Site-Specific Fragmentation following C:1s Core-Level Photoionization

Feb 1, 2001 - Site-Specific Fragmentation following C:1s Core-Level Photoionization of 1,1,1-Trifluoroethane Condensed on a Au Surface and of a 2,2 ...
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J. Phys. Chem. B 2001, 105, 1554-1561

Site-Specific Fragmentation following C:1s Core-Level Photoionization of 1,1,1-Trifluoroethane Condensed on a Au Surface and of a 2,2,2-Trifluoroethanol Monolayer Chemisorbed on a Si(100) Surface Shin-ichi Nagaoka* Institute for Molecular Science, Okazaki 444-8585, Japan

Shin-ichiro Tanaka Department of Physics, Graduate School of Science, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan

Kazuhiko Mase Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba 305-0801, Japan ReceiVed: August 21, 2000; In Final Form: NoVember 28, 2000

We used photoelectron spectroscopy, the energy-selected-photoelectron photoion coincidence (ESPEPICO) method, the Auger electron photoion coincidence (AEPICO) method, and the ab initio method to study sitespecific phenomena in the C:1s photoionization of 1,1,1-trifluoroethane (CF3CH3, TFEt) condensed on a Au surface. Site-specific excitation and occurrence of different chemical shifts at two carbon sites were evident in the total electron-yield spectrum and the photoelectron spectrum, and site-specific fragmentation was evident in the ESPEPICO spectrum. The fragmentation processes inferred from the ESPEPICO and AEPICO results were very different from those occurring in the vapor phase. We also studied the effect of the surface on the site-specific phenomena observed in a 2,2,2-trifluoroethanol (TFEtOH) monolayer chemisorbed on a Si(100) surface (CF3CH2OSi{substrate}). The molecular structure of TFEtOH is the same as that of TFEt except that it has a hydroxyl group substituted for one of the hydrogen atoms. Although site-specific phenomena were also observed in TFEtOH, the fragmentation process was very different from that of TFEt because of the chemisorption structure of TFEtOH on Si(100).

Introduction In contrast to valence electrons, which are often delocalized over the entire molecule, the core electrons are localized near the atom of origin. Although core electrons do not participate in chemical bonding, the energy of an atomic core level in the molecule depends on the chemical environment of the atom. Monochromatized synchrotron radiation can selectively excite the core electrons of an atom in a specific chemical environment, discriminating them from the core electrons of like atoms in different chemical environments. This site-specific excitation often results in site-specific fragmentation, which occurs around the site where the photoexcitation took place. These site-specific phenomena have been of considerable interest to many researchers.1-36 Site-specific fragmentation is of importance in understanding localization phenomena in chemical reactions. Site-specific fragmentation is also potentially useful for synthesizing materials through selective bond dissociation. Monochromatized synchrotron radiation could be used as an optical knife in this type of synthesis. When selective bond dissociation around an atomic site is required in the synthesis, one would use an optical knife (monochromatized synchrotron * To whom correspondence should be addressed. E-mail: nagaoka@ ims.ac.jp. FAX: +81-564-54-2254. Also Visiting Researcher of Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-8526, Japan.

radiation) that has a photon energy corresponding to the specific excitation of that site. The studies of site-specific fragmentation may offer guiding principles important in designing future molecular electric devices,37,38 the fabrication of which will require the control of very precisely localized reactions.39 Mu¨ller-Dethlefs and co-workers investigated site-specific fragmentation by studying the ionic dissociation processes following the C:1s photoionization of 1,1,1-trifluoroethane (CF3CH3, abbreviated hereafter as TFEt, Figure 1a) in the vapor phase.2,7,10,12 TFEt is the simplest of organic molecules with two carbon sites having different chemical environments: one of the carbon atoms is bonded to three hydrogen atoms (C[H]) and the other is bonded to three fluorine atoms (C[F]). Mu¨llerDethlefs and co-workers showed that site-specific fragmentation of TFEt takes place and that TFEt “memorizes” the site of the initial energy deposition, exhibiting the chemical memory effect.10 The experimental results on the site-specific fragmentation of TFEt were compared with theoretical predictions.11 Our group, on the other hand, studied site-specific phenomena in Si:2p core-level photoionization of X3Si(CH2)nSi(CH3)3 (X ) F or Cl, n ) 0-2) in the vapor phase19,28,29 and of the same molecules condensed on a Au or Si(111) surface.25,27,29,32 F3SiCH2CH2Si(CH3)3 (FSMSE), for example, has a silicon atom bonded to three methyl groups (Si[Me]) and a silicon atom bonded to three fluorine atoms (Si[F]), and we found that this molecule fragments site specifically on the surfaces: H+ and

