Site-specific dissociation of (CH3)3SiI involving I(4d), Si(2p), C(1s) and I(3d) inner-shell excitations in the range of 88–1000 eV

Journal of Electron Spectroscopy and Related Phenomena 128 (2003) 119–128 www.elsevier.com / locate / elspec

Site-specific dissociation of (CH 3 ) 3 SiI involving I(4d), Si(2p), C(1s) and I(3d) inner-shell excitations in the range of 88–1000 eV a, b Bong Hyun Boo *, Norio Saito a

Department of Chemistry, Chungnam National University, Daejeon 305 -764, South Korea b Electrotechnical Laboratory, 1 -1 -4 Umezono, Tsukuba-shi, Ibaraki 305, Japan

Received 18 February 2002; received in revised form 4 July 2002; accepted 17 July 2002

Abstract Dissociative multiple photoionization of iodotrimethylsilane [(CH 3 ) 3 SiI] has been investigated in the I(4d), Si(2p), I(4p), I(4s), C(1s), I(3p), and I(3d) core level excitation / ionization regions in the range from 88 to 1000 eV by time-of-flight (TOF) mass spectrometry and the photoion–photoion coincidence (PIPICO) method coupled to synchrotron radiation. The inner-shell spectra below 160 eV are dominated by a powerful giant resonance which has been identified as a resonance owing to the diffusion of the outgoing photoelectrons by the centrifugal barrier in the I(4d 9 ef) outgoing channel. In addition to the huge resonance, discrete photoabsorption bands around the Si(2p) (106.77 eV) and the C(1s) (289.95 eV) edges are observed in both the total photoion and PIPICO yield spectra. Site-selective dissociation patterns were elucidated by the coincident measurements of the fragment ions formed via a single photon absorption. At low energies below the Si(2p) threshold region (88–107 eV), dissociation pathways leading to the formation of H 1 –I 21 and H 1 –I 1 are dominant. Above 1 1 1 the energy, however, near total destruction giving rise to CH 1 ( p50–3), H 1 –SiH p1 ( p50–3), 3 –SiH , H –CH p 1 1 1 1 SiCH n –I , and H –SiCH p ( p50–5) are dominant.  2002 Elsevier Science B.V. All rights reserved. Keywords: Synchrotron radiation; Multiple photoionization; Si(2p) inner-shell excitation; I(4d) giant resonance; PIPICO

1. Introduction Inner-shell electronic excitation is found to be an attractive tool for the elucidation of atomic and electronic structures of molecules owing to the localization property of the core holes [1]. Therefore, elucidation concerning charge and energy redistribution following the core excitation / ionization pro*Corresponding author. Tel.: 182-42-821-6551; fax: 182-42823-1360. E-mail address: [email protected] (B.H. Boo).

vides a pivotal information on the energy dissipation and the dissociation processes of the core-excited molecules. Study of the fragmentation pathways in dissociative multiple ionization of molecule involves a variety of coincidence methods such as photoelectron–photoion (PEPICO), photoion–photoion (PIPICO), and photoelectron–photoion–photoion (PEPIPICO, PE2PICO) coincidence [2]. Among the coincidence methods, the triple coincidence PEPIPICO method was proven to be a powerful tool to elucidate the dynamics in dissociative double ionization processes [3–5]. Recent addition of

