Spectroscopy of HF and HF-containing clusters in solid parahydrogen

Area of Science:

• Solid-state physics
• Molecular spectroscopy
• Quantum chemistry

Background:

• Understanding molecular behavior in solid matrices is crucial for various scientific disciplines.
• Parahydrogen (pH(2)) offers a unique, weakly interacting quantum solid environment for studying guest molecules.
• Previous studies have explored hydrogen halides in solid rare gases, but less is known in solid parahydrogen.

Purpose of the Study:

• To investigate the vibrational and rotational dynamics of hydrogen fluoride (HF) and its deuterated form (DF) in solid parahydrogen (pH(2)).
• To characterize the spectral features of HF and DF monomers, polymers, and clusters within the pH(2) matrix.
• To elucidate the mechanisms of molecular migration, polymerization, and spectral broadening in this quantum solid.

Main Methods:

• Fourier Transform Infrared (FT-IR) absorption spectroscopy was employed to measure the spectra of HF and DF in solid pH(2).
• Analysis of absorption lines was performed to identify monomers, polymers, and clusters with other species (e.g., N(2), O(2)).
• Rotational constants were determined from cooperative transitions, and spectral line widths were analyzed.

Main Results:

• Numerous absorption lines were assigned to HF/DF monomers, polymers, and clusters with other guest molecules.
• Rotational constants indicate that HF and DF behave as nearly free rotors in solid pH(2).
• Time-dependent spectra revealed monomer migration and polymer formation, likely through tunneling reactions.
• HF monomers in pH(2) exhibited a broad line width (4 cm(-1)) compared to other hydrogen halides.
• This broadening is attributed to rapid rotational relaxation caused by resonant coupling with pH(2) phonons and rotational-translational coupling.

Conclusions:

• HF and DF molecules are nearly free rotors in solid parahydrogen, demonstrating the weakly interacting nature of the matrix.
• Monomer migration and polymerization occur in solid pH(2), suggesting dynamic processes even at low temperatures.
• The unusually broad spectral line width of HF monomers is explained by specific phonon-rotational energy level coincidences and coupling mechanisms.