The last excitation of the first element

Scientists have uncovered a previously hidden way that hydrogen molecules can move when subjected to extreme conditions.

Researchers at the Centre for Science at Extreme Conditions (CSEC) have observed a new type of quantum excitation in solid hydrogen, deuterium and the mixture of the two. Their results show that solid hydrogen molecules can only point in certain directions, and that anything in between is forbidden by the rules of quantum mechanics.

Hydrogen and quantum mechanics

The hydrogen molecule (H₂) is one of the simplest systems in quantum mechanics, and along with its well-defined spectroscopic properties, it makes a good textbook example for illustrating key concepts. The fundamental principle of quantum mechanics is that the energy of molecules is only allowed to have discrete values: quantum level. Until now, spectroscopic work detected two types of motion - vibrations and rotations. These are determined using a laser energy to speed up the rotation or vibration and measuring the ratio of the energy of quanta between hydrogen and deuterium. Fundamental quantum mechanics predicts that rotations vary by a factor of 2, vibrations by √2. This is because rotations have only kinetic energy, whereas vibrations have both kinetic and potential energy. 

A predicted but previously unseen excitation

However, a third type of excitation, with changes to potential energy only, was theorised by Professor Ackland. This is based on reorientation of the molecule while the total angular momentum is unchanged, so-called ΔJ = 0 or ‘zero-roton’ transitions where J labels the rotational energy. In a gas or liquid, such transitions are undetectable because there is no energy change to measure. However, when hydrogen enters a high-pressure solid state, the crystal field environment alters its symmetry: again quantum mechanics insists that the molecule orientates in one of a few fixed directions, nothing in between, but now these ΔJ = 0 reorientation transitions involve an energy change which can be detected by Raman Spectroscopy. However, because the energy change is small, the signal is exceptionally difficult to detect and had never been directly captured experimentally. This is despite the fact that the energy change is the same for both deuterium and hydrogen. 

Detecting an exceptionally faint signal

Using a sensitive Raman spectroscopy system, Professor Gregoryanz’s team conducted systematic spectroscopic measurements across a range of pressure and temperature conditions. They studied dense hydrogen, deuterium, and mixtures of the two, unambiguously observing this ‘hidden’ excitation signal for the first time. They found that in the gas and liquid state this excitation has, as theory suggests, zero Raman shift, but in the solid state, the crystal field drives it away from the zero value e.g. ~150 cm−1 at above 100 GPa and 20-45 K for both isotopes. Furthermore, the frequency of the ΔJ = 0 transition exhibits complete isotope independence, representing a novel excitation mode distinct from traditional harmonic oscillators and quantum rotors. The research further revealed the unique behaviour of this excitation during conversion between the different forms of hydrogen (known as ortho-para conversion). These observations provide crucial experimental clues for understanding the evolution of hydrogen's quantum states under extreme conditions.

From prediction to proof

Professor Gregoryanz said: 

We have known that the zero-roton excitation was there for more than 10 years, having observed it in previous work. But working on other effects meant that we never had the opportunity to pursue it. It is now very exciting to finally sort out and understand it – what we once saw as a nuisance turned out to be something fundamental.

Professor Ackland said: 

The prediction of the ΔJ = 0 transition was a central feature of the project, and in retrospect we even observed it in previous experiments: but to prove that the signal is indeed the zero-roton, and not some other effect, required incredibly precise work under extremely challenging experiments. As a theorist, it is thrilling to see the effect finally proven.

This discovery completes the final experimental observation of all allowed types of quantum transition in dense hydrogen, revealing the complete manifestation of Raman selection rules modified by a crystal field. It also provides a unique example showcasing the novel physics emerging in fundamental quantum systems under extreme conditions. 

These results were published in Physical Review Letters, and the work was supported by the EPSRC, ERC, and a range of Chinese Funding agencies.