Press Release: New Method Gives More Precise Information on Simple Molecules


By using x-ray absorption in studies of nitrogen gas, and starting from an ion rather than from a neutral molecule one may get out much more detailed information. It is possible to make deeper analyses than what the established ESCA-method has made possible. This shows a new study from, among others, Uppsala University published in Physical Review Letters.

– The method is much more precise, and we see new exciting details in the inner-shell vacancy states. We may study charged particles of simple gas molecules and we see clearly how the electrons interact. Now we can get out details that you could only guess before. This richness of detail has also contributed to the development of theoretical models to be able to describe our results. Our results is a stepping stone to be able to study more complicated molecules, says Rebecka Lindblad, postdoctoral at the Department of Chemistry – Ångström Laboratory at Uppsala University and the study’s first author.

In 1981, Kai Siegbahn (1918-2007) received the Nobel Prize in physics for the development of the ESCA-method, electron spectroscopy, where one accurately can measure the inner electrons. The point of the method is that the inner electrons tell us about the microscopic mechanisms which are the basis of the material properties. Kai Siegbahn and his co-workers built advanced electron spectrometers, instruments which can measure the electrons’ energy.

– X-ray absorption is usually a complement to ESCA, but with our method we reach the same state as with ESCA, without the need of an electron spectrometer. We do this by starting with removing an outer electron from the molecule and then make an inner electron take the empty place.

In the study, the researchers have looked at nitrogen gas, a simple molecule which has been studied with numerous methods during a long time. The new is that they have removed one of the loosest electrons and the ion then created they have irradiated with x-rays. The energy of the x-rays is transmitted to electrons in the nitrogen gas ion and thus the electrons can be excited, jump, in the molecule.

– Since the molecular ions we study also are found in space, our results might be used to interpret light coming here from other solar systems. We also get information that is useful to physicists in other contexts, processes that occur when, for example, using a very powerful laser. The figures we get are useful to other researchers.

Why is it important to find an alternative to ESCA?

– In most cases ESCA is an excellent method, but in some cases, for example if one wants to study a very small amount of a gaseous substance, our method is more sensitive than ESCA. A bonus is that we get the same information as ESCA, but certain effects are more prominent, which means that we can draw conclusions that one could not have done with ESCA. Even if the new method is simple in principle, it is probably too complicated in practice, for us to have a new ESCA-method useful in large scale.

The project has been possible to carry through in a large collaboration with researchers, among others, in Lund and at Helmholtz-Zentrum Berlin, where they have exactly the ion trap and the synchrotron needed.

Article reference:

Lindblad R, et al (2020); X-ray absorption spectrum of the N2+ molecular ion, Physical Review Letters, May 20, 2020. DOI:,


Rebecka Lindblad, Postdoctoral at the Department of Chemistry – Ångström Laboratory at Uppsala University,
email:, telephone: 070-738 72 20

Elin Bäckström
Translation: Johan Wall