Showing papers by "Ivan Powis published in 2010"

Effects of dimerization on the photoelectron angular distribution parameters from chiral camphor enantiomers obtained with circularly polarized vacuum-ultraviolet radiation

Abstract: As an intermediate state of matter between the free monomeric gas phase and the solid state, clusters may exhibit a specific electronic structure and photoionization dynamics that can be unraveled by different types of electron spectroscopies. From mass-selected ion yield scans measured for photoionization of (R)-camphor, the ionization potentials (IPs) of the monomer (8.66{+-}0.01 eV), and of the homochiral dimer ({<=}8.37{+-}0.01 eV) and trimer ({<=}8.30{+-}0.01 eV) were obtained. These spectra, combined with threshold photoelectron spectroscopy and velocity map ion imaging, allow us to show that the camphor monomer and dimer photoionization channels are decoupled, i.e., that the highest occupied molecular orbital (HOMO) of the dimer does not undergo a dissociative ionization process that would lead to a spurious contribution to the monomer ion channel. Therefore mass selection, as achieved in our imaging photoelectron-photoion coincidence experiments, leads to size selection of the nascent monomer or dimer species. Since both the monomer and dimer are chiral, their photoelectron angular distribution (PAD) not only involves the usual {beta} anisotropy parameter but also a chiral asymmetry parameter b{sub 1} that can generate a forward-backward asymmetry in the PAD. This has been investigated using circularly polarized light (CPL) to record the photoelectron circular dichroism (PECD)more » in the near-threshold vacuum-ultraviolet (VUV) photoionization region. Analysis of size-selected electron images recorded with left- and right-handed CPL shows that over the first 1.5 eV above the HOMO orbital ionization potentials (IPs), the {beta} parameter is not affected by the dimerization process, while the chiral b{sub 1} parameter shows clear differences between the monomer and the dimer, confirming that PECD is a subtle long-range probe of the molecular potential.« less

Photoelectron circular dichroism spectroscopy in an orbitally congested system: the terpene endoborneol.

Abstract: We have measured the photoelectron circular dichroism (PECD) of single enantiomers of endoborneol in the photon region from 9.9 to 23.6 eV by combining circularly polarized synchrotron radiation and a velocity map imaging technique. A photoelectron spectrum and the state-selected fragmentation curves of this terpene were also recorded. Unlike previous case studies, the broad featureless transitions encountered here preclude an orbital by orbital analysis of the PECD, although semiquantitative features of the highest-occupied molecular orbital PECD are identified and compared to full calculations. Despite our inability to further identify individual orbitals experimentally, we show that we are able to unambiguously assign the absolute configuration by comparison with realistic simulated PECD spectra. Furthermore, the calculations predict that for electron kinetic energies above 5 eV; the contributions of individual conformers to the PECD are nearly identical. Should this observation apply to bigger biological systems, the analysis could be greatly simplified by recording high kinetic energy electrons. On the other hand the contributions of the different conformers to the slow electron PECD seem to vary more significantly, and we deduce, within the theoretical limitations, a plausible 1:1:1 distribution of the three identified conformers.

Valence Photoelectron Circular Dichroism of Gas Phase Enantiomers

Abstract: To those broadly familiar with the theory or practice of chiral molecular spectroscopies, one of the more recently investigated chiroptical phenomena, photoelectron circular dichroism1 (PECD), may nevertheless be surprising when rst encountered because from randomly oriented, noninteracting molecular enantiomers it routinely yields asymmetry factors ranging from 0.01 to 0.3. Asymmetries of this magnitude exceed those encountered in more traditional circular dichroism (CD) measurements by several orders of magnitude, and so promise a number of advantages associated with the potentially greater ease of detecting such large effects.