G. Faigel

Dynamics in a cluster under the influence of intense femtosecond hard X-ray pulses

Abstract.In this paper we examine the behavior of small cluster of atoms in a short (10-50 fs) very intense hard X-ray (10 keV) pulse. We use numerical modeling based on the non-relativistic classical equation of motion. Quantum processes are taken into account by the respective cross-sections. We show that there is a Coulomb explosion, which has a different dynamics than one finds in classical laser driven cluster explosions. We discuss the consequences of our results to single molecule imaging by the free electron laser pulses.

Imaging light atoms by X-ray holography

We demonstrate here the imaging of light atoms by hard X-ray holography. Using intense synchrotron radiation for the excitation, the oxygen and nickel atoms in a nickel oxide sample are imaged in three dimensions with high resolution inside a volume of about 1,000 cubic ångströms. These remarkable characteristics mean that X-ray holography bridges the gap between diffraction, which relies on long-range order, and extended X-ray absorption fine structure, which gives information on the local environment of atoms.

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

The ingenious idea of single-molecule imaging by hard X-ray Free Electron Laser (X-FEL) pulses was recently proposed by Neutze et al. (Nature 406 (2000) 752). 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 Fienups algorithm successfully reconstructed the original atomic structure. The critical evaluation of these results gives guidelines and sets important conditions for future experiments aiming at single-molecule structure solution.

Structure and stability of crystalline C60 · n-pentane clathrate

Abstract Clathrate type single crystals with the composition of C60(n-C5H12)0.88(C7H8)0.05 were grown from C60-toluene- petroleum ether solution. The structure of the crystals, determined by X-ray diffraction is b-face centered orthorhombic. No phase transition, corresponding to the orientational ordering is observed at 250 K by differential scanning calorimetry. Around 420 K the solvent molecules are released from the clathrate as determined by thermogravimetry-mass spectrometry. An irreversible phase transition is observed simultaneously with the evolution of solvents, resulting in the face centered cubic structure of pure C60.

Structure and stability of covalently bonded polyfulleride ions in AxC60 salts

Abstract A comparative study of stabilities is presented for a variety of (C 60 n − ) p polyanions. Formation enthalpies of dimers and monomers were determined from those of AM1 optimized R m C 60 n − monomers using thermochemical considerations. In contrast to cycloadduct polymers, singly bonded polymers become more stable with increasing charge and a crossover occurs at around n =3. Geometries were analyzed in terms of bond and charge distributions. The polyanion geometries of Na 2 RbC 60 and Na 4 C 60 salts are in agreement with X-ray results.

Bulk structure of phototransformed C60

We have produced phototransformed C60 in powder form and in bulk quantities. The availability of several milligram batches allowed us to collect good quality X-ray powder diffractograms, infrared transmission spectra and differential scanning thermograms. Based on these data a disordered structural model is proposed which is built up from small closed oligomers. q 1999 Elsevier Science Ltd. All rights reserved.

Polymeric sheets in Mg4C60

Abstract We present preliminary results on the preparation and structure of Mg 4 C 60 , a new fulleride polymer. A series of Mg x C 60 compositions (0 x 4 C 60 . The X-ray diffraction pattern of Mg 4 C 60 was successfully indexed and fitted to a rhombohedral structure based on two-dimensional polymeric sheets of C 60 ions. This result extends the family of fulleride polymers to the salts of multiply charged cations and thus opens a new direction in this field.

Infrared and raman spectra of C60·n-pentane clathrate crystals

We used infrared and Raman spectroscopy to investigate single crystals of the compound C60(C5H12)0.88(C7H8)0.05. We observed the signatures of the individual components C60 and pentane in the infrared spectrum with minimal changes when compared to solvent-free solid or solution spectra. The corresponding Raman spectrum shows that while the molecular C60 frequencies remain unchanged, there are some additional lines, probably due to symmetry effects of the solid state. We conclude that the material is a true clathrate with no electronic interaction between the constituents. Unlike the infrared spectrum pentane lines are not observed in Raman spectra.

The effect of tamper layer on the explosion dynamics of atom clusters

AbstractThe behavior of small samples in very short and intense hard X-ray pulses is studied by molecular dynamics type calculations. The main emphasis is put on the effect of various tamper layers about the sample. This is discussed from the point of view of structural imaging of single particles, including not only the distortion of the structure but also the background conditions. A detailed picture is given about the Coulomb explosion, with explanation of the tampering mechanism. It is shown that a thin water layer is efficient in slowing down the distortion of the atomic structure, but it gives a significant contribution to the background.

Shift of the Optical Absorption Edge in C60 Clathrate Single Crystals

We measured the transmission between 1.6–2.2 eV of pure C60 and three clathrate single crystals containing different aliphatic guest molecules. The absorption edge in the clathrates shows a blue shift of 0.1 eV irrespective of the guest molecule, indicating that C60 is electronically isolated in these compounds.