The use of x-rays in materials research has been instrumental for our understanding of materials structure, both in the sense of ‘position of atoms’ and in the sense of the electronic structure, that is to say the nature of chemical bonding within materials.

Most instruments and apparatuses for x-ray research require a high grade vacuum and therefore experimental limitations early implied research on solid, non-gasing samples only. Now some facilities exist to use liquid and gaseous jets together with pinholes, differential pumping and high pumping speed installations for the main chamber. This works for many experiments but still limits what can be investigated. It is far more practicable to separate the gaseous or liquid sample from the vacuum equipment by a ‘window’ in the form of a membrane. However, with this method new limitations are introduced. For electrons the transmission through a solid is usually greatly attenuating. For x-rays, the transmission through a given material is highly dependent on the photon energy and so, in the soft x-ray region ( 100-1400eV), the absorption edges of the lighter elements stops the x-rays very efficiently.

Early in the use of membranes, silicon nitride was successfully used for x-ray emission studies from liquids. Specifically, with these membranes the oxygen edge (~530eV) became accessible, facilitating investigations of for example water and alcohols. Although the total transmission for a 100 nm silicon nitride membrane in the most common configuration (45 degrees in/45 degrees out) is just about 25-30%, the high molecular density of the liquid allowed high quality spectra to be collected. The main issue with using silicon materials for oxygen investigations is the propensity for silicon oxide formation, since the silicon oxide signal from the membrane surface interferes with the sample oxygen signal.

However, there are many interesting elements beside oxygen and it is also important to increase the transmission and durability of the membranes. Alternative membrane designs using other elements than silicon and nitrogen were therefore sought after. Due to a very high transmission around the absorption edges of several interesting elements, an attractive possibility seemed to be carbon. However, the processes for carbon were much less developed than those for silicon nitride, and so many attempts failed. Not until 2004 were the first successful measurements of a liquid made using carbon membranes. This was done by a participating research team led by Professor Joseph Nordgren (Uppsala University) at the Advanced Light Source. Since then a lot of important knowledge on electronic structure in liquids and gases has been gained through the use of carbon membranes. For some example publications, see the page Track record.