10.1021/jp002994l CCC: $20.00 © 2001 American Chemical Society Published on Web 02/01/2001

C:1s Photoionization of TFEt and TFEtOH

J. Phys. Chem. B, Vol. 105, No. 8, 2001 1555 TFEtOH is expected to be dissociatively chemisorbed like CF3CH2O-Si(100) (Figure 1b).41,42 The molecular structure of TFEtOH is exactly the same as that of TFEt except that it has a hydroxyl group substituted for one of the hydrogen atoms. This hydroxyl group is necessary for the chemisorption of TFEtOH on Si(100). Experimental Section

Figure 1. (a) Structure of TFEt. (b) Relative orientation of the timeof-flight (TOF) tube and TFEtOH chemisorbed on a Si(100) surface.

F+ ions were selectively desorbed coincidentally with the Si[Me]:2p and Si[F]:2p electrons, respectively.27,32 That is, the ionic fragmentation occurred around the atom where the photoionization had taken place. The energy-selected-photoelectron photoion coincidence (ESPEPICO) method was a very powerful tool in our study of site-specific fragmentation on the surfaces because the ion desorption yield could be measured for a selected electron emission. We found during these studies that, for the study of sitespecific fragmentation, the ESPEPICO experiment on a surface is often advantageous over that in the vapor phase.27,29,32 The advantages are that the coincidence count rate is high, the electric-field gradient applied across the ionization region is low, and the electric field does not smear the energy distribution of the photoelectrons. On the other hand, in contrast to what is seen in the vapor phase, excitations of surfaces by secondary electrons lead to ion desorption that does not show site specificity,40 and such ion desorption blurs the site-specific fragmentation process. The ESPEPICO spectrum, however, shows a peak for only the ion desorption initiated by a selected primary-electron emission, and ionic fragments formed as a result of the excitation by secondary electrons are observed only as a flat signal. Although neutralization by electron transfer from the substrate is efficient and ion desorption from a surface is not extensive,33 it would be worthwhile to compare the ionic site-specific fragmentation on a surface with that in the vapor phase. TFEt is suitable for such comparison because it is a prototypical example of a molecule showing site-specific fragmentation. Furthermore, site-specific fragmentation in a monolayer regime on a surface is interesting, because competition between surface reactions and electronic relaxation makes the fragmentation complex. Accordingly, in the present work, we used the ESPEPICO method to clarify site-specific fragmentation following the C:1s photoionization of TFEt condensed on a Au surface. The sitespecific excitation and the occurrence of different chemical shifts at the two carbon sites were studied using total electron yield (TEY) spectroscopy and photoelectron spectroscopy. The fragmentation processes are discussed here on the basis of the results of ab initio calculations, the ESPEPICO method, and the Auger electron photoion coincidence (AEPICO) method, in which the ion desorption yield can be measured for a selected Auger transition. Since the adsorption of TFEt on surfaces is negligible, we investigated the ion desorption of a 2,2,2trifluoroethanol (CF3CH2OH, abbreviated hereafter as TFEtOH) monolayer chemisorbed on a Si(100) surface in order to clarify the effect of the surface on the site-specific fragmentation.

