1,721,127 research outputs found
Soft x-ray and VUV photoexcitation of free radicals, short-lived species and chiral free molecules
No full textIntense 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
Inner-shell studies of free radical and transient species
No full textProgress in synchrotron radiation technology has provided sources with high photon flux and energy resolution. Consequently, the more favorable conditions have 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 core-hole 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 C(1s) photoabsorption and dissociation processes of the CH3 free radical, produced in a supersonic jet, by total-ion-yield and 3D-ion momentum imaging spectroscopy. Due to the high photon energy resolution clear vibrational spectral details in electronic transitions involving a significant molecular geometry change have been obtained. Different photofragment patterns have also been observed depending on the nature of the selected core-excited resonance. Theoretical studies on the C(1s) photoabsorption spectrum of the CH3 radical, which explicitly includes vibrational effects accompanying the photoexcitation process, have been carried out and the comparison between experiment and theory allows a detail characterization of the potential energy surface for the lowest lying core-excited state. The first study of umbrella-like motion in inner-shell spectroscopy in the case of CH3 will be presented.
The S(2p) photoabsorption and ionization processes in the CS transient molecule, produced by a MW plasma source, have been investigated. The fine spectral details obtained in this study allow the characterization of the vibronic structure and the potential energy curve of S(2p) core-excited and ionic states of the CS molecule. Experimental results have been crucially supported by ab initio relativistic calculations that provided 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)
Recent studies of radical, transient and chiral free molecules at ELETTRA
No full textUniversité Catholique de Louvain, Belgium, (invited lecture)
Synchrotron radiation studies of free radical and transient species
No full textHigh 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 core-hole 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 photofragment patterns have also been observed depending on the nature of the core-excited resonance selected. Theoretical studies on the C1s photoabsorption spectrum explicitly including vibrational effects accompanying the photoexcitation process in this open shell molecule have been accomplished and the comparison between experiment and theory allows a detail characterization of the potential energy surface for the lowest lying core-excited state. The first study of umbrella-like motion in inner-shell spectroscopy will be discussed in detailed in the case of CH3.
Fine spectral details have also been obtained by symmetry-resolved spectroscopy in studying S2p inner-shell photoabsorption processes in the CS transient molecule produced from a MW plasma discharge source. 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 have been supported crucially by ab initio relativistic calculations providing a firm basis for the spectral assignment. Excellent agreement is found between experiment and theory.
References
[1] S. Stranges, R. Richter, and M. Alagia, J. Chem. Phys., 116 (2002) 3676
Photoelectron spectroscopy (PES) of organometallic compounds with C-M (M = Ge, Sn, Pb) bonds",(capitolo)
No full textThe use of ab initio and DFT calculations in the interpretation of ultraviolet photoelectron spectra: the rotational isomerism of anisole and thioanisole as a case study
No full textThe rotational isomerism of anisole and thioanisole was investigated by means of several computational methods, in the framework of the Møller–Plesset and Density Functional theories. Total energy curves as a function of the C–O(S)–C–C dihedral angle were calculated. For anisole, all methods employed, with the exception of HF/6-31G*, predicted the planar conformer to be the only stable species, in agreement with experiment. For thioanisole, HF and Møller–Plesset calculations
predicted the perpendicular conformer to be the only stable species. Nearly all DFT methods predicted, instead, the existence of two minima, in agreement with experimental findings. Ionisation energies calculated according to the Koopmans’ theorem were compared with the values obtained from UP spectra. DFT ionisation energies, especially those obtained by the B3LYP, B1LYP, mPW1PW and B3PW functionals, were found to be in fair agreement with experimental values. HF ionisation energies showed only a rough agreement with experiment
High temperature reduction kinetics of ilmenite by hydrogen
No full textThe kinetics of H redn. of Norwegian ilmenite ore powder was investigated at 927-1280 K. The activation energy of a surface chem. reaction was 69.6 kJ/mol as calcd. from wt. loss-time measurements according to the shrinking-unreacted core model. Optical microscopy of synthetically prepd. FeTiO3 pellets at redn. degrees of 25 and 35% showed the interface between the reacted and unreacted zones. An electron probe qual. anal. of the reduced specimens indicated a segregated Fe phase present as spheroidal nodules and TiO2 phase present in vein-like arrangements
High-resolution inner-shell photoabsorption of the OH and OD free radicals
No full textThe O 1s photoabsorption ~ion-yield! spectra of OH (2Pi ,v950) and OD (2Pi ,v950) radicals have been studied in the gas phase for the first time. Spectroscopic parameters for the O 1s!p excited 2S1 state of OH and OD, observed at ;525.8 eV, have been obtained from a Franck– Condon analysis comparing the spectra of both molecules. The lifetime width of this state has been measured as G5147611 meV and G515667 meV for OH and OD, respectively. Excellent quantitative agreement is found between the experimental spectroscopic parameters re , ve , and T0 , measured in this work for the 2S1 state and recent theoretical results obtained for OH by high-level ab initio calculations. The spectroscopic parameters predicted by the equivalent-core model are close to the experimental values within a few percent. This model therefore provides an accurate description of the lowest-lying excitation process in both OH and OD molecules
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