Bell Labs: X-ray photoemission and absorption research
Paul Citrin at the X15B beamline at Brookhaven National Lab’s National Synchrotron Light Source facility.
Courtesy of Paul Citrin.
Reflections by P. H. Citrin
P. H. Citrin, “X-Ray Photoemission and Absorption Research” (.pdf, 3 mb)In this article, Paul Citrin looks back at work carried out at Bell Labs with x-ray photoemission and x-ray absorption spectroscopies during a period when both techniques were undergoing rapid experimental and conceptual development. Rather than a comprehensive review, it is a personal account of how specific electronic and structural problems were identified, pursued, and connected using these two complementary probes.
For x-ray photoemission (XPS), improved energy resolution and surface sensitivity exposed intrinsic effects that had previously been obscured, misinterpreted, or overlooked: asymmetric line shapes from many-body screening, phonon broadening, and distinct surface-atom core-level contributions. Understanding these features required separating initial- and final-state effects and quantitatively interpreting XPS observables with x-ray edge singularities and many-body calculations.
X-ray absorption (EXAFS) evolved in parallel. The availability of tunable, intense synchrotron radiation made it possible to move beyond bulk EXAFS to surface-sensitive and near-edge measurements. SEXAFS and NEXAFS provided direct, empirical access to local geometries and bonding features at surfaces and in dilute or disordered bulk systems, often addressing structural questions inaccessible to any other approach.
A recurring theme is the productive interplay between these x-ray techniques: insights from one informed the development of the other. That interplay was made possible by the Bell Labs environment—the close interaction between experiment and theory, early access to synchrotron facilities, and the freedom to pursue unresolved questions across disciplinary boundaries in the company of remarkable colleagues.
Cite this resource
P. H. Citrin, “X-ray Photoemission and Absorption Research,” History of Bell Labs Physical Research, 1980–2006 collection, edited by Arthur Ramirez, Cherry Murray, and David Bishop, American Institute of Physics, 2026, https://www.aip.org/history/bell-labs-physical-research-1980-2006/x-ray-photoemission-absorption.