Assessing the problem of excitation light scattering in high-viscosity jets used in serial crystallography sample delivery.

Serial crystallography for time-resolved structural studies of light-triggered reactions often employs high-viscosity jets to deliver crystals into an X-ray beam. A potential complication is that pump light can scatter within the jet to unintentionally irradiate yet-to-be-probed portions of the jet - a problem known as light contamination. Importantly, by transporting light out of the nominal interaction region, light scattering can reduce the effective irradiation energy density experienced by the diffracting crystal. This issue, which can even jeopardize an experiment, has proven rather controversial. To provide direct insight, we performed custom femtosecond transient absorption experiments with spatially displaced pump and probe beams directed onto actual jets under realistic experimental conditions, allowing the distribution of excited molecules along the flowing jet to be mapped out explicitly. To characterize the underlying light scattering properties of commonly used jet media, the Kubelka-Munk formalism was utilized. Our results show that, in contrast to flat-cell geometries, which we found to exhibit minimal light contamination, the cylindrical geometry of jets can facilitate a degree of light spill-over. The excitation energy density loss due to the scattering is less than 30% in realistic experimental conditions. This highlights the importance of carefully selecting jet media and laser parameters to minimize light-scattering-induced artefacts when undertaking pump-probe serial crystallography experiments.