Soft x-ray and VUV photoexcitation of free radicals, short-lived species and chiral free molecules

Intense and high-resolution synchrotron radiation has stimulated experimental and theoretical studies of inner-shell electron excitation and ionization processes in atoms and small molecules, including very low density and highly reactive free radicals and short-lived molecules. We report here on recent investigations of such processes in small hydrocarbon radicals (CH3 [1], CD3 [2] and C3H5) and the short-lived CS molecule. Emphasis is given to the first direct experimental evidence and detailed analysis of the vibrational fine structure related to the umbrella-like motion in inner-shell molecular spectroscopy, as observed in the core excited methyl radicals. In the case of the short-lived CS molecule, vibrationally resolved inner-shell photoabsorption and ionization spectra, together with results obtained by ab initio relativistic calculations, allow to discuss the chemical bond change upon electron excitation/ionization. Very few investigations using ionizing photons have been hitherto accomplished to study chiral molecular systems. Advanced synchrotron machines have provided intense sources with high degree of circularly polarized light to be used in experiments. This radiation can reveal Circular Dichroism in randomly oriented chiral molecules by means of angle-resolved photoelectron spectroscopy (ARPES) involving valence or inner-shell electrons. Recent results obtained in the valence regime by ARPES in studying methyl-oxirane enantiomers [3,4], as model chiral system, will be shown along with the most recent theoretical predictions. Prospects of these studies will also be highlighted. 1] M. Alagia et al., Phys. Rev. A, 76 (2007) 022509. 2] U. Ekström et al., J. Chem. Phys., 128 (2008) 044302. 3] S. Stranges et al., J. Chem. Phys., 122 (2005) 244303. 4] S. Turchini et al., Phys. Rev. A, 70 (2004) 14502.