Core Electron PECD

Powis Group

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Circular Dichroism in the Angle-Resolved C 1s Photoemission Spectroscopy of Gas-Phase Carvone Enantiomers

C. J. Harding, E. A. Mikajlo, I. Powis, S. Barth, S. Joshi, V. Ulrich and U. Hergenhahn, J. Chem. Phys. 123(2005), 234310.

The inner-shell C 1s photoionization of randomly oriented molecules of the chiral compound carvone has been investigated using circularly polarized synchrotron radiation up to 30 eV above threshold. Binding energies of the C=O and CH2= carbon 1s orbitals were determined to be 292.8±0.2 and 289.8±0.2 eV, respectively. The remaining C 1s levels substantially overlap under an intense central peak centered at 290.5±0.2 eV.

Picture of Carvone PECD Data

The angle-resolved photoemission from the carbonyl carbon CO core orbital in pure carvone enantiomers shows a pronounced circular dichroism of ~6% at the magic angle of 54.7° to the light beam propagation direction. This corresponds to an expected 0°–180° forward-backward electron emission asymmetry (Γ0) of ~10%. On changing between the R and S enantiomers of carvone the sense or sign of the asymmetry and associated dichroism effectively reverses. The observed circular dichroism, and its energy dependence, is well accounted for by calculations performed in the pure electric dipole approximation. ©2005 American Institute of Physics

Giant Chiral Asymmetry in the C 1s Core Level Photoemission from Randomly Oriented Fenchone Enantiomers

Volker Ulrich, Silko Barth, Sanjeev Joshi, Uwe Hergenhahn, Elisabeth Mikajlo, Chris J. Harding, and Ivan Powis J. Phys. Chem. A, 112(2008), 3544 -3549.

Measurements made with a dilute, non-oriented, gas-phase sample of a selected fenchone enantiomer using circularly polarized synchrotron radiation demonstrate huge chiral asymmetries, approaching 20%, in the angular distribution of photoelectrons ejected from carbonyl C 1s core orbitals. This asymmetry in the forward-backward scattering of electrons along the direction of the incident soft X-ray radiation reverses when either the enantiomer or the left-right handedness of the light polarization is exchanged.

Fenchone data

Calculations are provided that model and explain the resulting photoelectron circular dichroism with quantitative accuracy up to ~7 eV above threshold. A discrepancy at higher energies is discussed in the light of a comparison with the closely related terpene, camphor. The photoelectron dichroism spectrum can be used to identify the absolute chiral configuration, and it is more effective at distinguishing the similar camphor and fenchone molecules than the corresponding core photoelectron spectrum.©2008 American Chemical Society

Core Level PECD

A new form of CD — Photoelectron Circular Dichroism (PECD) — has been pioneered in Nottingham by the development of experimental and theoretical methods.Although the first calculations and experiments examined valence electrons, attention very quickly spread to an investigation of core 1s electron ionization. At first sight this might seem to be unpromising territory for observing facets of molecular chirality. Core electrons are known to be highly localised and their ionization is frequently viewed as atomic-like, and relatively immune to the specific molecular environment. Moreover, 1s electron orbitals, being spherical, are themselves intrinsically achiral.

Perhaps surprisingly, studies quickly established core electron PECD asymmetries can equal or even exceed those observed with valence electron ionization. This however can be understood as a consequence of final-state scattering of the outgoing photoelectron off the chiral molecular potential, which must clearly then be equally important as any intrinsic chirality of the ionizing orbital.

Whats So Special?

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 a very high relative intensity up to the few tens of percent range.

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.