0368-2048 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 02 )00184-6

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position-sensitive ion detection technology to the existing PEPIPICO spectroscopic tool makes it possible to investigate the detailed dynamics in the dissociation of doubly charged ion [4,6]. Until now, however, the PEPIPICO method is almost completely confined to the investigation of the three-body dissociation dynamics of relatively small dications. Additionally, Auger electron–ion coincidence studies could reveal the individual decay channels and intermediate states for the various doubly charged ionic configurations populated in the Auger decay of the core-hole states [7,8]. These studies also enable us to correlate the ionic dissociation products, their kinetic energies, and their electronic configuration with the hole configuration in the doubly charged molecule ion [7]. Furthermore, it is also possible to elucidate the effect of the core-hole decay on the site-selectivity in the bond fragmentation. However, these studies are also restricted to the elucidation of dissociation of small molecules following core photoexcitation / photoionization. Atomic and molecular iodine has been the subject of photoabsorption [9] and photoionization [10,11] and electron impact ionization studies [12]. The theoretical investigation of the photionization of the iodine atom and its ion shows that the photoionization cross sections, in the energy range above the 4d threshold are dominated by powerful giant resonances, which are similar to each other in shape and strength, but showing some shift in the energy dependence of the photoionization cross section for atomic iodine and its several multiply ions [11]. For a probe of competition between electronic relaxation and photochemical degradation of core-hole state, we have chosen (CH 3 ) 3 SiI molecule because intramolecular energy and charge transfer should follow excitation of the Si(2p) and the various iodine inner-shells, and thus a variety of dissociation patterns could be observed upon a specific core excitation / ionization. In this report, we presents the results of siteselective dissociation for (CH 3 ) 3 SiI involving the I(4d), Si(2p), I(4p), I(4s), C(1s), I(3d), and I(3p) inner-shells in the range of 88–1000 eV. Various dissociation patterns of the multiply charged parent ions are proposed on the basis of the measurements of the ion TOF differences in the PIPICO mode. Comparison of the dissociation patterns was made in

the I(4d), Si(2p), C(1s), and I(3d) core excitation regions.

2. Experimental Monochromatic vacuum ultraviolet and soft Xrays were obtained by dispersing synchrotron radiation from the TERAS storage ring at the Electrotechnical Laboratory using a Grasshopper monochromator. The principle and construction of the whole apparatus have been described in detail elsewhere [13,14], and thus only briefly described here. The monochromatized photons passed through two differential pumping stages and entered an ionization chamber equipped with a TOF spectrometer. Here, an electric field of 142.9 V/ cm was applied to the ionization chamber for ion and electron extraction. The main chamber which was equipped with the ionization cell, a photoelectron detector, and the TOF spectrometer, was evacuated down to ¯ 4310 27 Torr. When the gas molecules were introduced into the ionization cell, the pressure of the main chamber was maintained at ¯4310 25 Torr. The flight path length was 13 cm, and the ion detection angle was perpendicular to the direction of incident photon beam, and 558 to the photon beam polarization. The uncertainty in the photon beam energy lies within 60.1 eV. The total ion yield spectrum was obtained by recording the count rates of the total ions and the photon flux simultaneously while the photon wavelength was being scanned. For characterization of individual ions and ion pairs, the time-correlated ion counting technique was used in the PEPICO and PIPICO modes, respectively. The PEPICO mode utilizes a time-to-amplitude converter (TAC) for which the start pulses are provided by the photoelectron arrival signals at a microchannel plate (MCP), and the stop pulses are generated by the photoion detection signals at another MCP located at the end of the TOF tube. Note that we have sampled the photoelectrons for the start pulses flying in a direction opposite to the ion flight direction from the collision chamber. However, the PIPICO mode uses the start pulses provided by the lighter ions, and the stop pulses generated by the heavier ions of the counterparts. In the similar fashion, the total PIPICO

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yield spectrum was obtained by measuring the count rates of total PIPICO simultaneously while the photon wavelength was being scanned. For the measurement of the PEPICO count rates, the coincidence time range (the gate width in TAC) was set to be 0–10 ms. The PIPICO experiment also utilized the coincidence time range of 0–10 ms, because the TOF difference between any pair of ions formed from (CH 3 ) 3 SiI is found to fall within this time range. The individual ion and PIPICO yields at a given photon energy were directly obtained by measuring the integrated individual PEPICO and PIPICO intensities, respectively, and by simultaneously measuring the photon flux while the photon wavelength was being scanned. The corrected photon intensities were obtained by dividing the measured photoemission yields from a gold mesh photocathode placed between the exit slit of the monochromator and the experimental main chamber, by the published photoemission efficiency of a gold photocathode [15]. Uncertainties in the normalized ion and PIPICO yields may mainly come from this normalization processes and could be estimated to be less than ¯10%. The sample (CH 3 ) 3 SiI with nominal purity of 971% was purchased from Aldrich Chemical Co., and used without purification.

