# Optical Selection of Rotational and Parity-Resolved States for Rotationally Inelastic Scattering: NO(X2Π1/2, v = 1, j = 1.5e) with Ar and CH4

## TL;DR

This study uses precise optical methods to prepare and observe specific rotational states of NO molecules after collisions with Ar and CH4.

## Contribution

The paper introduces a novel method for preparing and detecting parity-resolved rotational states in vibrationally excited NO molecules.

## Key findings

- Differential cross sections for NO–Ar scattering agree closely with quantum scattering predictions.
- NO–CH4 scattering shows a negative correlation between NO rotational excitation and CH4 rotational energy.
- CH4 rotational energy correlates with scattering direction for NO j′ = 10.5.

## Abstract

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 F
1
e, 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 F
1
f and j′ = 10.5 F
1
f 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.