Noncollinear four-wave mixing with automated lens control for tunable, state-selective XUV detection in molecular scattering experiments

Quantum state-selective detection of reaction products with vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) light is crucial for understanding gas-phase reaction dynamics. However, existing VUV/XUV generation methods often face challenges with non-selective ionization from third-harmonic generation (THG) and limitations in beam separation. Presented here is a noncollinear four-wave mixing scheme that spatially separates the generated sum-frequency generation laser beam from the THG and fundamental input laser beams. This is achieved through independent and automated control of two focusing lenses, allowing flexible adjustment of each input laser's focal length. This enables precise control over focal positions and crossing angles, facilitating optimal spatial overlap despite significant wavelength differences, and critically, permits broad-range XUV scanning for state-selective ionization of diverse reaction products across multiple quantum states. The efficacy of this design is demonstrated through resonance-enhanced multiphoton ionization spectra of HF and N2, which show agreement with simulations. Furthermore, comparative velocity map images from inelastic scattering experiments unequivocally prove this noncollinear configuration effectively rejects THG, enabling sharply resolved state-selective product detection. This robust method enhances capabilities for detailed molecular beam investigations requiring stringent product quantum state selectivity across a wide range of species and quantum states.