3. Results and discussion We used a moderately low extraction field (142.9 V/ cm). The use of the low field enabled us to better evaluate the peak broadening owing to kinetic energy release in the dissociation event, and also allowed us to better detect the PIPICO channels such as 1 CH 1 n (n50–3)–CH 3 which have the similar masses. Note that the yield of the PIPICO channel which has very small TOF difference cannot be evaluated accurately owing to some interference effect in the pulse-processing. For these matters, it is desirable to use a low extraction field to lengthen the TOF difference. However, a use of too low field can decrease the detection sensitivity owing to a fast departing flight via a strong repulsion between ionic fragments. The mass discrimination effect of this kind gives rise to a problem of a quantitative analysis for the PIPICO yield.

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Fig. 1. Total photoion yield of (CH 3 ) 3 SiI recorded as a function of energy in the range of 88–1000 eV. For comparison, the photoionization efficiency of I 2 [16] is incorporated in the figure.

3.1. Core excitation spectra Fig. 1 presents the total photoion yield of (CH 3 ) 3 SiI vs. the photon energy in the range of 88–1000 eV. To elucidate the atomic source for the major contribution to the photoabsorption behavior related to the specific core excitation, we incorporated in the figure the total ion yield curve for I 2 obtained quite recently in our laboratory [16]. The thresholds of the various core electrons for (CH 3 ) 3 SiI molecule: Si(2p 3 / 2 )5106.7760.05 eV [17,18], C(1s)5289.9560.05 eV [17,18], and I(3d 5 / 2 )5625.9060.05 eV [17,18]; those for atomic iodine are: I(4d 5 / 2 )549.660.3 eV [19], I(4p)5 122.760.5 eV [19], I(4s)5186.460.3 eV [19], I(3d 5 / 2 )5619.460.3 eV, I(3p 3 / 2 )5874.660.3 eV [19], and I(3p 1 / 2 )5930.560.3 eV [19]. The I(4d 5 / 2 ) IE in (CH 3 ) 3 SiI was estimated as 56.1 eV, the value derived by adding the chemical shift of 6.5 eV, the value is the difference between I(3d 5 / 2 ) IE in (CH 3 ) 3 SiI and that in atomic iodine, to the I(4d 5 / 2 ) IE549.6 eV in atomic iodine. In the similar fashion, the IE values for the core electrons of (CH 3 ) 3 SiI were estimated as: I(4p)5129.2 eV, I(4s)5192.9 eV, I(3p 3 / 2 )5881.1 eV, and I(3p 1 / 2 )5937.0 eV. Note that some of the literature values and the estimated ones for the thresholds of the various core electrons are marked with hatched lines over the photoionization yield curve.

