Showing papers by "Pavle Juranić published in 2012"

Single-shot analysis of hard x-ray laser radiation using a noninvasive grating spectrometer

Abstract: We present a spectrometer setup based on grating dispersion for hard x-ray free-electron lasers. This setup consists of a focusing spectrometer grating and a charge-integrating microstrip detector. Measurement results acquired at Linac Coherent Light Source are presented, demonstrating noninvasive monitoring of single-shot spectra with a resolution of 2.0×10−4±0.3×10−4 at photon energy of 6 keV with more than 95% transmission of the main beam.

Photoemission of cooper pairs from aromatic hydrocarbons.

Abstract: We report the discovery of the formation of an electron Cooper pair approximately 40 eV above the double-ionization threshold in benzene, naphthalene, anthracene, and coronene after absorption of a single photon. We have measured the ratios of doubly to singly charged parent ions of the above mentioned molecules as well as pyrrole and furan by using monochromatized synchrotron radiation up to 100 eV above the corresponding thresholds. We also recorded photoelectron spectra of benzene and naphthalene at selected energies. The electron-pair formation is based on the specific structure of the molecules and does not exist for pyrrole and furan.

Large molecules reveal a linear length scaling for double photoionization.

Abstract: We present experimental results for the ratio of doubly to singly charged parent ions of structural similar molecules with different lengths. 1 The molecules under investigation are linearly fused benzene rings. The lengths of these molecules vary with respect to the structure of their carbon atoms from 2.8 A for benzene 2 to 14 A for pentacene 3 , which consists of five fused benzene rings and is also the longest stable chain of fused benzene rings. We discovered that the doubleto-total photoionization cross section ratio increases perfectly linearly with the lengths of the molecules for photon energies up to at least 30 eV above the double-ionization threshold (cf. Fig. 1). The dramatic increase of the ratio by a factor of 5 for pentacene as compared to the ratio of benzene can be understood in terms of the knock-out model that was previously introduced for atoms by comparing that ratio to the electron-impact single-ionization cross section of a singly charged ion 4 . Our results provide direct evidence of this model for the first time without the need for using electron-impact data for comparison. This discovery means that the structure of a molecule can have a significant influence on the ratio and that the origin of the two emitted electrons can be far apart. Moreover, the overall energy dependences of the molecules' ratios are very similar to that of He. This observation is another indication that the knock-out model is valid.