TFEt and TFEtOH were respectively obtained from PCR and Lancaster and were used without further purification. It is regrettable that the organic synthesis of 1,1-difluoroethanol (CH3CF2OH) is difficult and that this compound is not commercially available as far as we know. As the substrate for condensed TFEt, a thin Au film evaporated on a Cu surface was used without further cleaning. The Si(100), which was the substrate for TFEtOH monolayer, was of p-type. The experiments were performed using a single-pass cylindrical-mirror electron-energy analyzer (CMA) and a time-of-flight (TOF) ion-detection assembly43 coupled to a grasshopper monochromator (Mark XV) installed on the BL2B1 beamline of the UVSOR synchrotron-radiation facility in Okazaki.44 The setup and experimental procedures were described in detail in previous papers.27,32,43,44 The main chamberscontaining a sample with the Au or Si(100) surface, the CMA, and the TOF spectrometerswas evacuated to a pressure below 2 × 10-10 Torr (1 Torr ) 133 Pa). The background pressure in the main chamber during operation was less than 5 × 10-9 Torr. In the experiments with TFEt, the Au surface was cooled to about 50 K by flowing cold helium gas, and the sample was prepared by exposing the surface to TFEt gas at 150 L (1 L ) 1 × 10-6 Torr·s). In the experiments with a TFEtOH monolayer chemisorbed on the Si(100) surface, each day before starting the experiments the Si(100) surface was flash-heated (with an electric current) to about 1000 °C for 3 s to remove contamination and SiO2. After that, by using a sample pulse driver (General Valve Corporation, IOTA ONE), TFEtOH was chemisorbed up to saturation on the Si(100) surface at room temperature. We removed the time-structure background in the ESPEPICO spectrum by using fast-Fourier transformation.32 Computational Method and Procedures The ab initio molecular-orbital calculation for TFEt was performed using the Gaussian 94 program.45 The highest occupied molecular orbital (HOMO) of TFEt and the lowest unoccupied molecular orbital (LUMO) of TFEt2+ were calculated using the HF/6-31G* method46 and the molecular structures determined experimentally.47 The potential energy curve of the ground state of TFEt2+ was calculated using the CID/6311G**//MP2/6-31G* method:46 in the calculation, all bond lengths other than the C-C bond length (rC-C) and all the bond angles were optimized under the constraint of C3V symmetry and staggered conformation. Results and Discussion TFEt. Site-Specific Excitation and the Occurrence of Different Chemical Shifts. Figure 2 shows the TEY spectrum of TFEt condensed on the Au surface. Peak assignments in the figure were inferred by comparison with the electron yield spectrum10 and the inner-shell electron-energy-loss spectrum48 of TFEt vapor. The TEY spectrum shows that site-specific excitation indeed occurs in TFEt on the Au surface: the C:1s f σ* peak of TFEt is a doublet resulting from the C[H]:1s f σ* and C[F]:1s f σ* excitations.

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Figure 2. TEY spectrum of TFEt condensed on the Au surface. Channels were measured in 0.2-eV steps, and each channel was measured for 8 s. Arrow 1s indicates the position of the C:1s ionization threshold.

Figure 4. ESPEPICO spectra of TFEt condensed on the Au surface. The spectra were obtained at a photon energy of 393.4 eV, and the data collection time was 5000 s: (a) C[H]:1s electron emission; (b) C[F]:1s electron emission.

Figure 3. Photoelectron spectrum of TFEt condensed on the Au surface. The spectrum was taken at a photon energy of 393.4 eV. The inset shows the region of C:1s electron emission with an enlarged horizontal scale. Channels in the main spectrum were measured in 1-eV steps, and each channel was measured for 0.1 s. The respective values for the inset are 0.2 eV and 0.5 s. During these measurements, no electric field was applied across the ionization region.

Figure 3 shows the photoelectron spectrum of TFEt condensed on the Au surface. Peak assignments in the figure were inferred by comparison with the X-ray photoelectron and Auger spectra of C and Au.49,50 The photoelectron spectrum has two peaks in the region of C:1s electron emission. The lower energy and higher energy peaks were respectively assigned to C[H]:1s and C[F]:1s electron emissions by comparison with the zero-kineticenergy photoelectron-yield spectrum10 and the X-ray photoelectron spectrum48 of TFEt. The photoelectron spectrum of TFEt on the Au surface thus clearly shows that the chemical shifts (binding energies) at the two carbons are different. Site-Specific Fragmentation and ESPEPICO. The site-specific fragmentation following the C:1s core-level photoionization of TFEt condensed on the Au surface can be seen by using the ESPEPICO technique, and Figure 4 shows the ESPEPICO spectra obtained with emission of the C[H]:1s and C[F]:1s electrons. The intensity of the ESPEPICO signal corresponds to the ion desorption yield measured for a selected electron emission. H+, CH3+, and CF3+ ions are desorbed coincidentally with the C[H]:1s electrons (Figure 4a), and C2Hn+ and CFCHm+ ions are additionally desorbed with the C[F]:1s electrons (Figure 4b). These ESPEPICO spectra are very different from the mass spectrum obtained by electron impact in the vapor phase.51 It should be noted that only the C[F]:1s ionization induces the desorption of C2Hn+ and CFCHm+. H+, CH3+, and CF3+, in contrast, which do not contain a C-C bond, are desorbed with not only the C[F]:1s electrons but also the C[H]:1s electrons. The intensity ratios of the coincidental desorption with the C[H]:1s electron to that with the C[F]:1s electron are 1.1 for H+, 1.2 for CH3+, and 0.8 for CF3+. As shown in the next section, the predominant production of CH3+ following the C[H]:1s photoionization is also seen in the AEPICO result. Although the site specificity for H+, CH3+, and CF3+ in TFEt condensed on the Au substrate is less remarkable than expected

