Valence Electron PECD

Powis Group

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Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light

L. Nahon, G. A. Garcia, C. J. Harding, E. A. Mikajlo and I. Powis J. Chem. Phys. 125(2006), 114309.

An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, Image of RCamp_Gad this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry.

Several ionization channels have been studied using synchrotron radiation in the 8.85 – 26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. Image of RSCamp_img0 For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO-1 channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at ~20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve.

Newly optimized CMS-Xα photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization – where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect – in valence shell ionization there is a significant additional influence contributed by the initial orbital density. ©2006 American Institute of Physics

Chiral signatures in angle-resolved valence photoelectron spectroscopy of pure glycidol enantiomers

Gustavo A. Garcia, Laurent Nahon, Chris J. Harding and Ivan Powis Phys. Chem. Chem. Phys., 10(2008), 1628-1639.

Photoionization of the chiral molecule glycidol has been investigated in the valence region. Photoelectron circular dichroism (PECD) curves have been obtained at various photon energies by using circularly polarized VUV synchrotron radiation and a velocity map imaging technique to record angle-resolved photoelectron spectra (PES). The measured chiral asymmetries vary dramatically with the photon energy as well as with the ionized orbital, improving the effective orbital resolution of the PECD spectrum with respect to the PES. Typical asymmetry factors of 5% are observed, but the peak values measured range up to 15%. Glycidol Data The experimental results are interpreted by continuum multiple scattering (CMS-Xα) calculations for several thermally accessible glycidol conformers. We find that a nearly quantitative agreement between theory and experiments can be achieved for the ionization of several molecular orbitals. Owing to the sensitivity of PECD to molecular conformation this allows us to identify the dominant conformer. The influence of intramolecular hydrogen bond orbital polarization is found to play a small yet significant role in determining the chiral asymmetry in the electron angular distributions.

Reproduced by permission of the PCCP Owner Societies

Whats So Special?

A new form of CD — Photoelectron Circular Dichroism (PECD) — has been pioneered in Nottingham by the development of experimental and theoretical methods. PECD is already present in the pure electric-dipole approximation for the radiation-matter interaction approximation (unlike the "normal" absorption CD that requires higher order, and weaker, electric quadrupole or magnetic interaction terms) leading to unprecedentedly large chiral asymmetries in the photoelectron angular distributions that range up to several tens of percent — many orders of magnitude greater than encountered in absorption CD spectroscopy.

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But there are other new features — PECD develops from the continuum electron phase in a significantly different manner than the dipolar β parameter, and so provides a very much more sensitive probe of the molecular photoionization dynamics as well as of the molecular potential, leading to a very high sensitivity to molecular conformation and to the chemical environment.

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