Optical Selection of Rotational and Parity-Resolved States for Rotationally Inelastic Scattering: NO(X²Π_1/2, v = 1, j = 1.5e) with Ar and CH₄

We present new experimental measurements of rotationally inelastic scattering of vibrationally excited NO(X²Π) with Ar and CH₄. A molecular beam of NO was prepared in a single rotational and parity-resolved state, j = 1.5 F₁e, in the v = 1 vibrational level using mid-infrared radiation from a distributed feedback quantum cascade laser. Following collision with a crossed molecular beam of Ar or CH₄, rotationally excited NO(X, v = 1) in the isolated final rotational states j′ = 4.5 F₁f and j′ = 10.5 F₁f was detected by 1 + 1′ resonance-enhanced multiphoton ionization coupled with velocity-map imaging. Differential cross sections and rotational angular momentum polarization moments for inelastic scattering with Ar are in excellent, near-quantitative agreement with quantum scattering predictions on a literature potential energy surface. Images for scattering from CH₄ for both final states show clear evidence of significant rotational excitation in the CH₄. Overall, a negative correlation is observed in the NO–CH₄ rotational excitation, with higher average CH₄ rotational energy for final NO j′ = 4.5 than for j′ = 10.5. For NO j′ = 10.5, higher rotational energies of CH₄ are surprisingly correlated with forward hemisphere scattering, while lower CH₄ rotation is correlated with backward hemisphere scattering. These measurements demonstrate the importance of the preparation of an initial rotational and parity-selected state, and the varied and surprising dynamics that remain underexplored in molecule–molecule inelastic scattering.