from the results obtained when investigating FSMSE condensed on Au27 (see the Introduction), the predominant fragmentation processes following the C[H]:1s and C[F]:1s photoionizations are as follows:

where n.p. stands for neutral product. When a C:1s core electron is emitted, the core hole thereby created is usually filled by a valence electron through a KVV normal Auger process, which involves the emission of one more electron (reactions 1a and 2a). When a C:1s core electron is initially emitted (one hole) and a normal Auger transition follows it, the molecule is left in a final state with two holes in valence orbitals (TFEt2+ in reactions 1a and 2a). Subsequently, fragmentation and ion desorption begin. Then an ion pair should arise from the Coulomb repulsion between the two holes. When one ion of the pair is released into the vacuum owing to the Coulomb repulsion between the two ions, the other is pushed toward the substrate and is neutralized. As a result, a fragment ion and a neutral product are produced in reactions 1b,c and 2b-d. Which ion of the pair is released into the vacuum and which ion is neutralized? The answer depends on the product of the photoionization cross section in the vapor phase52 multiplied by the ion desorption probability on the surface53,54 (a kind of condensed-phase effect), which is not easily evaluated. The C[H]:1s ionization induces the breaking of the C-H bonds (reaction 1b) and the C-C bond (reaction 1c), and the C[F]:1s ionization induces the breaking of the C-C bond (reaction 2b) and the C-F bonds (reactions 2c,d). Thus, sitespecific fragmentation occurs around the carbon atom where the photoionization has taken place. The KVV normal Auger transition is localized, and energy randomization destroying the

C:1s Photoionization of TFEt and TFEtOH

Figure 5. AEPICO spectra of TFEt condensed on the Au surface. (a) C[H]:KVV normal Auger process. This spectrum was obtained at a photon energy of 292.7 eV for an electron kinetic energy of 260 eV, which corresponds to the high-energy edge of the C[H]:KVV normal Auger peak (Figure 6a). The data collection time here was 1080 s. (b) C[H]:KVVσ* resonant Auger process. This spectrum was obtained at a photon energy of 288.5 eV for an electron kinetic energy of 265 eV, which corresponds to the high-energy edge of the C[H]:KVVσ* resonant Auger peak (Figure 6b). The data collection time here was 1200 s.

memory of the ionization process does not take place extensively before the fragmentation. TFEt moderately memorizes the site of the initial energy deposition, exhibiting the chemical memory effect.10 Although a strong effect from the site of the initial energy deposition is observed for the fragments C2Hn+ and CFCHm+ released from TFEt condensed on the Au surface, the site specificity for H+, CH3+, and CF3+ is, as mentioned above, less remarkable than that previously shown for FSMSE on Au.27 We cannot yet explain the fragmentation process for CF3+ in the C[H]:1s ionization and for H+ and CH3+ in the C[F]:1s ionization, but the reason for the lesser degree of site specificity for H+, CH3+, and CF3+ in TFEt is thought to be that the two carbon sites are close to each other. In fact, we previously showed that site specificity in X3Si(CH2)nSi(CH3)3 (X ) F or Cl, n ) 0-2) decreases with decreasing the distance between the two silicon sites.32 In the Introduction, we mentioned that site-specific fragmentation is potentially useful for synthesizing new materials through selective bond dissociation. The present results, however, as well as those for X3Si(CH2)nSi(CH3)3 (X ) F or Cl, n ) 0-2),32 show that if this process works well, the atomic site of interest must be far from any atomic site at which bond dissociation is undesirable. C[H]:KVV Normal Auger Transition and AEPICO. The C[H]: KVV and C[F]:KVV normal Auger transitions respectively follow the C[H]:1s and C[F]:1s electron emissions, and the ion desorption yields for these transitions of TFEt can be evaluated by using the AEPICO method because the intensity of the AEPICO signal corresponds to the ion desorption yield measured for a selected Auger transition. Figures 5a and 6a respectively show, for TFEt condensed on the Au surface, the AEPICO spectrum and the AEPICO yield spectrum for the C[H]:KVV normal Auger process. These spectra were obtained at a photon energy between the C[H]:1s ionization threshold (≈290 eV) and the C[F]:1s f σ* resonance (Figure 2). At this energy, only the C[H]:1s ionization and the subsequent C[H]:KVV normal Auger transition take place, and neither the C[F]:1s ionization and the subsequent C[F]:KVV normal Auger transition nor the C[H,F]:1s f σ* excitation and

