Site-specific ion desorption from condensed F3SiCD2CH2Si(CH3)3 induced by Si-2p core-level ionizations studied with photoelectron photoion coincidence (PEPICO) spectroscopy, Auger photoelectron coincidence spectroscopy (APECS) and Auger electron photoion coincidence (AEPICO) spectroscopy☆
Highlights
► Ion desorption from condensed F3SiCD2CH2Si(CH3)3 by Si-2p ionization was studied. ► PEPICO, APECS and AEPICO spectroscopies were used. ► Site-specific F+ desorption and non-site-specific H+ desorption were observed.
Introduction
After the concept of desorption induced by electronic transitions (DIET) was put forward by Menzel and Gomer [1] and Redhead [2], DIET has developed into an active field in surface science [3], [4], [5]. Isikawa [6], [7] and Ohta [8], [9] also carried out pioneering works. One of the recent topics in this field is site-specific desorption induced by core-electron excitations of surface molecules containing inequivalent atoms of the same element [10]. Site-specific chemical bond scission by core-electron transitions using synchrotron radiation is a candidate for the control technique of chemical reactions, because a core-electron is highly localized on a specific atom [11]. Actually, such a site-specific bond scission was demonstrated for N2/Ru(001) by Romberg et al. [12]. Since detection of an ion is much easier than that of a neutral, site-specific ion desorption induced by core-electron transitions has been investigated extensively [11].
Site-specific ion desorption has been investigated mainly based on measurements of the desorption yield as a function of the excitation energy so far, and Auger stimulated ion desorption (ASID) mechanism was suggested to be responsible for the site-specificity [10], [11]. The ASID is described as the following three step processes: 1) a core-level ionization (~ 0.1 fs), 2) an Auger transition leaving two valence holes (1–10 fs), and 3) ion desorption due to the electron missing from bonding orbitals and hole–hole Coulomb repulsion (10–100 fs). A particular ASID mechanism was proposed by Knotek and Feibelman for O+ desorption induced by Ti-2p ionization of a TiO2 surface, where the driving force was attributed to the Coulomb repulsion between Ti2 + and O+ created by an inter-atomic shake-off Auger process [13]. Later, the ASID was used to explain ion desorption from covalently bonded systems [14].
After an electron ion coincidence (EICO) analyzer for surface studies was developed [15], [16], photoelectron photoion coincidence (PEPICO) spectroscopy has become an ideal tool for studies of ion desorption induced by photoionization. In EICO spectroscopy a desorbed ion is measured in time coincidence with its associated electron. Thus, an ion desorbed in the same excitation event that emits the specific electron is measured selectively [17]. PEPICO spectroscopy was used to study site-specific ion desorption of various systems, such as condensed F3SiCH2CH2Si(CH3)3 [18], condensed X3Si(CH2)nSi(CH3)3 (X = F or Cl, n = 0–2) [19], condensed CF3CH3 [20], CF3CH2OH chemisorbed on Si(100) [20], CF3CD(OH)CH3 chemisorbed on Si(100) [21], Si(111) fluorinated by XeF2 [22] and H2O dissociatively chemisorbed on oxidized Si(111) [23]. Another key technique for ASID studies is Auger electron photoion coincidence (AEPICO) spectroscopy that clarifies correlation between Auger processes and ion desorption [17]. Using AEPICO technique various ion desorption mechanisms were investigated, such as normal-ASID [17], [24], spectator-ASID [17], [25], [26], ion desorption involving bond extension within the core-hole lifetime [17], [25], [26], [27], ion desorption from three-hole states resulting from multi-electron excitation/decay [23], [28], [29] and ion desorption involving inter-atomic charge transfer [23], [29].
Auger photoelectron coincidence spectroscopy (APECS) [30], [31], [32], [33] is a unique technique to study the correlation between site-specific photoionization and Auger decays. APECS was widely applied for studies such as 1) Auger corresponding to individual peak of multiplet structures [30], [34], [35], 2) Coster-Kronig Auger [30], [34], [35], 3) Auger free from secondary electrons [36], 4) off-site Auger [37], [38], 5) surface-specific Auger [37], [38], [39], 6) photoelectron peak free from core-level lifetime broadening [40], and 7) determination of the average emission depth of individual Auger and photoelectrons [41]. In order to apply APECS to site-specific ion desorption, we have developed a new APECS analyzer [42], [43], [44] that is compatible with our EICO analyzer. Recently, we have developed an electron electron ion coincidence analyzer that can be used for both APECS and EICO spectroscopy [45].
