Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens

Area of Science:

• High-pressure physics and chemistry
• Molecular spectroscopy
• Condensed matter physics

Background:

• Understanding molecular behavior under extreme conditions is crucial.
• Raman spectroscopy is a powerful tool for probing molecular vibrations and rotations.
• Previous studies on hydrogen isotopes at high pressures have focused on vibrational and distinct rotational modes.

Purpose of the Study:

• To investigate the ΔJ=0 excitation in hydrogen (H2) and deuterium (D2) under a wide range of pressures and temperatures.
• To characterize the behavior of this excitation in different phases (gas, fluid, solid).
• To determine the isotope dependence of the ΔJ=0 transition.

Main Methods:

• High-pressure Raman spectroscopy measurements.
• Systematic variation of pressure (up to 50 GPa) and temperature (down to 10 K).
• Analysis of spectral shifts and mode behavior across different phases.

Main Results:

• The ΔJ=0 excitation was observed in H2, D2, and their mixture across various pressure-temperature conditions.
• In the solid state, the crystal field shifted the ΔJ=0 Raman shift to ~75 cm⁻¹.
• The excitation frequencies were found to be isotope-independent, deviating from typical rotational or vibrational scaling.
• The ΔJ=0 mode in deuterium exhibited splitting in phase II, indicating a complex molecular environment.

Conclusions:

• The observed ΔJ=0 transition in hydrogen isotopes is a unique phenomenon.
• Its isotope independence suggests it is fundamentally different from harmonic oscillator and quantum rotor models.
• The findings provide new insights into the complex molecular behavior of hydrogen and deuterium under extreme pressure conditions.