Vibrational relaxation of trapped molecules in solid matrices: OH(A 2Sigma+; v = 1)/Ar

Area of Science:

• Physical Chemistry
• Condensed Matter Physics
• Spectroscopy

Background:

• Vibrational relaxation dynamics of small molecules in solid matrices are crucial for understanding energy transfer processes.
• Hydroxyl radical (OH) is a key species in atmospheric and combustion chemistry, and its behavior in solid environments provides insights into fundamental interactions.

Purpose of the Study:

• To investigate the vibrational relaxation mechanism of OH(A (2)Sigma(+);v=1) embedded in solid argon.
• To determine the rate constants and temperature dependence of this relaxation process.

Main Methods:

• Computational modeling of OH local motions within an argon cage (face-centered cubic).
• Solving equations of motion for OH-first shell atom interactions and employing Langevin dynamics for energy propagation.
• Utilizing a semiclassical procedure with calculated energy transfer data to derive rate constants.

Main Results:

• OH vibrational energy initially transfers to intramolecular libration-rotation, then to first shell and chain atom vibrations on a picosecond timescale.
• Libration-to-rotational transitions facilitate energy dissipation into small packets suitable for lattice phonon propagation.
• Energy transfer to the bulk argon heat bath occurs on a longer timescale (10 ns or more).
• The calculated rate constant is approximately 10^6 s^-1 and shows weak temperature dependence across the studied range (4-80 K).

Conclusions:

• The study elucidates the multi-step vibrational relaxation pathway of OH in solid argon.
• The findings highlight the importance of cage dynamics and lattice interactions in determining energy transfer rates.
• The weak temperature dependence suggests that the relaxation process is primarily governed by intramolecular couplings and cage structure.