In the present paper we describe a study of site-specific ion desorption of condensed 1-trifluorosilyl-2-trimethylsilylethane-d2 (F3SiCD2CH2Si(CH3)3) using PEPICO, APECS and AEPICO measurements. Site-specific ion desorption of condensed F3SiCH2CH2Si(CH3)3 was investigated using PEPICO spectroscopy previously [18], [19]. The F3Si− site (Si[F]) and the Si(CH3)3 site (Si[Me]) of F3SiCH2CH2Si(CH3)3 were clearly distinguished using photoelectron spectroscopy in the Si-2p region, and prominent site-specific F+ and H+ desorptions were reported to be observed in the Si[F]-2p and Si[Me]-2p ionizations, respectively [18], [19]. Based on the results, F+ and H+ desorptions were concluded to take place in the vicinity of the Si[F] and Si[Me] sites, respectively [18], [19]. An ASID mechanism was suggested to be responsible for the site-specific ion desorption [19]. There was, however, an ambiguity in the previous study, that is, H+ desorption was observed also in the Si[F]-2p ionization [18], [19]. The H+ desorption was suggested to be derived from the CH2 bonded to the Si[F] site [19]. To avoid the ambiguity we have prepared F3SiCD2CH2Si(CH3)3 in the present study, because it contains no H atoms in vicinity of the Si[F] site. Furthermore, the previous study suffered from the poor signal-to-background (S/B) ratio and the low resolution of electron kinetic energy [18], [19]. The aim of the present study is to clarify the details of the site-specific ion desorption induced by the core-level ionizations based on improved and detailed coincidence measurements.
Section snippets
Experiments
The experiments were performed in an ultrahigh vacuum chamber with a mu-metal magnetic shield (base pressure = 1.3 × 10− 8 Pa) connected to the soft X-ray beamline 1C of Photon Factory. The typical photon energy resolution (E/ΔE) was > 2000. A Si(111) wafer was mounted to a cryo-manipulator, which can be cooled to ~ 80 K with liquid nitrogen [46]. The incident angle of synchrotron radiation with p-polarization was 84° from the surface normal.
F3SiCD2CH2Si(CH3)3 was prepared as outlined in Fig. 1.
Computational method and procedures
The computational method and procedure used in the present ab initio molecular-orbital (MO) calculations of the Si-L23VV normal Auger transition probability of isolated F3SiCH2CH2Si(CH3)3 were described in detail in a previous paper [53]. The Gaussian 98 program [54] was used for geometry optimization, without any symmetry constraints, at the HF/cc-pVDZ level. The MOs for the ground and core-hole states were produced within the density functional theory framework by StoBe-deMon code using
Results and discussion
Fig. 2 shows a typical photoelectron spectrum (PES) of condensed F3SiCD2CH2Si(CH3)3 measured with the ASMA of the EEICO apparatus at hν = 230 eV. Si[F]-2p and Si[Me]-2p photoelectron peaks are clearly resolved because three F atoms bonded to Si[F] are strongly electron-accepting, while three methyl groups bonded to Si[Me] are electron-donating [18]. Si-L23VV Auger structures, however, are smeared because of superposing of the Si[F]-L23VV and Si[Me]-L23VV Auger components together with
Conclusions
Based on these PEPICO, APECS and AEPICO results, we propose an ASID mechanism for the observed site-specific F+ desorption: 1) Si[F]-2p ionization, 2) Si[F]-L23VV normal Auger leaving two valence holes in the vicinity of Si-F bonds, and 3) F+ desorption due to the electron loss in bonding orbitals with a σSi-F character and hole–hole Coulomb repulsion. For the observed non-site-specific H+ desorption the following mechanism is thought to be responsible: 1) Si[Me] (Si[F]) 2p ionization, 2)
Acknowledgments
The authors would like to thank Dr. Masaki Mitani (Mie University), Mr. Junya Seo and Mr. Narihiko Fujita (Yokohama National Univ.), and Miss Megumi Hino (Ehime Univ.) for their valuable support. We also express our sincere thanks to the members of the Photon Factory for their valuable help during the course of the experiments. This work was supported by PRESTO (Structure Function and Measurement Analysis) from the Japan Science and Technology Agency (JST), Research for the Future Program
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This paper is dedicated to Dr. Mitsuru Nagasono of RIKEN SPring-8 Center who passed away on August 27, 2012 at the age of 44 years as a result of a bicycle accident.
- 1
Present address: Kyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu 841-0005, Japan.
- 2
Present address: Central Research Laboratory, Hitachi, Ltd., Hatoyama 350-0395, Japan.
- 3
Present address: Department of Chemistry, Faculty of Science and Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan.
- 4
Present address: Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan.