., however the DW factors have to be more important. In recent high-energy XPS (ω k >5keV) analyses, we find that recoil effects in the direct term A are affected by phonon excitation [13][14][15][16].…”