Chiral organic molecules are of widespread interest in all natural sciences , from the biological homochirality of sugars and aminoacids to the selective activity of only one enantiomer in pharmaceutical drugs. It is therefore evident that the study of their optical properties is not only relevant in basic research but also for applied purposes. A pioneering activity using a spectroscopic technique based on ionizing circularly polarised synchrotron radiation (SR) was developed by Bowering et al. to investigate this class of free molecules . Initially known as Circular Dichroism in the Angular Distribution (CDAD), and later named as Photoelectron Circular Dichroism (PECD) by Powis et al. , this technique is based on the asymmetry observed in the angular distribution of photoelectrons emitted by randomly oriented pure enantiomers. In such case the photoelectron intensity I(ω,θ) is a function of the emission angle θ, the energy-dependent (ω) photoemission cross section σ(ω),the asymmetry parameter β(ω), and the dichroism parameter D(ω),the latter related to the chiro-optical properties of a specific enantiomer . The measured photoelectron intensity shows a forward-backward asymmetry, with respect to the radiation propagation axis, in the photoelectron angular distribution, that depends on the value of D(ω). This experimental methodology, combined with theoretical spectral simulations calculated at sufficiently high level of theory, provides a powerful tool to establish the absolute configuration of the specific enantiomer. In the perspective of using this technique in a joined experimental-theoretical investigation for studying the enantiomeric recognition on model chiral molecules, we have measured for the first time the photoelectron asymmetry parameters β(ω), and the relative photoemission cross sections, σ(ω),of the epichlorohydrin chiral free molecule by linearly polarised VUV SR. The experiments were performed at the CIPO Beamline of the ELETTRA Synchrotron Facility, and the experimental results are supported by state-of-art TDDFT calculations. Both experimental and theoretical data show an interesting dynamic behaviour of the β(ω) parameters in the Cl 3p Cooper minimum region, this depending on the nature of the specific Molecular Orbital involved in the photoionization process. The present study of the photoemission observables σ(ω)and β(ω) will be completed by the investigation of the photoemission dichroism parameters D(ω), that is in progress.
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