Precision Ultranarrow-linewidth Resonance Excitation (PURE) preparation of a molecular beam of nitric oxide molecules for inelastic scattering with argon

We recently reported a novel technique that utilizes narrow-linewidth lasers to selectively excite molecules into a pure quantum state with a well-resolved, tunable distribution of velocities. (J. Phys. Chem. Lett. 2024, 15, 50, 12455-12463) We refer to the preparation of molecules by this technique as Precision Ultranarrow-linewidth Resonant Excitation (PURE) preparation. Here, we pair this PURE preparation methodology with active frequency stabilization of the quantum cascade laser (QCL) that drives the excitation. In doing so, highly velocity-resolved packets of molecules are possible, with the limitations of this velocity resolution set by the stability of the laser locking and the spectral linewidth of the transition. We show that side-of-fringe locking to a Doppler-broadened absorption spectrum of our target molecule is sufficiently stable for PURE preparation of molecules for long timescales (several hours) with highly resolved velocity spreads (σ