The Advanced Photon Source
a U.S. Department of Energy Office of Science User Facility

Stephen Southworth Elected to APS Fellowship

Argonne Senior Scientist Stephen Southworth has been named a Fellow of the American Physical Society, an honor limited to no more than one-half of one percent of the society's membership of more than 50,000.

He is recognized for “pioneering the development of atomic and molecular spectroscopies with third- and fourth-generation light sources including such new effects as higher multipole asymmetries, double -shell photoionization, and femtosecond electronic response of atoms to ultra-intense x-rays.”

Southworth is the leader of the Atomic, Molecular, and Optical Physics Group in the Argonne X-ray Science Division at the Advanced Photon Source (APS). His work has had a major impact on the field of atomic and molecular physics.

In synchrotron-based work, he did innovative experiments on the anisotropy of polarized x-ray emission from molecules, where distinctly different angular distributions were obtained for x-ray emission involving molecular orbitals of different symmetry. He also performed seminal experiments on double K-shell photoionization of atoms and non-dipole asymmetries in photoelectron angular distributions.

In recent work, x rays are being used to explore atoms and molecules that are excited, aligned or ionized by optical lasers. Two particularly challenging experiments included the development of an x-ray microprobe at the APS, and the observation of ultra-fast laser induced x-ray transparency at the Advanced Light Source at Lawrence Berkeley Laboratory.

Southworth was also a key player in the first experiments at the Linac Coherent Light Source, which studied femtosecond electronic response of atoms to ultra-intense x-rays. In this experiment, fully stripped neon was obtained in the course of a single femtosecond x-ray pulse, creating a condition of x-ray transparency induced by the intense pulse of x-rays.

Southworth emphasizes the importance of teamwork and is grateful to have worked with excellent collaborators on every project.

The Advanced Photon Source at Argonne National Laboratory is one of five national synchrotron radiation light sources supported by the U.S. Department of Energy’s Office of Science to carry out applied and basic research to understand, predict, and ultimately control matter and energy at the electronic, atomic, and molecular levels, provide the foundations for new energy technologies, and support DOE missions in energy, environment, and national security. To learn more about the Office of Science x-ray user facilities, visit

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