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The shapes of the energy profiles for the total ion yield curves are similar to each other, indicating that the iodine core excitation could play a decisive role in the photoionization cross section. Here, we have observed significant features, peaks A and B at 105.4 and 122.6 eV, respectively, over a structureless giant background resonance spanning 88–160 eV, and a discrete feature, peak C at 293.4 over a huge peak ranging from ¯160 to ¯620 eV. Previous studies show that the excitation of Si(2p) core electron to the first antibonding orbital occurs when the incident photon energy reaches ¯103 eV, the energy being more or less shifted with different ligands. The chemical shifts of photoelectron and Auger lines are linearly dependent on the average differences in Pauling’s electronegativities to the nearest neighbors, D] xP [20]. The binding energy of Si(2p 3 / 2 ) electron of the molecule has been reported as 106.7760.05 eV, the value determined by photoelectron spectroscopy [17,18]. Hence, the transition below 106.77 eV (peak A) is attributed to the excitation of the Si(2p 3 / 2 ) core electron to the unoccupied antibonding and Rydberg orbitals. The peak at 122.6 eV (peak B) may correspond to the shape resonance above the Si(2p) threshold coupled to the Rydberg and continuum. The resonance arises from the excitation of the Si(2p) to a quasi-bound potential well which is created by a backscattering of the excited electrons by the methyl and iodine ligands. The huge resonance below 160 eV has been identified as a resonance owing to the diffusion of photoelectron in the centrifugal barrier in the 4d 9 ef outgoing channel [21]. This is responsible for the delayed onset and for the enhancement of the photoionization cross section. The initial core MO is almost pure 4d orbital which overlaps with continuous MO mainly in the region of the atomic core. The resultant orbitals are described by the ef atomic orbitals [9], the excitation to which is responsible for the enhancement of the photoionization cross section. Therefore, the repulsive force is increasingly important with increasing the angular momentum quantum number , [21,22]. The giant resonance in the 4d inner-shell region in Xe is interpreted well in terms of a broad collective resonance in the 4d 10 shell in Xe [23], which is associated with dipolar and oscillatory motion of the entire 4d 10 inner-shell. This collective phenomenon

can be thought of in terms of inhomogeneous electron gas confined to a volume of atomic dimensions. The coupling gives rise to a collective interaction of the electrons with the incident radiation. It has been shown that the agreement between theory and experiment turns out to be excellent if a strong coupling among all the 4d electrons is taken into account [23]. Numerous studies show that the huge feature in the I(4d) threshold region is not peculiarity of the iodine atom and molecule, but a characteristic behavior of the photoabsorption involving 4d innershell electron, more generally inner electrons with large orbital angular momentum quantum number such as , $ 2 [21]. The other huge peak spanning 160–620 eV is clearly observed, however, in the same energy region, there is no clear indication for sizeable contribution from iodine core such as I(4p) and I(4s) to the photoionization of I 2 . Therefore, it seems evident that the huge peak in the (CH 3 ) 3 SiI system in the energy range of 160–620 eV arises from the shake-off process above the Si(2p) threshold rather than the iodine I(4p) and I(4s) core excitation. It is presumed that the nonresonant shake-off process brings about the double ejection of the Si(2p) fast electron and another electron as its counterpart. This simultaneous ejection of two or more electrons from a molecule may end up with fast relocation of energy and charge on the whole molecule, thereby yielding a variety of decomposition products with relatively high yields. In this shake-off process, site-selectivity in the fragmentation is not specifically observed. The peak at 293.4 eV (peak C in Fig. 1) may correspond to the excitation of the C(1s) to the unoccupied and Rydberg orbitals because the peak C appears near the C(1s) threshold of 289.95 eV. There is another significant feature in the photoionization efficiency curve for I 2 above the I(3d 5 / 2 ) edge. In this energy range, however, there seems no significant contribution from the core excitation / ionization corresponding to the I(3d 5 / 2 ), to the photoionization of (CH 3 ) 3 SiI. This might happen because of the competition between the excitation efficiency of the iodine core and the favored excitation efficiencies for the other cores. Fig. 2 presents the total PIPICO yield curve for comparison with the total ion yield curve. The shape of the total PIPICO yield spectrum looks similar to

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Fig. 2. Total photoion–photoion yield of (CH 3 ) 3 SiI as a function of energy in the range of 88–1000 eV.

that of the total ion yield spectrum. Here, the two features at ¯107.8 eV (peak A) and ¯125.0 eV (peak B) also appear, indicating that the two resonances occur leading to the formation of ion-pairs. Fig. 3 shows TOF mass spectra taken in the PEPICO mode at 88, 110, 130, 160, 200, 300, 640, and 890 eV, where the core orbitals to be excited or ionized are marked with hatched lines below the photon 1 energies. Various monocations, H 1 n (n 5 1–2), CH n 1 1 (n 5 0–3), C 2 H n (n 5 0–3), SiH n (n 5 0–3), SiCH n1 (n 5 0–5), and SiC 2 H n1 (n 5 0, 7), I 21 , SiC 3 H 91 , and I 1 are clearly observed. The variation of the PIPICO intensity with energy is shown in Fig. 4.

