Femtosecond response of polyatomic molecules to ultra-intense hard X-rays

Upon exposure to ultra-intense, hard X-ray pulses, polyatomic molecules containing one heavy atom reach a much higher degree of ionization than do individual heavy atoms, contrary to previous assumptions. Ultrafast molecular response to intense X-rays X-ray free-electron lasers offer many new applications such as the ability to structurally probe fast biological processes. This requires the use of hard and intense X-ray pulses, but the behaviour of matter under such conditions has not been fully explored. Artem Rudenko et al . show that when exposing small polyatomic molecules that contain one heavy atom to hard X-ray pulses with ultra-high intensities, the response is qualitatively different from what is seen in experiments carried out under less extreme conditions. The observed ionization of the molecule is considerably enhanced compared to that of an individual heavy atom under the same conditions, owing to ultrafast charge transfer within the molecule that replenishes the electrons removed from the heavy atom, enabling further ionization. Being able to account for this effect will aid further use of X-ray free-electron lasers for studying biological systems. X-ray free-electron lasers enable the investigation of the structure and dynamics of diverse systems, including atoms, molecules, nanocrystals and single bioparticles, under extreme conditions 1 , 2 , 3 , 4 , 5 , 6 , 7 . Many imaging applications that target biological systems and complex materials use hard X-ray pulses with extremely high peak intensities (exceeding 10 20 watts per square centimetre) 3 , 5 . However, fundamental investigations have focused mainly on the individual response of atoms and small molecules using soft X-rays with much lower intensities 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . Studies with intense X-ray pulses have shown that irradiated atoms reach a very high degree of ionization, owing to multiphoton absorption 8 , 12 , 13 , 18 , which in a heteronuclear molecular system occurs predominantly locally on a heavy atom (provided that the absorption cross-section of the heavy atom is considerably larger than those of its neighbours) and is followed by efficient redistribution of the induced charge 14 , 15 , 16 , 17 , 19 , 20 . In serial femtosecond crystallography of biological objects—an application of X-ray free-electron lasers that greatly enhances our ability to determine protein structure 2 , 3 —the ionization of heavy atoms increases the local radiation damage