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

Abstract

Some important phonon effects observed in X-ray absorption and X-ray photoemis-sion spectra are discussed on the basis of nonequilibrium Green's function theory. For the pre-edge structures, the intensity associated with forbidden electric dipole transition is sensitive to temperature compared with allowed electric quadrupole transition. We also discuss the FC and their interference, which have negligible contribution to pre-edge intensity and energy shift. The quasi-particle energy is also influenced by the core displacement which can be responsible for the peak shift of the pre-edges. We also discuss the photoelectron angular distribution caused by the thermal atomic vibration. Research HighlightsPage 2 of 32 A c c e p t e d M a n u s c r i p t Phonon Effects on X-ray Absorption and X-ray Photoemission Spectra Abstract Some important phonon effects observed in X-ray absorption and X-ray photoemission spectra are discussed on the basis of nonequilibrium Green's function theory. This theoretical framework allows us to incorporate phonon effects, such as DebyeWaller (DW) factors, Franck-Condon (FC) factors and electron-phonon interactions in a natural way. In the case of core level excitations, we can take into account the core-hole effects in lesser Green's function g < and photoelectron propagation in greater Green's function g > . For the core-hole propagation we derive some formulas to describe the thermally displaced core functions: we have p components even for deep core s orbital due to the thermal motion. We should notice that the thermal fluctuation is quite small but it is already in the order of the spread of the core functions. Applying Mermin's theorem, we can calculate the thermal average of the hole propagator g < : Here an important ingredient is the Debye-Waller factor used in X-ray and neutron diffraction. For the pre-edge structures, the intensity associated with forbidden electric dipole transition is sensitive to the temperature compared with allowed electric quadrupole transition. We also discuss the FC and their interference, which have negligible contribution to pre-edge intensity and energy shift. The quasi-particle energy is also influenced by the core displacement which can be responsible for the peak shift of the pre-edges. We also discuss the changes of the photoelectron angular distributions caused by the thermal atomic vibration.

Temperature and radiation effects at the fluorine K-edge in LiF

“…In SrTiO 3 at the Ti K-edge the effect of temperature was shown to lead to an increase in a similar preedge feature [31]. In fact, this effect has been attempted to be generalized both with respect to direct phonon effects captured in DFT and based on nonequilibrium Green's functions [32,33]. The method has been extended to include zero point motion and other quasiharmonic approximations producing an excellent match with experiment for corundum [34].…”

Phonon effects on x-ray absorption and nuclear magnetic resonance spectroscopies

“…A substantial theoretical work proved that vibrations could be represented as the convolution of the x-ray absorption cross section, calculated for the equilibrium configuration, with the phonon spectral function. 26,27 This theory has been applied to reproduce room-temperature experimental XANES spectra in Ref. 20.…”

Determination of Ferro‐ and Antiferroelectricity Using the Temperature Dependence of the Pre‐Edge Features in the XANES Spectra: XANES Study of Tetragonal and Cubic ATiO3 (A = Sr, Ba, and Pb) and PbZrO3

“…For temperatures higher than 450 K, the gradient of the A2 peak intensity for BaTiO 3 is similar to that for TiO 2 anatase. This increase is thought to be because of deformation vibration in thermal vibration . The absorption intensity of X‐rays (Figure ) linearly changes with temperature in a wide temperature range, and even at low temperature where the quantum effect should occur.…”

“…A large temperature dependence of the pre‐edge peaks results in a quadrupole transition and a large density of d states below the p states. The temperature dependence is not only because of a phase transition or local geometrical distortion of the titanium site but also because of the thermal vibration of the absorbing and scattering atoms . The temperature change of the X‐ray absorptivity at each peak position fluctuates by the hybridized orbital proportion and site geometry.…”