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Figure 6. AEPICO yield spectra and photoemission spectra of TFEt condensed on the Au surface. These spectra were plotted as a function of electron kinetic energy in the region of the carbon Auger electron emission. During these measurements, an electric field was applied across the ionization region. Channels in the photoemission spectrum were measured in 0.5-eV steps, and each channel was measured for 0.5 s. (a) C[H]:KVV normal Auger process. These spectra were obtained at a photon energy of 292.7 eV. The data collection time for each data point in an AEPICO yield spectrum was 480 s. The peak around 279 eV in the photoemission spectrum is due to the valence electron. (b) C[H]:KVVσ* resonant Auger process. These spectra were obtained at a photon energy of 288.5 eV. The data collection time for each data point in an AEPICO yield spectrum was 1200 s. The peak around 275 eV in the photoemission spectrum is due to the valence electron.

the subsequent resonant Auger transition (C[H,F]:KVVσ* transition) take place. CH3+ ions are extensively desorbed coincidentally with the C[H]:KVV normal Auger electrons (Figure 5a), and the coincidence count rate is much larger than those obtained in the other coincidence measurements in this work. This result is consistent with reaction 1c given in the preceding section. The AEPICO yield of CH3+ shows a peak at the high-energy edge of the Auger peak (Figure 6a). In the ESPEPICO result, H+ ions were extensively desorbed coincidentally with the C[H]:1s electrons (reaction 1b), but the coincidence count of H+ ion is negligible in Figures 5a and 6a. The AEPICO yield spectrum of H+ seems to be very broad, and the peak of H+ in the AEPICO spectrum (Figure 5a) seems to be buried under the background noise, that is, under the accidental coincidences55,56 coming from CH3+. The C[H]:KVV normal Auger process takes place at every photon energy above the C[H]:1s ionization threshold, and CH3+ ions are extensively desorbed coincidentally with the C[H]:KVV normal Auger electrons. The C[H]:KVV normal Auger spectrum is superimposed on the C[F]:KVV normal Auger spectrum obtained at a photon energy above the C[F]:1s ionization threshold (≈295 eV), and one cannot resolve the two spectra. Accordingly, although remarkable coincidental desorption of CH3+ with these Auger electrons is induced by excitation at a photon energy above the C[F]:1s threshold, this ion is thought to be desorbed as a result of the C[H]:KVV normal Auger process. At this photon energy, the peaks due to the other ions are unfortunately buried under the background noise, that is, under the accidental coincidences55,56 coming from CH3+. Thus, the fragmentation products following the C[F]:KVV normal Auger process could not be clearly revealed in this experiment. Next we focus our attention on our finding that the AEPICO yield of CH3+ shows a peak at the high-energy edge of the C[H]:KVV normal Auger peak. The energy difference between the HOMO and the C[H]:1s orbital is larger than the energy

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Figure 8. Potential energy curve of the ground state of TFEt2+ and plots of rC-F and rC-H against rC-C.

Figure 7. Energy level diagram and schematic C[H]:KVV normal Auger spectrum of TFEt. The solid arrows show the HOMO f C[H]: 1s hole transition and the subsequent Auger electron emission from the HOMO. The broken arrows correspond to the case that an occupied orbital other than the HOMO participates in the Auger process.

differences between the other occupied orbitals and the C[H]: 1s orbital. Accordingly, in normal Auger processes following the C[H]:1s electron emission, the HOMO f C[H]:1s hole transition gives the largest amount of kinetic energy to the normal Auger electron (Figure 7). Furthermore, the Fermi level of the substrate defines the zero of the Auger electron kinetic energy,57 and the energy difference between the Fermi level and the HOMO is less than the energy differences between the Fermi level and the other occupied orbitals. Accordingly, in normal Auger processes following a valence f C[H]:1s hole transition, the normal Auger electron emitted from the HOMO has the largest amount of kinetic energy (Figure 7). We therefore think that the normal Auger electron at the high-energy edge of the normal Auger peak is emitted from the HOMO after the HOMO f C[H]:1s hole transition (solid arrows in Figure 7) and the final state of this normal Auger process has two holes in the HOMO. In the result of the ab initio calculation, the HOMO of TFEt has a character of C-C bonding. Accordingly, the final state of the normal Auger process at the high-energy edge of the normal Auger peak has two holes in the C-C bonding orbital. In the result of the ab initio calculation, the HOMO of TFEt (C-C bonding) corresponds to the LUMO in the ground state of TFEt2+. That is, TFEt with two holes in the HOMO (C-C bonding) corresponds to TFEt2+ whose LUMO is the C-C bonding orbital. Since the AEPICO yield of CH3+ shows a peak at the high-energy edge of the Auger peak in TFEt (Figure 6a), the CH3+ ion is thought to be formed from TFEt2+ whose LUMO is the C-C bonding orbital. To study the dynamics of the TFEt2+ produced by the normal Auger process at the high-energy edge, we calculated the potential energy curve of the TFEt2+ (Figure 8). Since the potential energy decreases as rC-C increases, the dissociation of the C-C bond is likely to take place extensively in the TFEt2+. This is reasonable because the TFEt2+ has no electrons in the C-C bonding orbital. When rC-C increases, one of the two holes is localized in the CH3 moiety, the other is localized

