Measuring the impossible: X-ray photoelectron spectroscopy of hydrogen and helium

The results disprove the misconception that it’s impossible to obtain X-ray photoelectron spectroscopy (XPS) spectra of hydrogen and helium, the two lightest elements of the Periodic Table. This was thought to be the case due to low probabilities of electron ejection from these elements induced by X-rays. Unparalleled beam brightness at the National Synchrotron Light Source-II significantly increases the probability of a photon colliding with a gas atom at ambient pressures. The beamline makes it possible to use XPS to directly study the two most abundant elements in the universe. Also, this work helps describe the limits of XPS, opening a broader scope for one of the most useful techniques in materials science.

Summary

X-ray photoelectron spectroscopy (XPS) is one of the most powerful techniques in materials science. However, the literature is filled with claims stating that it’s impossible to use XPS to study the two lightest and most abundant elements in the universe, hydrogen and helium. This work demonstrated that ambient pressure X-ray photoelectron spectra of hydrogen and helium can be obtained when a bright-enough X-ray source is used, such as at the National Synchrotron Light Source II. In the case of helium gas, the spectrum shows a symmetric peak from its only orbital. In the case of hydrogen gas molecules, an asymmetric peak is observed, which is related to the different possible vibrational modes of the final state. The hydrogen molecule vibrational structure is evident in the H₂ 1s spectrum.

Funding

The research carried out in part at the Center for Functional Nanomaterials and the CSX-2 beamline of the National Synchrotron Light Source II, Brookhaven National Laboratory, was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences. J.Q. Zhong and M. Wang were supported by a Laboratory Directed Research and Development Project at Brookhaven National Laboratory, and W.H. Hoffmann was supported by the DOE Science Undergraduate Laboratory Internships. This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis, an Energy Frontier Research Center funded by DOE, Office of Science, Basic Energy Sciences. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility supported by the DOE Office of Science.