Effect of Oxygen on the Soft X-Ray Appearance Potential Spectrum of Chromium

Soft x‐ray appearance potential spectra have been obtained for clean and oxygen‐exposed chromium surfaces. The measurements were made with a simplified spectrometer which uses the walls of the vacuum chamber as the photoelectric soft x‐ray detector. The oxygen exposure gives rise to a multipeaked structure corresponding to the excitation of the oxygen K shell, with a threshold at $\text{529.1±0.5}\mathrm{eV}$ . In addition, the detailed spectra for the L excitation of chromium are markedly altered. The L₃ threshold is increased from 573.5 to 574.1 eV, and the L₃ peak width, which is related to the width of the unfilled d band, increases by 0.8 eV. Although the absolute thresholds are accurate to only about $\text{± 0.5}\mathrm{eV}$ , changes of less than 0.1 eV are easily observed. The detailed form of the fine structure between the L₂ and L₁ excitations, which is due to characteristic loss replicas of the major peaks, is also found to change as a result of the oxygen exposure.