3.2. Dissociation behaviors Fig. 5 presents PIPICO spectra taken by excitation at the various photon energies of 88, 110, 130, 160, 200, 300, 640, and 890 eV as used for the mass spectra in Fig. 3. Note that the PIPICO spectra were taken under the same condition for the spectra shown in Fig. 3. At least 12 classes for the ion pair formation crop up in the PIPICO spectra (Fig. 5). The PIPICO channels are listed in the order of the 1 increasing TOF difference: CH 1 n –CH p (n, p 5 0–3), 1 1 1 CH n –SiH p (n, p 5 0–3), CH 3 –SiCH 31 , H 1 –CH p1 ( p 5 0–3), CH n1 –SiC 2 H p1 (n 5 0–3, p 5 0–7), H 1 – SiH p1 ( p 5 0–3), SiCH n1 (n 5 0–3)–I 1 , H 1 –SiCH 1 p ( p 5 0–5), H 1 –SiC 2 H p1 ( p 5 0–3), H 1 –I 21 , CH 31 –

Fig. 3. TOF mass spectra of (CH 3 ) 3 SiI taken by excitation at various photon energies of 88, 110, 130, 160, 200, 300, 640, and 890 eV. The core orbitals related to the photon energies are marked in the individual spectra.

I 1 , and H 1 –I 1 . Also note that the PIPICO channels for the C 2 H n1 (n 5 0–3) ion and its ionic counterpart are included in those for the SiH 1 n ion and its ionic partner. The most efficient process at 88 eV, the lowest energy examined here, is the ion pair formation of H 1 –I 1 . Fig. 6 shows the variation of the PIPICO yield with energy. At low energies, the most efficient processes are found to be reactions (1) and (2): (CH 3 ) 3 SiI 1 hn → (CH 3 ) 3 SiI 21 1 2e → H 1 1 I 1 1 NP 1 2e

(1)

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Fig. 4. Partial ion yield spectra of (CH 3 ) 3 SiI in the range of 88–1000 eV. The spectral intensities (Iphotoion /Itot2photon ) are presented on the same relative intensity scale.

(CH 3 ) 3 SiI 1 hn → (CH 3 ) 3 SiI 31 1 3e → H 1 1 I 21 1 NP 1 3e

(2)

Throughout this text, NP denotes the corresponding neutral products as undetected neutral fragments. The I(4d) core excitation led to the detection of the ion pairs of H 1 –I 21 and H 1 –I 1 in the significant yields. The formation of such a H 1 –I 21 ion pair indicates that local multiple ionization of the specifically excited bare iodine atom follows rapid dissociation of the bond between the central silicon atom and the core-excited I atom. The detection of the ionic rearrangement products 1 1 1 such as H 1 (n#5) 2 , C 2 H n , SiH n , and SiCH n supports the stepwise or the sequential model in the specific dissociation event which involves the photochemical degradation and rearrangements (vide infra). The stepwise mechanism seems to be more

plausible than the spontaneous Coulomb explosion in the valence ionization event. The variation of the partial ion and PIPICO yields with energy is discussed in conjunction with the relevant electronic states, on the basis of the observations of the ion and PIPICO intensities as functions of energy.

