Tetsuji Sekitani

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.

Study of ion desorption induced by a resonant core-level excitation of condensed H2O using Auger electron photo-ion coincidence (AEPICO) spectroscopy combined with synchrotron radiation

Ion desorption induced by a resonant excitation of O 1s of condensed amorphous H2O has been studied by total ion and total electron yield spectroscopy, nonderivative Auger electron spectroscopy (AES) and Auger electron photo-ion coincidence (AEPICO) spectroscopy. The spectrum of total ion yield divided by total electron yield exhibits a characteristic threshold peak at hν = 533.4 eV, which is assigned to the 4a1 ← O 1s resonant transition. The AES at the 4a1 ← O 1s resonance is interpreted as being composed of the spectator-AES of the surface H2O, and the normal-AES of the bulk H2O, where the 4a1 electron is delocalized before Auger transitions. H+ is found to be the only ion species in AEPICO spectra measured at the 4a1 ← O 1s resonance and at the O 1s ionization (hν = 560 eV). The electron kinetic energy dependence of the AEPICO yield (AEPICO yield spectrum) at the 4a1 ← O 1s resonance is found to be greatly different from that at the O 1s ionization. The peak positions of the AEPICO yield spectrum at the 4a1 ← O 1s resonance are found to correspond to those of the spectator-AES of the surface H2O, which is extracted from the AES at the 4a1 ← O 1s resonance. Furthermore, the AEPICO yield is greatly enhanced at the 4a1 ← O 1s resonance as compared with that at the O 1s ionization. On the basis of these results, a spectator-Auger-stimulated ion desorption mechanism and/or ultra-fast ion desorption mechanism are concluded to be responsible for the H+ desorption at the 4a1 ← O 1s resonance. The enhancement of the H+ yield is ascribed to the OH anti-bonding character of the 4a1 orbital.

Auger electron photoion coincidence technique combined with synchrotron radiation for the study of the ion desorption mechanism in the region of resonant transitions of condensed H2O

The Auger electron photoion coincidence (AEPICO) technique has been applied for the study of H+ desorption induced by resonant excitations of O 1s of condensed H2O. The peak positions of the AEPICO yield spectrum at the 4a1←O 1s resonance (hν=533.4 eV) are found to correspond to spectator-Auger transitions leaving (O 2s)−2(4a1)1, (O 2s)−1(O 2p)−1(4a1)1, and (O 2p)−2(4a1)1 states. The H+ AEPICO yield is greatly enhanced at 4a1←O 1s while it is suppressed at 3p←O 1s (hν=537 eV) as compared with that at the O 1s ionization (hν=560 eV). On the basis of these results, the ultrafast ion desorption mechanism is suggested to be favorable for the H+ desorption at 4a1←O 1s, that is, the repulsive potential energy surface of the (O 1s)−1(4a1)1 state is responsible for the H+ desorption. For H+ desorption at 3p←O 1s, a spectator-Auger stimulated ion desorption mechanism is concluded to be probable. The suppression of the H+ AEPICO yield is ascribed to the reduction of the hole–hole repulsion due to the shield effect ...

Ion desorption from molecules condensed at low temperature: A study with electron-ion coincidence spectroscopy combined with synchrotron radiation (Review)

This article reviews our recent work on photostimulated ion desorption (PSID) from molecules condensed at low temperature. We have used electron–ion coincidence (EICO) spectroscopy combined with synchrotron radiation. The history and present status of the EICO apparatus is described, as well as our recent investigations of condensed H2O, NH3, CH3CN, and CF3CH3. Auger electron photon coincidence (AEPICO) spectra of condensed H2O at the O:1s ionization showed that H+ desorption was stimulated by O:KVV Auger processes leading to two-hole states (normal-Auger stimulated ion desorption (ASID) mechanism). The driving forces for H+ desorption were attributed to the electron missing in the O–H bonding orbitals and the effective hole–hole Coulomb repulsion. The normal ASID mechanism was also demonstrated for condensed NH3. The H+ desorption at the 4a1←O(N):1s resonance of both condensed H2O and condensed NH3 was found to be greatly enhanced. Based on the AEPICO spectra the following four-step mechanism was propose...

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.

Auger-electron-ion coincidence study of photon-stimulated ion desorption for condensed acetonitrile

Abstract Auger-electron-photoion coincidence (AEPICO) studies of photon-stimulated ion desorption (PSID) for condensed acetonitrile induced by carbon core excitation have been performed to elucidate the desorption mechanism related to the Auger process. We have detected only the H+ ion in AEPICO spectra. The total ion yield spectrum divided by the total electron yield shows that the desorption efficiency is largely increased at the resonant excitation to CH∗. We have also measured the Auger electron spectrum and the AEPICO yield spectrum at the CH∗ excitation. The AEPICO yield spectrum shows enhancement at 245–250 eV electron energy. This seems to be related to the spectator resonant Auger stimulated ion desorption. That is, H+ desorption is enhanced due to a two-hole-one-electron state at which the electron is in an anti-bonding CH∗ orbital and the two holes can be in a 1π bonding orbital localized on the CH3 group. We have also measured similar spectra for other resonant excitation ( π∗ CN , σ∗ CC , σ∗ CN ). The results are discussed in connection with the bonding/anti-bonding character and localized character of the excited state.

Site specific photochemical reaction of PMMA and related polymers by inner shell electron excitation

Abstract The photoion desorption by inner shell electron excitation from solid films of poly(methyl methacrylate), poly(methyl acrylate) and poly(methacrylic acid) was studied. The assignment of the features observed in the absorption spectrum of PMMA was elucidated by a systematic comparison of the H+ yield spectra of the three polymers. Detailed information about the specific excitations that promote the desorption of CH3+ ions was obtained both in the carbon and oxygen K-edge regions. It was observed that most of the PMMA CH3+ ions are desorbed by the fragmentation at the side chain.

Mechanism of ion desorption reaction of PMMA thin film induced by core excitation

Abstract Auger electron spectra of PMMA (poly-methylmethacrylate) which show remarkable site-specific ion desorption are calculated using ab initio molecular orbital (MO) calculation and compared with the results of Auger electron-photoion coincidence experiments. Methyl isobutyrate and its trimer are used as model molecules of PMMA. The calculation is based on the single configuration state function and limited configuration interaction methods, and Auger transition probabilities are estimated by overlap between core and valence MOs. The calculated normal Auger spectra obtained on specific atoms reproduce well the experimental resonant Auger spectra for both the carbon and oxygen sites. The careful analysis of the calculated Auger spectra indicates that the desorption of CHn+ ions is strongly concerned with the Auger final states which have holes in the O–CH3 bonding orbitals.

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...

Control of chemical bond scission by using site-specific photochemical surface reactions

Abstract Using continuous variable synchrotron radiation, site selective core excitation can be achieved. The site-specificity of the consequent chemical reactions has been studied for adsorbates, thin films and molecules condensed onto a substrate. During the present research, a site-specific reaction was found in the case of photon stimulated ion desorption (PSID) of PMMA (polymethylmethacrylate) thin films. The PSID of condensed acetonitrile was also examined in order to elucidate the Auger process that follows resonant core excitation, and the ion desorption mechanism related to this Auger process. We discuss from the experimental results obtained so far, the possibility of controlling chemical bond scission through the use of site-specific photochemical surface reactions.