in CF3, and CH3+ and CF3+ are likely to be produced. Since the amount of CF3+ produced is negligible and only CH3+ is observed in the AEPICO spectrum (Figures 5a and 6a), we think that the CH3+ in the ion pair CH3+-CF3+ is released into the vacuum owing to the Coulomb repulsion between CH3+ and CF3+ when the molecule is oriented with the CH3 part protruding outside. The CF3+, on the other hand, is then pushed toward the Au substrate and is neutralized. Plots of the C-F and C-H bond lengths (rC-F and rC-H, respectively) against rC-C are also shown in Figure 8. rC-F and rC-H do not increase when rC-C increases, so the dissociation of the C-F and C-H bonds is unlikely to be extensive. From the results so far presented, the fragmentation process in question can be described in the following way:

C[H]:KVVσ* Resonant Auger Transition and AEPICO. When monochromatized synchrotron radiation excites a C:1s core electron to a vacant orbital σ*, the core hole thereby created is usually filled by a valence electron through a KVVσ* resonant Auger process that involves the emission of one electron.58 Then, as shown in Figure 6, in the photoemission spectrum, the kinetic energy of the peak of the resonant Auger electron is greater than that of the normal Auger electron.58 The C[H]:KVVσ* and C[F]:KVVσ* resonant Auger processes respectively take place following the C[H]:1s f σ* and C[F]:1s f σ* resonant excitations. At the photon energy of the C[F]:1s f σ* excitation, not only the C[F]:KVVσ* resonant Auger transition but also the C[H]:KVV normal Auger transition takes place. Accordingly, as in the case of the C[F]:KVV normal Auger processes, the fragmentation products in the C[F]:KVVσ* resonant Auger process could not be clearly revealed in the AEPICO experiment. Next we focus our attention on the C[H]:KVVσ* resonant Auger transition. Figures 5b and 6b respectively show the AEPICO spectrum and the AEPICO yield spectrum for the C[H]:KVVσ* resonant Auger process in TFEt condensed on the Au surface. These spectra were obtained at a photon energy for the C[H]:1s f σ* resonance (Figure 2). As in the case of the C[H]:KVV normal Auger process, CH3+ ions are desorbed

C:1s Photoionization of TFEt and TFEtOH

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TABLE 1: Site-Specific Fragmentation Products of TFEt condensed TFEt (this work) C[H]:1s C[F]:1s a

+

+

H , CH3 C2Hn+,a CFCHm+,a CF3+

TFEt vapor10 +,

CFH2 CF2CH2+,a CF3+ C+, CF+

These fragments have a C-C bond.

coincidentally with the C[H]:KVVσ* resonant Auger electrons. Since H+ ions are additionally desorbed in the C[H]:KVVσ* resonant Auger transition, the σ* orbital is likely to have a character of C-H antibonding. The AEPICO yield spectra for H+ and CH3+ show peaks at the high-energy edge of the Auger peak and between 245 and 250 eV. The excited electron in the σ* orbital sometimes participates in the Auger transition (participant decay), and sometimes does not (spectator decay). At the high-energy edge of the resonant Auger peak, the spectator decay predominates as noted previously,59,60 and the final state of the Auger process has two holes in HOMO as described in the preceding section. Since the AEPICO yield spectra for H+ and CH3+ show peaks at the highenergy edge of the Auger peak (Figure 6b), its fragmentation process following the C[H]:1s f σ* excitation and the subsequent C[H]:KVVσ* spectator Auger transition can be described in the following way:

