High-Resolution Time-Resolved PEPICO with Tunable Vacuum Ultraviolet Photoionization

Recently we presented a new time-resolved, double-imaging photoelectron photoion coincidence (i²PEPICO) spectrometer for the study of chemical reactions using fixed frequency, single-photon vacuum ultraviolet ionization. Here we describe new capabilities and insights from this instrument when coupled with tunable ionizing radiation. We interrogate the gas expansion dynamics of a side-sampled chemical reactor tube, revealing clear evidence for viscous flow in the expansion before ionization. Cation imaging can be used to restrict detected signal to only the direct molecular beam, removing contributions from background and reflected gases. We characterize the peak shape and mass resolution of the instrument, provide new insight and clarification regarding collection efficiencies, and consider the noise sources and resulting signal-to-noise in PEPICO experiments. We quantify the temporal instrument response function and show that velocity map imaging of cations may be used to eliminate the transit time delay and reduce the temporal blurring inherent in ex situ sampling geometries. The resulting upper bound of time resolution is 7 μs, a significant improvement compared to previous instruments. We discuss methods to quantify and address the ubiquitous problem of background electrons ejected from metal surfaces in photoelectron spectrometers. Finally, we compare variants of photoion mass-selected photoelectron spectroscopy, provide an example of quantitative analysis using PEPICO, and present evidence for unexpected products in the 193 nm photodissociation of CH₃OH that underscores the value of universal imaging approaches.