The molecule 'beryllium dimer' has mystified chemists for many years. Recent measurements have helped scientists identify 11 vibrational levels, and now a US-Czech team of researchers sheds light on a 12th level. The findings were published in the journal Science.
Made up of two atoms, beryllium dimer is a solid, toxic metal found in minerals that can be used as an alloy with other metals in various applications like parts of nuclear weapons, explained Drs Krzysztof Szalewicz and Konrad Patkowski of the University of Delaware in the US, and Vladimír Spirko, a chemist from the Academy of Sciences of the Czech Republic.
Researchers had long speculated that the two atoms that make up the beryllium dimer bounced off each other. Dr Patkowski, the lead author of the study, said these ideas were based on a basic theory in chemistry which explains how electrons in a molecule occupy different orbitals. But more than 40 years ago, scientists found that in fact these two atoms actually bond with each other.
However, attempts to study the forces binding the beryllium atoms together came up with very different results. Flash-forward to May 2009 and a team from Emory University in the US discovered the vibrational energy of the bonding atoms for 11 levels. Finally, researchers had succeeded in reconciling the theoretical and experimental models.
'A molecule vibrates, so the distance between atoms changes in time. A molecule can't just sit there and not vibrate,' Dr Patkowski said. 'The more vibrational energy a molecule has, the farther its atoms stray from their equilibrium positions.'
In this latest study, the researchers confirmed a 12th and highest vibrational level for the beryllium molecule. Key to their discovery was the 'morphing' work carried out by Dr Spirko. Scientists can make simple changes to the theoretical interaction energy curve to agree with experimental findings, the team said. 'Morphed versions of this potential energy, fitted to experimental data, closely reproduce the observed spectra,' they explained.
'[The Emory team's] results agreed with our study, so it was really gratifying to see the previous mysterious disagreement between experimental and theoretical numbers from the past disappear,' Dr Patkowski said. 'Their work showed up we were going in the right direction.'
'The beryllium dimer is commonly used in benchmarking studies in experimental and theoretical physics, yet the molecule is anything but common,' he pointed out. 'It's a prototype system that is small and nasty, both for experimental studies, because of its toxicity and reactivity, and for theoretical studies, because standard quantum chemistry methods work very poorly here,' he added.
'The interesting thing about this molecule is that basic chemistry knowledge tells us that the atoms are not going to bond, but they do — and it's a pretty strong one. It's a nice model for developing new theories of molecular physics.'
The study was funded in part by the Academy of Sciences of the Czech Republic and the Czech Ministry of Education, Youth and Sports.