High-resolution core-level spectroscopy of free radical and transient species

High photon flux and energy resolution provided by SR undulator beam lines have recently stimulated experimental and theoretical studies of inner-shell ionization and excitation processes in small free molecules including highly diluted reactive species [1]. Fine spectral details revealed by using corehole sub-natural lifetime resolution provide an insight into the photoabsorption and ionization energetics and dynamics. Results have been obtained for the first time on the C1s photoabsorption and dissociation processes of CH3 free radical produced in a supersonic beam by Total-Ion-Yield and 3D-ion momentum imaging spectroscopies. Due to the high photon energy resolution clear vibrational details in electronic transitions involving a significant molecular geometry change have been obtained. Different photofragmentation patterns have also been observed depending on the nature of the core-excited resonance selected. Theoretical studies of the C1s photoabsorption spectrum including explicitly vibrational effects accompanying the photoexcitation process in this open shell molecule have been accomplished and the comparison between experiment and theory allows a detailed characterization of the potential energy surface for the lowest lying core-excited state. The first direct study of umbrella-like motion in inner-shell spectroscopy will be discussed in detail in the case of CH3. Fine spectral details have also been observed by symmetry-resolved spectroscopy in the S2p inner-shell photoabsorption processes in the CS transient molecule produced by a MW plasma discharge source. In some cases the vibrationally resolved structure gives direct information on the potential energy curve of the excited state and the chemical bond change upon inner-shell excitation. S2p photoionization processes in CS have also been studied allowing the characterization of the vibronic structure and the potential energy curve of the ionic states. Experimental results are supported by ab initio relativistic calculations providing a firm basis for the spectral assignment. Excellent agreement is found between experiment and theory. [1] S. Stranges, R. Richter, and M. Alagia, J. Chem. Phys., 116 , 3676 (2002)