Imaging atom - clusters by hard x - ray free electron lasers

PACS. 61.10.-i – X-ray diffraction and scattering. PACS. 32.80.-t – Photon interactions with atoms. PACS. 36.40.-c – Atomic and molecular clusters. Abstract. – The ingenious idea of single molecule imaging by hard x-ray Free Electron Laser (X-FEL) pulses was recently proposed by Neutze et al. [1]. However, in their numerical modelling of the Coulomb explosion several interactions were neglected and no reconstruction of the atomic structure was given. In this work we carried out improved molecular dynamics calculations including all quantum processes which affect the explosion. Based on this time evolution we generated composite elastic scattering patterns, and by using Fienup's algorithm successfully reconstructed the original atomic structure. The critical evaluation of these results gives guidelines and sets important conditions for future experiments aiming single molecule structure solution. Introduction. – Structure solution without crystals was not conceivable for nearly a century. However, Linac based X-FEL-s will soon become operational and their extremely short and intense pulses offer the chance for a new type of experiment. The concept of single molecule imaging [1] is based on fast measurement of a " slowly " exploding system. In the proposed experiment the molecule does not have time to deteriorate during a single x-ray pulse and enough elastically scattered photons can be collected to give information on the unmodified structure. There are two distinct theoretical parts of the problem: the Coulomb explosion and the image reconstruction process. In a previous paper we have given a detailed description of the Coulomb explosion [2]. In the present work we analyse the effect of the explosion on the image reconstruction. Up to now only two papers have been published on the Coulomb explosion initiated by hard x-ray pulses. In the first, the explosion of a lysozime molecule has been modelled [1]. In the second, 100-1500 atom carbon clusters exploded under the influence of a single pulse [2]. These works are special molecular dynamics calculations of charged particles, which take into account various quantum processes initiated by the photoeffect. From this point of view, the second set of calculations is closer to reality, because it handles all processes which change the dynamics of the system. Therefore, our treatment of Coulomb explosion is based on this work.