Chemists at a major x-ray laser facility in the US have examined a reaction in unprecedented detail, observing individual valence electrons as ammonia dissociates. ‘For the first time here, we managed to follow just a single valence electron,’ states Ian Gabalski, a PhD candidate at Stanford University in California. This is likely the first of many similar experiments that will aid scientists in developing a better understanding of the processes involved in synthesizing the chemicals essential to us.
Valence electrons are the outermost electrons of an atom that dictate its chemical properties. They ‘are shared among atoms, effectively driving all chemical reactions,’ Gabalski emphasizes. ‘They operate on ultra-fast time scales and traverse very small distances.’ To measure them requires light pulses shorter than the time it takes for the electrons to shift. The distance between peaks in the lightwave, commonly referred to as the wavelength, must also be comparable to the size of the electron orbital. ‘That ultimately leads to an x-ray,’ Gabalski notes.
Gabalski was part of a research team at the SLAC National Accelerator Laboratory in California, directed by Mike Glownia and Philip Bucksbaum. The researchers utilized ultraviolet laser light to prompt individual hydrogen atoms to dissociate from ammonia molecules. They subsequently directed x-ray beams from SLAC’s Linac Coherent Light Source (LCLS) at the molecules after an infinitesimally brief delay.
By gradually altering the length of the delays, the team investigated the valence bonding electrons over a span of a few hundred femtoseconds. To put that into perspective, there are 10^15 femtoseconds in one second, while the universe is about 7 x 10^15 minutes old.
### Activating and deactivating valence electrons
In prior research, scientists observed brief indications of shifts in valence electrons in similar experiments. Nonetheless, they focused on more complex molecules, which presented unexpected challenges. ‘That brief pulse of x-rays ultimately captures a picture of all the electrons in your system,’ Gabalski clarifies. ‘Most of the electrons in the molecule are localized around the heavier atoms, making them the primary contributors to the scattering.’
To identify a system where this issue did not arise, the LCLS scientists collaborated with Nanna List at the KTH Royal Institute of Technology in Stockholm, Sweden, and the University of Birmingham, UK. List points out that such experiments have been inspired by researchers demonstrating that it should theoretically be feasible to obtain x-ray scattering measurements attributable to valence electrons.
List employed similar theoretical calculations to ‘select a target system,’ specifically ammonia. These calculations were part of a comprehensive proposal the scientists submitted to secure highly sought-after time on the LCLS x-ray laser.
The scientists documented the changes in the angles of x-ray beams scattered by ammonia molecules. From this data, they could determine the position of the electrons binding the atoms within the molecule as it transitions from a pyramidal to a planar configuration, prior to a hydrogen atom detaching.
List’s theoretical models enabled the LCLS team to ascertain the contribution of the valence electron, activating and deactivating the effect of its presence. Gabalski explains that their findings align with a scenario where the ultraviolet laser renders the valence electron significantly more delocalized. Under this condition, it becomes easier for hydrogen to dissociate, an effect that lasts approximately 100 fs.
Adam Kirrander from the University of Oxford regards the selection of the simple target system as clever, as it simplifies the appreciation of the electronic rearrangements as distinct from the structural changes in the signal. ‘It exemplifies the increasingly precise mapping of time-dependent dynamics in molecules that is now achievable,’ he states.