Hydrogen atom diffraction through free-standing single-layer graphene

What was measured

A free-standing graphene monolayer, monocrystalline over the illuminated area and with nothing behind it to scatter from, was placed in a beam of neutral hydrogen atoms. The transmitted atoms were collected on a position-sensitive detector far enough downstream that the milliradian deflections of the first diffraction orders separate cleanly from the direct beam.

The pattern that appears is the reciprocal lattice of graphene. That it appears at all is the result: it means a substantial fraction of the atoms cross a sheet of matter without losing the phase coherence that makes diffraction possible.

Why it was not obvious

An atom passing through a monolayer travels within an ångström of carbon nuclei. Every electronic excitation it provokes on the way, and every quantum of energy left in the lattice, marks which path it took and destroys the interference. The open question was whether an elastic channel survives at all at these energies, and how much of the beam stays in it.

Where it led

Establishing the effect made the intensities usable as a measurement in their own right, which is what the transmission work has been doing since: using the relative brightness of the diffraction orders to constrain the projected atom–surface interaction potential, and extending the method past graphene to other two-dimensional targets.

This page is a summary, written for whoever scanned the code on the poster. The poster itself carries the data, the error bars and the conditions; the paper below carries the argument in full.