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Resonant Circular Dichroism in Photoelectron Spectroscopy

The chiral molecules, from the ancient greek cheir (hand), are a class of molecules that can not be superimposed on their mirror image: amino acids, proteins, sugars and about 50% of the active principles of medicines are chiral molecules. The study of the electronic structure of chiral molecules is carried out by spectroscopies employing the circularly polarized light (right and left), a probe that is not symmetric for specular reflections. The chiral molecules, in fact, respond differently to the two circular polarizations, and the difference (dichroism) provides information on the structural and electronic properties of molecules. The spectroscopy that gave rise to the development of stereochemistry is Natural Circular Dichroism (NCD) in absorption in the wavelength interval ranging from IR to UV.
With the development of synchrotron light sources, which offer a range of wavelengths from visible to X-rays, it was possible to extend the dichroism to the emission of electrons by photoionization, which is the process describing the emission of an electron after the absorption of  an ultraviolet photon, thus combining the study of the molecular orbitals with the chiral character. The probability of emitting an electron from the chiral molecule is dependent on the polarization of light, and the normalized difference between these probabilities (the dichroic D parameter) produces a characteristic modulation of the oscillating photo-emitted intensity as a function of the electron kinetic energy. This signal represents a true 'fingerprint' of the orbital and molecular structure of a chiral molecule.

In the previous studies the effectual points outlined by Photoelectron Spectroscopy Circular Dichroism (PECD) are related to: 1) molecular orbital assignment in the experimental spectrum, 2) sensitivity to conformational effects, 3) electronic character of the molecular orbital.
The circular dichroism in the photo-electron angular distribution is due to the dipole interaction term, while in the absorption process it is related to the second order interference terms, the electric dipole-electric quadrupole and electric dipole-magnetic dipole. Therefore, the circular dichroism in the photoelectron angular distributionis about one order of magnitude more intense.
The work explored the PECD of the valence states in the 3p→3d autoionization resonance region, to extend PECD analysis to the correlated electronic description.

Figure 1:  Left panel: Schematical representation of direct ionization and autoionization processes for HOMO state. Right panel: upper part, experimental dichroic dispersion (red dots) and TDDFT calculated D profile (grey line) in the Co 3p→3d autoionization resonance region, lower part, experimental HOMO photoemission intensity (blue dots) and TDDFT calculated cross section (grey line) in the Co 3p→3d autoionization resonance region.

The investigation was focused on Δ-cobalt(III) tris-acetylacetonate [Co(acac)3], which is a chiral metal complex.
The experiment was performed at the Circular Polarization beamline at Elettra. The calculation of the autoionizing resonant PECD is based on the Linear Combination of Atomic Orbitals (LCAO) B-spline method in the framework of the Time Dependent Density Functional Theory (TDDFT).
Figure 1 reports the dispersion of the constant initial state (CIS) intensity and of the dichroism (D parameter) for the Highest Occupied Molecular Orbital (HOMO). A sizable enhancement of the circular dichroism (5% maximum) is observed. In correspondence of the autoionization resonance, the very intense discrete Co 3p→3d transition is coupled with the HOMO direct ionization and the dichroic dispersion provides new insights into the electronic excitations in chiral molecules.
These findings open new physical aspects of photoelectron circular dichroism that now can be interpreted not only via the simple direct ionization, but also through more complex electron correlation processes. The satisfactory agreement between theory and experimental results proves that the TDDFT procedure can also account for the structural and electronic sensitivity of the circular dichroism in the resonant ionization regime.

This research was conducted by the following team:

  • Daniele Catone, Nicola Zema, Giorgio Contini, Tommaso Prosperi and Stefano Turchini – Istituto di Struttura della Materia – CNR, Italy
  • Mauro Stener and Piero Decleva - Dipartimento di Scienze Chimiche, Università di Trieste, Italy
  • Vitaliy Feyer and Kevin Charles Prince - Sincrotrone Trieste, Italy

Reference

D. Catone, M. Stener, P. Decleva, G. Contini, N. Zema, T. Prosperi, V. Feyer, K. C. Prince and S. Turchini, "Resonant Circular Dichroism of Chiral Metal-Organic Complex", Physical Review Letters, 108, 083001 (2012), doi: 10.1103/PhysRevLett.108.083001


 

Last Updated on Thursday, 30 August 2012 15:36