When we cut a corner from a sheet of paper, we usually use a pair of scissors (Fig. 1). When the piece of paper is small, a knife is necessary. However, when the target becomes smaller and smaller towards the size of a molecule, a molecular assembly, or a nanoscale device, we have not yet found an appropriate cutting tool. As a candidate for such a molecular knife, we may consider localized photoexcitation. In contrast to valence electrons delocalized over the entire molecule, core electrons can be localized near the nucleus of one particular atom. Accordingly, controlled bond breaking following photoexcitation or photoemission of localized core electrons can be considered a molecular knife. To select an atom not only of one specific element, e.g., carbon but also in a specific chemical environment inside the molecule, the corresponding core-electron binding energy differences, that is, chemical shifts in ESCA, can be used. This quantity has been used to study site-specific fragmentation [1], where bonds around the site of core-ionized atoms are dissociated selectively. In this work, we employed an energy-selected- photoelectron photoion/photoion-photoion coincidence (PEPICO/PEPIPICO) method [2] to study site-specific fragmentation caused by Si:2 p core-level photoionization of F 3 SiCH 2 CH 2 Si(CH 3 ) 3 (FSMSE) in the vapor phase. The coincidence apparatus was installed at the c branch of BL27SU [3]. FSMSE was chosen for this study because the chemical environment of a Si atom bonded to three F atoms (as denoted here Si[F]) is very different from that of Si bonded to three methyl groups (Si[Me]). Therefore, the two peaks in the photoelectron spectrum (PES) of FSMSE vapor assigned to Si[Me]:2 p and Si[F]:2 p photoelectron emission [4] are clearly separated in Fig. 2(a). The PEPICO spectra measured in coincidence with the Si[Me]:2 p and Si[F]:2 p photoelectrons are shown in Figs. 2(b) and 2(c), where site-specific fragmentation is clearly evident. For example, the site selectivity for the production of F 3 SiCH 2 CH 2 + and F + is almost 100%. The bond dissociations mostly occur at the Si site where the photoionization has taken place. Site-specific 114 A A M Mo ol le ec cu ul la ar r K Kn ni if fe e - - C Co on nt tr ro ol l o of f C Ch he em mi ic ca al l R Re ea ac ct ti io on ns s u us si in ng g C Co or re e I Io on ni iz za at ti io on n - - Chemical Science Core ionization Fig. 1. Molecular knife. fragmentation is also revealed in ion-pair formation. Figure 3 shows the PEPIPICO counts of F 3 SiCH 2 CH 2 + –Si(CH 3 ) 3 + and SiF + –SiCH 3 + versus the photoelectron binding energy, together with the PES of FSMSE. The site selectivity for the production of these ion pairs was also almost 100%. Site-specific fragmentation thus offers an approach to control chemical reactions by selecting the sites at which holes are created. The elucidation of the details of the fragment production mechanism brings the goal of a molecular knife one step closer to realization. 115 0 104 106 108 110 112 50 100 Binding Energy (eV) 0 50 100 Counts 0 10000 20000 Si[Me]:2 p Si[F]:2 p (b) (c) F 3 SiCH 2 CH 2 + – Si(CH 3 ) 3 + SiF + – SiCH 3 + PES (a) Fig. 2. (a) Si:2 p PES of FSMSE vapor. (b) and (c) PEPICO spectra measured in coincidence with the Si[Me]:2 p and Si[F]:2 p photoelectrons. The insets show the regions of F + and F 3 SiCH 2 CH 2 + with enlarged scales. Fig. 3. (a) Si:2 p PES of FSMSE vapor. (b) and (c) Plots of the PEPIPICO counts of F 3 SiCH 2 CH 2 + –Si(CH 3 ) 3 + and SiF + –SiCH 3 + versus the photoelectron binding energy. S. Nagaoka a, *, G. Prümper b and K. Ueda b a Graduate School of Science and Engineering, Ehime University b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University *E-mail: nagaoka@ehimegw.dpc.ehime-u.ac.jp References [1] S. Nagaoka et al. : J. Chem. Phys. 129 (2008) 204309 and references cited therein. [2] G. Prümper et al .: J. Electron Spectrosc. Relat. Phenom. 144-147 (2005) 227. [3] H. Ohashi et al .: Nucl. Instrum. Methods A 467-468 (2001) 529. [4] S. Nagaoka, G. Prümper, H. Fukuzawa, M. Hino, M. Takemoto, Y. Tamenori, J. Harries, I. H. Suzuki, O. Takahashi, K. Okada, K. Tabayashi, X.-J. Liu, T. Lischke and K. Ueda: Phys. Rev. A 75 (2007) 020502(R). 0 50 100 600 400 200 200 SiF 2 + SiF 3 + SiF + Counts 600 400 0 800 F + Si[Me]:2 p Si[Me]:2 p Si[F]:2 p Si[F]:2 p 10 5 0 104 106 108 110 112 10 -3 Counts Binding Energy (eV) PES (c) (a) (b) Si(CH 3 ) 2 + Si(CH 3 ) 3 + F 3 SiCH 2 CH 2 + m / e
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