Comparison with the Results Obtained in the Vapor Phase. As mentioned in a previous section, the site-specific excitation and the occurrence of different chemical shifts (different binding energies) of TFEt on the surface that were obtained in the present work are consistent with those obtained in the vapor phase.10 However, the site-specific fragmentation products of TFEt on the surface and in the vapor phase are very different from each other (Table 1). In the vapor phase, the fragment ions CFH2+, CF2CH2+, CF3+, C+, and CF+ were the ones most sensitive to the site of the initial energy deposition:10 The C[H]:1s ionization led to much higher counts of CFH2+ and CF2CH2+ than did the C[F]:1s ionization. The C[H]:1s ionization or excitation enhanced the production of CF3+, whereas the C[F]:1s ionization or excitation enhanced production of C+ and CF+. The C-C bond of TFEt vapor was more easily broken by C[F]:1s ionization or excitation than by C[H]:1s ionization or excitation. The fragmentation products of TFEt condensed on Au, in contrast, show that those most sensitive to the site of the initial energy deposition are C2Hn+ and CFCHm+ (Figure 4): of the fragments whose signals are evident in the spectra in Figures 4 and 5, only C2Hn+ and CFCHm+ have a C-C bond, and those ions are desorbed only after the C[F]:1s ionization. The C-C bond in TFEt on the surface is thus more easily broken by C[H]:1s ionization than by C[F]:1s ionization. In contrast to what is observed in the vapor phase, CF3+ is the predominant product of desorption induced by C[F]:1s ionization on the surface (reaction 2b). We do not know why the site-specific fragmentation of TFEt on the surface differs from that of TFEt in the vapor phase, but we can suggest a possible explanation. Tinone et al.15 previously noted that the reneutralization path is less probable in the vapor phase, all the ions produced are collected by ion detection apparatus, and the resulting spectrum is an average of all

Figure 9. Photoelectron spectrum in the region of the C:1s electron emission of the TFEtOH monolayer chemisorbed on the Si(100) surface. The spectrum was obtained at a photon energy of 407.1 eV. Channels were measured in 0.2-eV steps, and each channel was measured for 1 s. During these measurements, an electric field was applied across the ionization region.

Figure 10. ESPEPICO spectra of the TFEtOH monolayer chemisorbed on the Si(100) surface. The spectra were obtained at a photon energy of 407.1 eV, and the data collection time was 6000 s: (a) C[H]:1s electron emission; (b) C[F]:1s electron emission.

decomposition paths, but that in excitation on a solid surface, the ions produced through the fast and energetic path are detected selectively. The above-mentioned difference in fragmentation process between TFEt vapor and condensed TFEt may also originate from the difference suggested by Tinone et al. Further investigations on the dynamics following the corelevel ionization of TFEt are clearly needed. TFEtOH. Site-Specific Excitation and the Occurrence of Different Chemical Shifts. Figure 9 shows the photoelectron spectrum of the TFEtOH monolayer chemisorbed on the Si(100) surface. Like the photoelectron spectrum of TFEt condensed on Au, it has two peaks in the region of C:1s electron emission. The lower energy and higher energy peaks are respectively assigned to C[H]:1s and C[F]:1s electron emissions, so we conclude that the chemical shifts (binding energies) at the two carbon sites are different. As noted in the next section, sitespecific excitation also occurs in TFEtOH chemisorbed on Si(100). Effect of Surface on Fragmentation Process. Figure 10 shows the ESPEPICO spectra obtained with emission of the C[H]:1s and C[F]:1s electrons. F+ ions are desorbed coincidentally with the C[F]:1s electrons, but coincidental ion desorption is negligible with the C[H]:1s electrons. These ESPEPICO spectra are very different from the mass spectrum obtained by electron impact in the vapor phase.61 In the TFEtOH monolayer chemisorbed on the Si(100) surface, the fragmentation processes following the C[H]:1s and C[F]:1s photoionizations are as follows:

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Figure 11. Carbon Auger electron yield spectrum and TIY spectrum of the TFEtOH monolayer chemisorbed on the Si(100) surface. Channels were measured in 0.2-eV steps, and each channel was measured for 5 s. These spectra were normalized by the total electron intensity. During these measurements, an electric field was applied across the ionization region. (a) Carbon Auger electron yield spectra obtained for an electron kinetic energy of 260 eV. (b) TIY spectrum.

