S.A. Sardar

Control of chemical reactions by core excitations

Abstract Recent progress in synchrotron radiation techniques in a soft X-ray region has developed a new field in chemistry. Using tunable synchrotron radiation in a soft X-ray region, site selective core excitation can be achieved. The site-specificity of the consequent chemical reactions is one of the attractive issues and has been studied for various kinds of molecules. During these studies, a remarkable site-specific reaction has been found in the case of photon stimulated ion desorption (PSID) of polymethylmethacrylate (PMMA) thin films. The PSID of PMMA has been examined in order to elucidate the reaction mechanism using total electron yield, total and partial ion yield, Auger electron, and Auger electron–photoion coincidence spectroscopies. In this paper we discuss the possibility of controlling chemical bond scission by using the site-specific photochemical surface reactions mainly based on the recent experimental results.

Photon-stimulated ion desorption for PMMA thin film in the oxygen K-edge region studied by Auger electron-photoion coincidence spectroscopy

Auger electron-photoion coincidence (AEPICO) measurements have been performed for poly-methylmethacrylate 1 (PMMA) thin films induced by oxygen core excitation. The dominant productions of CH ( n51-3) ions were observed at n 1 resonant excitation to O-CH anti-bonding state, and OCH ion was observed at resonant excitation to C-OCH 3 3 anti-bonding state. Their AEPICO yields were strongly dependent on Auger electron kinetic energy and shows enhancement around 505 eV Auger electron energy for each ion. This seems to be related to the spectator resonant Auger-stimulated ion desorption. The correlation between ion desorption and Auger final state for PMMA thin films in the oxygen K-edge region is discussed.

Study of ion desorption induced by carbon core excitation for poly-methylmethacrylate thin film using electron–ion coincidence spectroscopy

We have developed a new electron–ion coincidence apparatus combined with synchrotron radiation in order to examine the various ion desorption mechanisms related to the Auger process induced by core excitation. Photon stimulated ion desorption (PSID) of a poly-methylmethacrylate (PMMA) thin film has been investigated by this apparatus. The PSID of PMMA induced by carbon core excitation has been examined using Auger electron yield, total ion yield, resonant Auger electron, and Auger electron–photoion coincidence (AEPICO) spectra. The spectrum of the total ion yield divided by the Auger electron yield shows that the desorption efficiency is largely increased at the resonant excitation of carbon 1s electron in the O–CH3 side chain to σ*(O–CH3) orbital. In AEPICO measurement, H+ and CHn+ (n=1–3) ions are observed at various resonant excitations. The AEPICO signal intensity depends on the Auger electron energy. Particularly, the CH3+ ion desorption in coincidence with Auger electron at 270 eV shows strong enhan...

Development of a time-of-flight mass spectrometer for ion desorption studies at HiSOR

Abstract We have developed a time-of-flight mass spectrometer which is now under operation at HiSOR storage ring for research of photon stimulated ion desorption (PSID). The employment of the pulsed high voltage method as a trigger allowed us to perform the investigations at a multi bunch operation of the storage ring. The performance of this spectrometer was evaluated by applying to the PSID measurements of PMMA (poly-methylmethacrylate) thin films. The results are compared with those obtained at Photon Factory by using pulsed synchrotron radiation in a single bunch operation. The capabilities of the apparatus for ion desorption studies are discussed.

Adsorption behavior of sulfur-containing amino acid molecule on transition metal surface studied by S K-edge NEXAFS

Abstract Adsorption behavior of a sulfur-containing amino acid L-cysteine molecule on transition metal surface have been investigated by S K-edge near-edge X-ray absorption fine structure. The L-cysteine molecule for first adsorption layer was found to dissociate on polycrystalline nickel surface, whereas molecularly adsorbed on copper surface at room temperature. Most of the L-cysteine molecules have been dissociated on nickel surface in annealing condition up to 353 K. On the other hand, the L-cysteine molecule did not dissociate on copper surface and the elongation of the S–C bonding occurred at 353 K.

Thioethanol on Cu(111) at room temperature: A near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy study

Orientation and charge transfer upon adsorption of thioethanol on copper surface at room temperature has been studied by near edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS) techniques. It is found that thioethanol adsorbs dissociatively on the Cu(111) surface. Polarization dependent S K-edge NEXAFS indicates that the S–C bond is tilted 47±7° from the surface. Above 400 K, the S–C bond breaks and creates atomic sulfur. An amount of charge transfer (one electron) from copper to thioethanol molecules was measured using S 1s XPS.