Characterizing the Intrinsic Background in XPS Using the Narrow-Shirley Approach

The background signal in X-ray Photoelectron Spectroscopy (XPS) consists of extrinsic and intrinsic components. This study focuses on characterizing the intrinsic background, particularly its structure across an extended binding energy (BE) range beyond the vicinity of the main photoelectron peak. The structure of the intrinsic background in the extended region can be reproduced through a rich set of wide Gaussian peaks. In the near-peak region, the intrinsic background is successfully characterized using the empirical Shirley algorithm. Since the Shirley algorithm fails when applied to extended energy regions by overshooting the experimental signal, a modified version must be employed that decays beyond the near-peak region. We found that a functional form flat near the peak (as in the Shirley algorithm) decays in a Gaussian-like manner for higher binding energies, satisfactorily reproduces the experimental data, and allows for revealing the rest of the rich structure of the intrinsic background. For this reason, we termed it a narrow-Shirley (NS) background. The characterization of the structure of the extended region of the intrinsic background enables the qualitative exploration of its physical origin. Building on previous work linking the near-peak Shirley signal to Interchannel Coupling with Valence Band Losses (ICVBL), we propose and provide qualitative evidence that this ICVBL mechanism is responsible for the entire intrinsic background structure across the extended energy range; this mechanism involves the absorption of a photon by a participating core level. Two of the predictions of the ICVBL mechanism are tested by comparing the structure of the intrinsic background with Auger Electron Spectroscopy (AES) and X-ray Absorption Spectroscopy (XAS) data; a third prediction, the expected modulation of the intrinsic background with photon energies around the threshold of the participating core level, is tested through synchrotron experiments.