Polarization and steric effects in inelastic collisions of NO(X ²Π) with Ar and He

Abstract

Differential m_j resolved cross-sections were determined for collisions of NO(X²Π) with Ar and He based on full close-coupled calculations and abinitio potential energy surfaces. As a function of the scattering angle, the predicted alignment becomes increasingly negative as the scattering angle increases. Integrated over all scattering angles, the sign of the product alignment becomes increasingly negative as Δj increases. For collisions with Ar, the product alignment is more negative for spin–orbit changing, as compared to spin–orbit conserving, transitions. For collisions with He, the calculated product alignment, both differential and integral, is in excellent agreement with experiment.

A sizable steric effect is also seen. For scattering by Ar, collisions in which the NO molecules are initially oriented so that the O end preferentially points toward the Ar atom are more effective in promoting spin–orbit changing transitions. The stronger steric asymmetry for spin–orbit changing transitions is consistent with the asymmetry of the difference (V_A-V_A) potential energy surface. For moderate to large Δj, regardless of the scattering partner (Ar or He) the steric asymmetry is large and positive for even Δj while negative for odd Δj. The origin of this effect is not yet obvious.

To our knowledge this is the first fully-quantum, differential scattering study of polarization and steric effects in collisions of a open-shell system.

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