3.2.1. H 1 and H 21 channels As seen in Fig. 4, the H 1 , I 21 , and I 1 ions are abundant in the I(4d) threshold region, and the energy dependence of the ion intensities are quite similar to each other. Also the energy profile of the PIPICO channel corresponding to H 1 –I 21 is similar to that of H 1 –I 1 . However, the shape of the ion yield curve for H 1 2 is quite different from that for H 1 , indicating the different reaction pathways for the formation of the two ions. At low energies near the

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(CH 3 ) 3 Si 21 → → CH 3 1 H 1 1 SiC 2 H 1 5

(5)

(CH 3 ) 3 Si 1 → → CH 3 1 H 1 –SiC 2 H 5

(6)

1

1

Ion pair of H –SiC 2 H p is observed at 88 eV, as illustrated in Fig. 5, which reveals the validity of process (5). Mechanistic interpretation for the complexation between Si and ethene is further discussed below.

3.2.2. CH 1 n ( n 50 – 3) channels As shown in Fig. 4, the ion intensity of CH 1 3 begins to increase after ¯103.8 eV, the energy corresponding to the Si(2p) excitation, and then rise again from the C(1s) threshold. The increase compensates with the decrease of the H 1 ion intensity (Fig. 4), indicating that the two reactions are competitive. Scheme 1 outlined below can account for the formation of CH 1 n (n50–3): (CH 3 ) 3 SiI 21

Fig. 5. PIPICO spectra of (CH 3 ) 3 SiI taken by excitation at the various photon energies of 88, 110, 130, 160, 200, 300, 640, and 890 eV as used for the TOF mass spectra in Fig. 3. The core orbitals related to the photon energies are marked in the individual spectra.

I(4d) edge, Reactions (3)–(6) can account for the 1 1 21 formation of the H ion and the ion-pairs of H –I 1 1 and H –I : (CH 3 ) 3 SiI 1 hn → (CH 3 ) 3 SiI 31 1 3e → (CH 3 ) 3 Si 21 1 I 1 1 NP 1 3e

(3)

(CH 3 ) 3 SiI 1 hn → (CH 3 ) 3 SiI 31 1 3e → (CH 3 ) 3 Si 1 I 1

21

1 3e

(4)

→ → → → → →

1 CH 1 3 1CH 3 1NP 1 CH 3 1SiH 1 1NP 1 CH 1 3 1SiCH 3 1NP 1 1 CH 3 1H 1NP 1 CH 1 3 1SiC 2 H p ( p50, 6)1NP ??? Scheme 1

3.2.3. C2 H 1 n ( n 50 – 4) channels We have observed the rearranged C 2 H n1 ions over the entire energy range, not showing specific preference of the core excitation leading to the formation of the rearrangement product ions. The formation of these ions indicates that intramolecular rearrangement involving double H-transfers from the carbon atoms to the silicon atom can occur prior to the bond cleavages as shown below in Scheme 2: (CH 3 ) 3 SiI 21 [H 2 (CH 3 )Si? ? ?uu] 21

→ [H 2 (CH 3 )Si? ? ?uu] 21 1I 1 → CH 1 3 1C 2 H 4 1NP 1 1 → H 1C 2 H 4 1NP → H 2 SiCH 31 1C 2 H 41 1NP → ??? Scheme 2

1 The PIPICO channels of CH 1 ( p50–3) 3 –C 2 H p 1 1 and H –C 2 H p ( p50–3) are not purely isolated,

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Fig. 6. Partial PIPICO yield spectra of (CH 3 ) 3 SiI in the range of 88–1000 eV. The spectral intensities (IPIPICO /Itot2photon ) are presented on the same relative intensity scale.

because of the closeness of the C 2 H p 1 ( p50–3) and 1 SiH n ions. The previous PIPICO study employing Ge(CH 3 ) 3 indicates that the ion pairs are clearly observed [24]. This kind of hydrogen shift and the concomitant C–C bond formation leading to the formation between the Si atom and the ethene ligand are supported by previous experimental and theoretical studies of silicon-containing compounds [25– 30].

