Soft X-Ray and Extreme Ultraviolet Photons Meeting Liquids
In 1987, Yang and Kirz used soft X-ray transparent silicon nitride windows to separate a thin water film from the vacuum surrounding. For a membrane such as of 100 nm thickness approximately 20% of the incident photons is transmitted. However, radiation induced sample damage remains as a crucial problem, leading to the development of flow cells, which allow for the continuous refreshment of the sample during the measurements. For transmission mode measurements requiring a sample thickness in the micrometer range, flow cells exist which allow refreshment between every XA spectrum or even every measuring point. It has to be kept in mind that membranes are not ideally transparent in all energy ranges. Absorption edges of the elements that are contained in the membrane material cannot be investigated. Furthermore, the sample may react with the membrane and distort the experimental results. Membrane coatings such as gold can protect the membrane from reacting with radicals created by X-ray irradiation.
A different approach to bring liquids together with soft X-rays was shown by H. Siegbahn and K. Siegbahn who were the first to introduce the "liquid beam" technique to X-ray photoemission experiments on continuously fresh liquid samples. With a jet-diameter of approximately 0.2 mm their approach was limited to the investigation of liquids with vapor pressures lower than 1 Torr. Faubel and co-workers developed the idea further to the liquid micro-jet technique which allows obtaining stable vacuum conditions also for liquids with higher vapour pressure. This was achieved by using a liquid jet diameter of, e.g., only ~20mm produced in a glass nozzle and flow velocities between 30 and 120 m/s.
In large parts the liquid from the micro-jet is prevented from evaporating by freezing it inside a liquid nitrogen cooled container. Additional cryotraps and turbo-pumps are required to obtain the necessary vacuum conditions. Recently, techniques for recycling the solution from the micro-jet became available upon collecting the liquid after irradiation inside a differentially pumped container, which can be depleted from outside. This is advantageous especially for expensive or rare samples; however, possible sample damage has to be considered then. Originally developed for methods based on electron detection, the liquid micro-jet technique is currently also used for photon-detection-based approaches. However, effects of evaporative cooling have to be considered when using the jet-technique.
Figure taken from the review of Lange et al.