femtosecond laser setup

Researchers in Germany and Austria used femtosecond laser pulses to excite and then eject electrons from molecules, allowing them to keep track of the orbital tomograms through time. [Image: Philipps-Universität Marburg / Till Schürmann]

One major goal of chemists is to be able to track chemical reactions as they happen in both time and space. New work from researchers in Germany and Austria shows how this could be possible, having combined femtosecond spectroscopy with a recently developed tomomgraphic technique to observe how electrons move between orbitals in molecules above a metal substrate (Science, doi: 10.1126/science.abf3286).

Scientists regularly use ultrafast laser pulses to probe electron dynamics during reactions. To date, however, these experiments lack spatial information—they record the intensity of light given off when electrons are ejected from molecules but do not chart how the electron wavefunctions evolve. The idea with the latest work was to show how to incorporate this second element by monitoring electronic orbitals in momentum space.

Imaging momentum-space dynamics

The research, carried out by Ulrich Höfer at Philipps-Universität Marburg, Germany, Stefan Tautz of Forschungszentrum Jülich, Germany, and colleagues, relies on the theory of what are known as frontier orbitals. The idea is that, in many cases, reaction dynamics can be explained simply in terms of what happens between two adjacent molecular orbitals—the highest occupied one and, immediately above it, the lowest unoccupied one.

More specifically, the researchers made use of what is known as photoemission orbital tomography. Developed by Peter Puschnig and Michael Ramsey in 2009, this technique involves liberating electrons from molecules laid down on a metal surface and then measuring the distribution of the electrons’ energy and momenta. Those distributions in turn reveal how the electrons are spread out in space in the molecular orbitals.

Höfer, Tautz and co-workers carried out their experiments using the organic dye molecule perylene-tetracarboxylic-dianhydride separated from a copper substrate by a very thin layer of oxygen. They first exposed the molecules to femtosecond pulses of visible light that was finely tuned to excite electrons. They then ejected the electrons from the molecule using a second, higher-energy femtosecond pulse at ultraviolet wavelengths produced via high-harmonic generation.

The researchers used what is known as a momentum microscope to record the electrons’ momenta in two dimensions, while employing the time-of-flight technique to measure the particles’ energy. Combining these three quantities with the time delay between the two pulses, which they varied, they were able to build up a four-dimensional “data cube” and from that work out the excitation pathways in time and space.

Considering practical applications

In this way, the researchers were able to confirm that the multi-electron wave function of the excited state in fact corresponded to that of a single electron jumping between the highest occupied orbital and the lowest occupied one. This, they say, would not have been possible using conventional femtosecond spectroscopy - which yields information on electrons’ energy but not momentum.

“The key result of our work is that we can image the orbital tomograms with ultrahigh resolution over time,” Robert Wallauer, group leader at the University of Marburg said in a press statement.

Höfer argues that the findings “represent a crucial breakthrough towards the goal of tracing electrons through chemical reactions in space and time.” He also reckons that the work could have practical applications by optimizing interfaces in a range of technologies. As he points out, it is by creating an interface between two different materials in organic solar cells that excited electrons can be better siphoned off and therefore made to generate an electrical current.