Measuring The Impossible: Structure of Hydrogen and Helium Atoms Using X-Rays

  • Hydrogen and helium were previously considered to be impossible to measure by X-ray. 
  • Contrary to this belief, researchers have measured the vibrational structure of both elements using Ambient Pressure X-Ray Photoelectron Spectroscopy. 

It’s impossible to detect hydrogen and helium, the two most abundant elements in the Universe, using X-Ray Photoelectron Spectroscopy (XPS). Both elements have an extremely small photoelectron cross-section and they hardly share their electron to form other compounds. This is what previous studies have taught us so far.

Contrary to popular belief, researchers at Brookhaven National Laboratory, Stony Brook University, NC State University, and Harvard University have measured the vibration structure of both elements using XPS.

XPS is a useful measurement technique that evaluates the elemental composition, electronic state and chemical state of the elements within a material. It can detect all elements in the parts per thousand range.

The new findings disprove all scientific literature that claims it’s impossible to obtain XPS spectra of elements lighter than lithium due to the low probability of electron photo-ejection from these elements.

What Makes This Possible?

Using ambient pressure XPS (AP-XPS) measurements, researchers have demonstrated that gas phase XPS spectra of helium and hydrogen can be obtained via a very bright X-ray source.

The exceptional brightness of a beam at the National Synchrotron Light Source-II dramatically raises the chances of a photon colliding with a gas atom at ambient pressures.

The spectrum of helium gas molecules shows a symmetric peak from its orbit, whereas hydrogen gas spectrum shows an asymmetric peak, which is associated with multiple possible vibrational modes of the final phase.

Reference: | doi:10.1063/1.5022479 | US Department of Energy

These outcomes have important implications to better understand the limitations of the XPS spectra. Furthermore, this research makes the XPS technique useful for direct studies of hydrogen and helium.

Technical Details

The use of synchrotron radiation is important for observing hydrogen and helium in XPS. Typically, these sources have photon fluxes in the order of 1013, which is 3 order of magnitude higher than conventional lab sources.

X-ray triggering photo-ejection of an electron from hydrogen (left) and helium (right) | Credit: Brookhaven National Laboratory 

The synchrotrons should provide configurable photon energies, for example, Coherent Soft X-ray Scattering and Spectroscopy beamline at the National Synchrotron Light Source II has energies, ranging from 250 eV to 2,000 eV.

At the low energy range, the cross-section of the 1s orbitals of hydrogen is nearly 1 kBarn. Together, the 1s photoelectron intensity of hydrogen is 6 orders of magnitude more intense with synchrotron-based XPS than with lab-based XPS.

Read: What’s The Relation Between Periodic Table and Superconductivity?

The potential of detecting hydrogen is specifically relevant for AP-XPS. Using this methodology, which has greatly improved in the last 20 years, liquid or solid samples are being exposed to gas surroundings with pressures up to a few Torr. In this study, researchers leveraged both AP-XPS and synchrotron light to obtain photoelectron spectra of hydrogen and helium in the gas phase.

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