Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.
We present kinematically complete measurements of the photo double ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photo electrons and the nuclear kinetic energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels.The reason for such a striking difference is explained in part on the basis of a propensity rule which influences the population of states in the photo double ionization of a centrosymmetric closed shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.
Even though ion/atom-collision is a mature field of atomic physics great discrepancies between experiment and theoretical calculations are still common. Here we present experimental results with highest momentum resolution on single ionization of helium induced by 1 MeV protons and compare these to different theoretical calculations. The overall agreement is strikingly good and already the first Born approximation yields good agreement between theory and experiment. This has been expected since several decades, but so far has not been accomplished. The influence of projectile coherence effects on the measured data is shortly discussed in line with an ongoing dispute on the existence of nodal structures in the electron angular emission distributions.
The absolute configuration of individual small molecules in the gas phase can be determined directly by light-induced Coulomb explosion imaging (CEI). Herein, this approach is demonstrated for ionization with a single X-ray photon from a synchrotron light source, leading to enhanced efficiency and faster fragmentation as compared to previous experiments with a femtosecond laser. In addition, it is shown that even incomplete fragmentation pathways of individual molecules from a racemic CHBrClF sample can give access to the absolute configuration in CEI. This leads to a significant increase of the applicability of the method as compared to the previously reported complete break-up into atomic ions and can pave the way for routine stereochemical analysis of larger chiral molecules by light-induced CEI.
We investigate the contribution of Interatomic Coulombic Decay induced by ion impact in neon and argon dimers (Ne 2 and Ar 2 ) to the production of low energy electrons. Our experiments cover a broad range of perturbation strengths and reaction channels. We use 11.37 MeV/u S 14+ , 0.125 MeV/u He 1+ , 0.1625 MeV/u He 1+ and 0.150 MeV/u He 2+ as projectiles and study ionization, single and double electron transfer to the projectile as well as projectile electron loss processes.The application of a COLTRIMS reaction microscope enables us to retrieve the three-dimensional momentum vectors of the ion pairs of the fragmenting dimer into Ne q+ /Ne 1+ and Ar q+ /Ar 1+ (q = 1, 2, 3) in coincidence with at least one emitted electron.
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