Thus, the fragmentation of TFEtOH is very different from that of TFEt. In TFEtOH chemisorbed on Si(100), coincidental ion desorption with the C[H]:1s electron is negligible (reaction 5). The ion desorption efficiency following the C[H]:1s ionization or excitation is very small, which can also be seen in the total ion yield (TIY) spectrum as shown below. Figure 11 shows the carbon Auger electron yield spectrum and the TIY spectrum. Like the TEY spectrum of TFEt condensed on Au (Figure 2), the Auger electron yield spectrum shows that site-specific excitation occurs in the TFEtOH monolayer chemisorbed on the Si(100) surface: the C:1s f σ* peak of TFEtOH is a doublet resulting from the C[H]:1s f σ* and C[F]:1s f σ* excitations. The TIY spectrum, however, shows a peak only at the energy of the C[F]:1s f σ* excitation, and the TIY at the C[H]:1s f σ* excitation energy is very small. The AEPICO result shows that F+ ions are produced following the C[F]:1s f σ* excitation (data not shown), as in reaction 6. As noted above, the ion desorption efficiency following the C[H]:1s ionization or excitation is very small in TFEtOH chemisorbed on Si(100), and the C[F]:1s ionization or excitation produces F+ ions. Thus, although ion desorption from the C[H] site is efficient in TFEt, it is not efficient in TFEtOH. As shown in FSMSE27 and as described in the case of TFEt in a previous section, the site-specific ion desorption occurs predominantly from the site where the core ionization or excitation has taken place. Accordingly, C[F]:1s ionization or excitation might be expected to elicit efficient ion desorption from C[F] and inefficient desorption from C[H]. Why, then, is ion desorption from the C[H] site of TFEtOH not seen after C[H]:1s ionization or excitation, as it is in TFEt? This can be explained by the chemisorption structure of TFEtOH on Si(100), which is shown

Nagaoka et al. schematically in Figure 1b. The bulky CF3 group shields the C[H] site from the TOF tube like an umbrella. Although the photoelectron and the Auger electron can escape from the C[H] site,57 the bulky CF3 group keeps the coincidentally desorbed ions from reaching the TOF tube. The ions released from the C[H] site are likely to be pushed back toward the substrate and neutralized. The fragment ions desorbed in the C[F]:1s ionization of condensed TFEt (C2Hn+ {m/e ∼ 26}, CFCHm+ {m/e ∼ 45}, and CF3+ {m/e ) 69}, reactions 2b-d) are different from that in the C[F]:1s ionization of the TFEtOH monolayer (F+ {m/e ) 19}, reaction 6). We cannot explain why they are different, but the reason may be due to the electronic effect of the -OSi(substrate) moiety. In fact, Sekiguchi and co-workers36,62,63 found that ion desorption yields of massive fragments (m/e g 25) following core ionization or excitation are negligible in monolayers even though in condensed molecules they are similar to or much greater than those of light fragments (m/e e 19): this difference was explained in terms of the electronic effect of the substrate in monolayers.36,62,63 Conclusion We used photoelectron spectroscopy, the ESPEPICO method, and the AEPICO method to study the site-specific phenomena in the C:1s photoionization of TFEt condensed on the Au surface and of the TFEtOH monolayer chemisorbed on the Si(100) surface. The electron-yield spectra and the photoelectron spectra of these molecules showed that site-specific excitation occurs and that the chemical shifts at the C[H] and C[F] sites are different. The site-specific fragmentation was also evident in the ESPEPICO spectra. The fragmentation processes of TFEt were evaluated on the basis of the results of ESPEPICO, AEPICO, and ab initio calculations and were found to differ between TFEt condensed on the Au surface and TFEt in the vapor phase. Because of the chemisorption structure of TFEtOH on Si(100), only F+ ions were desorbed in the C[F]:1s photoionization and ion desorption from the C[H] site was negligible. The fragmentation process of TFEtOH was very different from that of TFEt. Site-specific phenomena in the monolayer regime are interesting, but the TFEtOH monolayer chemisorbed on Si(100) is not suitable for studying them because its chemisorption structure inhibits ion desorption from the C[H] site. Accordingly, we are investigating the site-specific fragmentation of 1,1,1trifluoro-2-propanol,35 which is expected to be dissociatively chemisorbed on Si(100) like CF3CH(OSi{substrate})CH3. In this chemisorption, neither the C[H] site nor the C[F] site is shielded by other functional groups. Acknowledgment. We express our sincere thanks to the members of the UVSOR facility for their valuable help during the course of the experiments. S.N. also thanks Professor Inosuke Koyano of the Himeji Institute of Technology for his continuous encouragement. This work was supported by a Grantin-Aid for Scientific Research on the Priority Area “Manipulation of Atoms and Molecules by Electronic Excitation” (11222206) from the Japanese Ministry of Education, Science, Sports and Culture. We thank the Computer Center of the Institute for Molecular Science for the use of their Fujitsu VPP5000, SGI 2800, NEC SX-5, HPC, and IBM SP2 computers and the Gaussian 94 Library Program. References and Notes (1) Eberhardt, W.; Sham, T. K.; Carr, R.; Krummacher, S.; Strongin, M.; Weng, S. L.; Wesner, D. Phys. ReV. Lett. 1983, 50, 1038.

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