Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens

Abstract

Raman measurements performed on dense H2, D2 and H2+D2 in a wide pressure-temperature range reveal the presence of the ΔJ=0 rotational excitation. In the gas/fluid state this excitation has zero Raman shift, but in the solid, the crystal field drive s it away from the zero value e.g. 75 cm-1 at around 50 GPa and 10 K for both isotopes and their mixture. In the case of deuterium, the ΔJ=0 mode splits upon entering phase II suggesting a very complex molecular environment of the broken symmetry phase (BSP). In the fluid state and phases I and II the frequencies (energies) of the ΔJ=0 transition for H2 and D2 do not scale either as rotational (by factor of 2) nor vibrational (by square 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.

Figures (3)

• Figure 1: Representative Raman spectra of the rotational excitations of H$_2$, D$_2$ and H$_2$+D$_2$ as function of pressure at 10 K. (a), (b) and (c) show the evolution of the rotational modes of H$_2$, D$_2$ and H$_2$+D$_2$ respectively. The black dotted curves - phase I, blue - phase II for all panels. The fits to the $\Delta J$=0 modes are shown in red in all panels and its split components in D$_2$-II are in orange and dark red. The new librational mode of D$_2$-II is shown in dark yellow. The S$_0$(0) mode was fitted with 3 peaks corresponding to 3 m$_J$ components (see text). Due to the significant overlap and large number of the m$_J$ components making up the S$_0$(1) mode SoM, it was fitted with only 2 or 3 peaks, which have no physical meaning. Asterisks ($\ast$) mark the lattice (phonon) mode. The vertical dashed line marks the cut-off of the elastic laser line masking the signal.
• Figure 2: Measured frequencies of the $\Delta J$=0 rotational mode as a function of pressure at 10 K. (a) H$_2$: phase I - the brown empty circles, phase II - the solid circles. Inset: the intensity of the zero roton with pressure in H$_2$ and H$_2$+D$_2$. Note the frequency decrease in H$_2$ due to the ortho-para conversion. (b) D$_2$: phase I - the red empty circles, phase II - the solid circles. The areas of phase II in D$_2$ are shaded gray. The empty blue circles indicate the frequency of the $\Delta J$=0 mode of H$_2$+D$_2$ in both panels. The "+" indicate the librational mode appearing in phase II.
• Figure 3: The rotation excitation modes of H$_2$+D$_2$ and H$_2$ as a function of temperature. Representative Raman spectra of the H$_2$+D$_2$ mixture at 31 GPa (a) and H$_2$ at 96 GPa (b) over a wide temperature range. The black dotted curves - phase I, blue - phase II for (a) and (b) panels. (c) The frequency of the zero roton mode as function of temperature at 31 and 96 GPa. D$_2$: phase I - the brown empty circles, phase II - the solid circles. The areas of phase II in D$_2$ are shaded gray. H$_2$+D$_2$: the empty blue circles. Inset: The intensity of zero roton in H$_2$+D$_2$ at 31GPa.