Electron Momentum Spectroscopy of Molecular Systems

Electron Momentum Spectroscopy (EMS) is an incisive technique for probing the three-dimensional momentum distributions of electrons in molecules with orbital-specific resolution. In an EMS experiment, a high-energy electron beam impinges on a gaseous target and ejects a bound electron; the energies and angles of both scattered and ejected electrons are measured in coincidence (the (e,2e) method). This yields the electron momentum profile, which relates directly to the Fourier transform of the molecular orbital wavefunction, thus providing a quantum-mechanical image of bonding, orbital hybridisation and many-body interactions. EMS can reveal departures from the independent-particle picture, capture electron correlation effects and test the accuracy of ab initio calculations. Realising the full potential of EMS demands high-resolution spectrometers, meticulous calibration of kinematics and sophisticated theoretical models to interpret the momentum spectra. Applications range from validating electronic‐structure theories to guiding the design of functional materials and, in emerging studies, visualising ultrafast electron dynamics when coupled with pulsed electron sources.

Research from Nature Portfolio

Recent studies have demonstrated the capacity of EMS to image both molecular geometry and electron density distributions simultaneously. By analysing multicentre interference patterns in momentum profiles, researchers have extracted interatomic distances in small molecules with sub-ångström precision, using a single spectrometer to map orbital shapes and bond lengths. This dual capability offers a route to resolve transient structures in chemical reactions and track bond-making or bond-breaking events in real time. Another line of work has extended many-body perturbation theory to interpret EMS data, revealing the pivotal role of electron–vibration coupling in shaping momentum spectra and highlighting the limitations of the Born–Oppenheimer approximation in strongly correlated systems. These landmark contributions underpin efforts to enhance temporal resolution and sensitivity in next-generation EMS experiments.

Electron Momentum Spectroscopy of Molecular Systems publication trend

The graph below shows the total number of articles in electron momentum spectroscopy of molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Electron Momentum Spectroscopy (EMS): A spectroscopic method in which an incident electron collides with a target, ejecting a bound electron; the momenta of both outgoing electrons are measured in coincidence to determine the initial electron’s momentum distribution.

(e,2e) method: The scattering process in EMS where one incoming electron yields two detected outgoing electrons, enabling kinematic reconstruction of the bound electron’s momentum and binding energy.

Momentum profile: A one-dimensional projection of the three-dimensional electron momentum density of a molecular orbital, reflecting its wavefunction in momentum space.

Quasi-particle: An electron whose effective properties are modified by interactions with its environment, leading to renormalised energy and lifetime compared to a free electron.

References

1. Imaging molecular geometry with electron momentum spectroscopy. Scientific Reports (2016).