Sample charging in x-ray photoelectron spectroscopy

Sample charging remains one of the most serious obstacles in x-ray photoelectron spectroscopy of poorly conducting materials. It shifts spectral peaks, distorts line shapes, compromises energy referencing, and, thus, makes the entire data acquisition and analysis process challenging. This review examines charging from a physical perspective. Particular emphasis is placed on the factors that determine the steady-state surface potential, including intrinsic and x-ray-induced conductivity, x-ray attenuation length, photoionization cross sections, secondary-electron yield, specimen thickness, mounting geometry, excitation conditions, and the role of flood guns. This article further discusses how charging can be recognized experimentally, how it may be mitigated or, in favorable cases, eliminated, and how controlled charging can even be exploited to provide useful information about analyzed samples. Most results are based on thin film specimens supported on conducting substrates. This model geometry is well suited for studies of basic phenomena because mechanisms operating over length scales from nanometers to several micrometers can be investigated separately. The concepts developed here, based on such deliberately idealized configurations, should, therefore, be regarded not as an exhaustive description of every charging scenario but rather as a framework that can guide the analysis of charging behavior across a broad range of materials and sample types.