Photoemission Investigation of the Electronic Structure of Chromium

Abstract

Photoemission measurements of the quantum yield and energy distributions are obtained from evaporated films of Cr in the spectral range from 4.68 (threshold) to 11.8 eV. Data are obtained at room temperature, at liquid-air temperature, and above the Néel temperature. No evidence is found that the conservation of the Bloch-state wave vector is important in the optical excitations. The temperature dependence shows that the excitations are not phonon-assisted. The nondirect transition model is used to obtain the optical density of filled states, $N_v$, and of empty states, $N_c$. The matrix elements for the excitation are approximately constant. $N_v$ has a peak at $E-E_F=-5.5\mathrm{}\pm 0.3$ eV that corresponds to the anomalous peak observed in Fe, Co, and Ni, which is not predicted by band calculations. $E_F$ is the Fermi energy. $N_v$ has peaks at $E-E_F$ equal to -1.1±0.2 and -2.2±0.2 eV and a shoulder at 0.2 eV. The properties of $N_v$ show that the rigid-band model is invalid for obtaining the Cr density of states from that for Fe. $N_v$ near $E_F$ correlates with specific-heat results. No structure is observed in $N_c$ above the vacuum energy and the empty $d$ bands are found to be approximately 1.5 eV wide by analyzing the quantum yield. The photodiode is used to obtain a semiquantitative measure of the reflectance, which contains structure that is consistent with the anomalous peak in $N_v$. The quantum yield and reflectance show a feature at 9.4 eV that is suggestive of a surface or impurity plasmon and show no significant effects on the energy distributions of the emitted electrons. Studies above and below the Néel temperature show that electronic-structure changes due to magnetic ordering are too small to be observed, i.e., no greater than 0.3 eV.