FOM PhD student measures fundamental vibrations in molecules
FOM PhD student Gareth Dickenson, MSc will gain his doctorate from the VU University Amsterdam on 8 May 2013 for his thesis entitled 'Laser and synchrotron spectroscopic studies of molecular hydrogen'. On the same day an important result from his work will be published in Physical Review Letters.
The FOM researcher determined the exact amount of vibrational energy that a vibrating hydrogen molecule contains by making measurements with a high-precision, titanium-sapphire laser. This energy is directly related to the frequency of the molecule. As the hydrogen molecule is the simplest molecule that exists then this is a measurement of the most fundamental vibration in molecules.
The special titanium-sapphire laser researchers used to vibrate the molecule was constructed in the LaserLaB of VU University Amsterdam. The frequencies were subsequently measured using a different laser, a so-called frequency comb laser, which was also built in the LaserLaB. The frequency comb laser is linked to a caesium atomic clock. This allowed the researchers to precisely determine the vibrational time of the molecule. The measured vibrational frequency of the hydrogen molecule was found to be 124748628 ± 5 MHz.
Theory meets practice
Dickenson's extremely accurate measurements are comparable to the theoretical calculations made for this elementary system. Prof.dr. Krzysztof Pachucki from the University of Warsaw has calculated the vibrational frequencies of hydrogen with a comparable accuracy of eight figures. He did this by using the Schrodinger equation to calculate the energy levels of the hydrogen molecule. He then had to perform relativistic corrections on the outcomes to allow for Einstein's theory of relativity. Finally, he had to allow for the interaction of the molecule with the vacuum, as 'ghost particles' are always briefly appearing in the vacuum.
The calculated vibration frequency was found to agree perfectly with the measured frequency. That is not just the case for the H2 molecule, but also for the molecules in which a hydrogen nucleus is replaced by a deuterium nucleus (HD and D2).
Fundamental
Extending similar measurements and calculations to larger molecules would be interesting as that would provide a better understanding of the energy balance in chemical reactions. The measurements are also important for fundamental physics. As for the relatively simple system of the hydrogen atom very precise calculations exist, the agreement between theory and practice also provides an answer to the search for new physical forces. Physicists have long been searching for a 'fifth' physical force and the hydrogen molecule is the ideal candidate to search in for this. As all of the calculations have been performed with the electromagnetic force, and the researchers found an exact agreement between theory and experiment, the existence of a fifth force can be excluded for the time being, at least up to a certain level of accuracy.
Reference
Dickenson, G.D., M.L. Niu, E.J. Salumbides, J. Komasa, K.S.E. Eikema, K. Pachucki, and W. Ubachs. 'The Fundamental Vibration of Molecular Hydrogen'. Physical Review Letters (2013).
A prepublication is available online at arXiv.
Contact
Prof.dr. Wim Ubachs, LaserLaB VU University Amsterdam, +31 (0)20 598 79 48.