Stable kilohertz rate molecular beam laser ablation sources

Abstract

We describe a stable kHz rate laser ablation/desorption supersonic molecular beam source for use in kHz rate laser experiments. With the development of modern lasers that typically operate at kHz rates, a need has arisen for stable molecular beam laser ablation/desorption sources for the study of involatile species. Many biomolecules of interest cannot be brought into the gas phase without thermal decomposition by simply heating the substrate and most (especially refractory) metals have melting and boiling points that are impossible to reach with conventional ovens. The source is based upon strong nonresonant interaction of a dithering laser focus with a rotating and translating solid rod, hydrodynamic transport of the ablated/desorbed material in helium or argon, and subsequent supersonic expansion. Further design details include flexible and easy adjustment of the source for rapid prototyping and optimization for kHz rate performance. Due to the high rate of sample removal, a major concern is clogging of the nozzle and laser input channel due to both material condensation and debris formation. In order to illustrate the range of applications, we demonstrate (1) the kHz laser ablation of a high temperature refractory metal (niobium) for use in studies of metal clusters; and (2) the kHz laser desorption and jet cooling of an involatile biomolecule (the DNA base guanine) for use in spectroscopic and dynamical studies. This kHz source design has been shown to be stable for over 12 continuous hours of operation (>4×107 laser shots) and can be readily scaled to even higher repetition rates (>10 kHz).