Elastic depolarization of OH(A) by He and Ar: a comparative study

Area of Science:

• Chemical Physics
• Molecular Spectroscopy
• Collision Dynamics

Background:

• Understanding molecular interactions is crucial for chemical kinetics and atmospheric science.
• Collisional processes significantly influence the state distribution of molecules in gas-phase reactions.

Purpose of the Study:

• To measure the collisional depolarization of hydroxyl radicals (OH) in excited electronic states (A(2)Sigma(+)) by helium (He) and argon (Ar).
• To determine cross sections for rotational energy transfer (RET) and elastic depolarization in OH + Ar collisions.
• To compare experimental findings with theoretical calculations.

Main Methods:

• Two-color polarization spectroscopy to measure collisional depolarization of OH(A(2)Sigma(+), v = 1) by He and Ar.
• Zeeman quantum beat spectroscopy to determine RET and elastic depolarization cross sections for OH(A, v = 0) + Ar.
• Dispersion of emission to observe single fluorescence transitions.

Main Results:

• Significant elastic depolarization of OH(A) by Ar was observed, with rotational alignment loss exceeding orientation loss.
• For OH(A) + He, RET was found to be more significant than elastic depolarization in reducing the polarization signal.
• Experimental data for OH(A) + Ar showed good agreement with quasi-classical trajectory and quantum mechanical scattering calculations.
• Elastic depolarization cross sections for OH(A) + Ar were similar to OH(X) + Ar at low rotational quantum numbers (N) and insensitive to N from 1-14.

Conclusions:

• Collisional depolarization of OH radicals by noble gases is a complex process influenced by both elastic and inelastic (RET) scattering.
• The anisotropic potential energy surface and the nature of elastic depolarizing collisions contribute to the observed depolarization behavior.
• Theoretical methods, including classical and quantum mechanical approaches, provide reliable predictions for these collision dynamics.