A research team led by Profs. Eugene Gregoryanz, SHARPS/HPSTAR and Xiaodi Liu, Hefei Institute of Physical Science of Chinese Academy of Sciences (ISSP), has observed the ΔJ = 0 rotational excitation in dense hydrogen, deuterium and their mixture. In the fluid state and phase I the frequencies (energies) of the ∆J=0 transition for H2 and D2 do not scale either as rotational (by factor of 2) nor vibrational (by √2) modes and appear to be completely isotope independent. This independence on mass marks this transition as unique and a fundamentally different type of excitation from the commonly considered harmonic oscillator and quantum rotor. These interesting results were published in Physical Review Letters (link to: https://link.aps.org/doi/10.1103/7m51-n6w7).
The hydrogen molecule (H2) is a paradigm of a fundamental system in quantum mechanic. Its simplicity, combined with the well-defined spectroscopic properties, makes it a textbook example for illustrating key concepts. In its gas/fluid states, its rotational transitions follow Raman selection rules (ΔJ = ±2). However, when hydrogen enters a high-pressure solid state, the crystal field environment alters its symmetry, leading to degeneracy lifting and theoretically permitting ΔJ = 0 transitions. However, because the excitation frequency is so close to the laser line and thus obscured by the Rayleigh scattering, this transition, which simply comes out of the selection rules, had never been directly captured experimentally.
The first author, a PhD student of the ISSP and USTC, Ms Jie Feng said: That was a cool PhD project, which taught me the basics of the Raman spectroscopy and physics of molecular hydrogen. It is great to be the first to observe the last unobserved excitation in such iconic system as hydrogen.
Utilizing a high-performance, low-wavenumber, high-pressure, and low-temperature Raman spectroscopy system the team conducted systematic Raman spectroscopic measurements on dense hydrogen (H₂), deuterium (D₂), and their mixtures across a broad range of pressure and temperature conditions, unambiguously observing this "hidden" excitation signal for the first time. They found that in the gas/fluid state this excitation has zero Raman shift as theory suggests, but in the solid, 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 behavior of this excitation during the ortho-para hydrogen conversion process, providing crucial experimental clues for understanding the evolution of hydrogen's quantum states under extreme conditions. Prof. Gregoryanz said: In fact, we have known that the “zero roton” (the name we have given to this excitation) is there for more than 10 years. We have observed it in our previous works going back to at least 2017 but we were always interested in something else and never had time to pursue the “zero roton”. It was very exciting to finally sort out and understand the effect, which kept popping up in our experiments being a nuisance.
The reported discovery thus not only completes the final experimental observation of all allowed rotational transitions in the dense hydrogen, revealing the complete manifestation of Raman selection rules modified by a crystal field, but also provides a unique example showcasing the novel physics emerging in fundamental quantum systems under extreme conditions.
The above work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Ministry of Science and Technology, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Director’s Fund of the Hefei Institutes of Physical Science.
Figure 1. The translons behind the ΔJ = 0 excitation. The left part illustrates the rotational energy level diagram of hydrogen molecules in the gas/fluid state, while the right part is a schematic diagram showing the energy level splitting caused by the hexagonal crystal field.
由上海前瞻物质科学研究院的Eugene Gregoryanz教授带领的研究团队观测到稠密氢、氘及其混合物中的ΔJ=0转动激发。在流体状态和阶段I中，H2和D2的∆J=0跃迁的频率（能量）既不以旋转（按系数2）缩放，也不以振动（按√2）模式缩放，并且似乎完全与同位素无关。这种对质量的独立性标志着这种转变是独特的，是与通常认为的谐振子和量子转子根本不同的激发类型。该研究成果以Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens为题发表在《物理评论快报》上。