3.2.4. SiH 1 n ( n 50 – 3) channels The formation of the ions such as SiH 1 and SiH 1 2 indicates again the occurrence of the hydrogen transfers from the carbon atoms to the silicon atom as shown in processes (7) and (8): (CH 3 ) 3 Si 21 → [H 2 Si ? ? i] 21 1 CH 3 → SiH 1 1 H 1 C 2 H 41 1 CH 3

(7)

→ SiH 21 1 C 2 H 41 1 CH 3

(8)

1

However, the Si ion is presumed to be formed directly via the Si–C and Si–I bond cleavages. In the Si(2p) and C(1s) regions, the SiH 1 n intensities are enhanced, indicating that the rearrangement is quite dominated by the core excitation. As shown in Fig. 4, the sizeable contribution to the individual ion yield of the SiH 1 n ion is observed at energies above the I(3d) threshold. In this energy region, however, the I 21 and I 1 ion intensities are shown to have negligibly low intensities. This implies that excitation of the relatively deep core gives rise to near total destruction due to excessive energy left in the molecule, thereby yielding a variety of small ions.

3.2.5. SiCH 1 n ( n 50 – 5) channels Fig. 6 indicates that the major dissociation channels for the formation of SiCH n1 (n 5 0–5) ions

B.H. Boo, N. Saito / Journal of Electron Spectroscopy and Related Phenomena 128 (2003) 119–128 1 involve SiCH 1 and H 1 –SiCH p 1 corresponding n –I to processes (9) and (10):

(CH 3 ) 3 SiI 1 hn → (CH 3 ) 3 Si 21 1 I 1 1 3e (CH 3 ) 3 Si 21 → H 1 1 SiCH n1 (n 5 0–5) 1 NP

(9) (10)

The ion intensity is observed to rise from the Si(2p), C(1s), and I(3d) thresholds as shown in Fig. 4. At the low energies below about 200 eV, the formation of the SiCH 31 and SiCH 51 ions prevail, by contrast, at the high energies above about 200 eV, the SiC 1 and SiCH 1 ion intensities are competitive (Fig. 3).

3.2.6. SiC2 H 1 n (n 50,7) channels As seen in Fig. 3, the SiC 2 H n1 ions appear in the two TOF regions. One is for the smaller n (around 0), and the other is for larger n (around 7). We presume that the ions with smaller n are formed via core excitation followed by consecutive eliminations of molecular and / or atomic hydrogen, and that those with the larger n are produced in the off-resonance valence ionization region. This consecutive elimination of molecular hydrogen was also observed in the photoionization of (CH 3 ) 2 SiH 2 , where the SiC 2 H 1 and the SiC 2 H 1 ion intensities are dominated among the SiC 2 H n1 (n50–7) channels at energies above the Si(2p) threshold [30]. 3.2.7. SiC3 H 91 channel The ion intensity is observed to rise rapidly from the Si(2p) threshold, where as its possible ionic counterparts, the I 21 and I 1 ions are observed to have negligibly low intensities (Fig. 3). Also, the ion pairs of SiC 3 H n 1 –I 1 are not observed in the whole energy range. This implies that the Si(2p) and C(1s) core excitation / ionization leads to solely the formation of internally hot SiC 3 H 91 ion and neutral atomic iodine (or atomic iodine ion), and then the hot SiC 3 H 91 ion undergoes further decomposition yielding H 1 , CH 31 , SiCH n 1 (n#5), and etc.. 3.2.8. I 21 and I 1 channels The I 21 and I 1 ion intensities are dominant in the I(4d) threshold region. Above the Si(2p) threshold, the ion intensities are significantly diminished presumably because of the strong competition between the I(4d) and Si(2p) inner-shell excitation / ionization

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efficiencies. At the lowest energy examined here (88 eV), the I 21 and I 1 ion intensities are dominant. The presence of the doubly charged atomic ion indicates that fast dissociation followed by autoionization upon the I(4d) excitation may occur as shown below in Scheme 3: (CH 3 ) 3 SiI1hv (CH 3 ) 3 Si–I * (CH 3 ) 3 Si 21 [(CH 3 )(H 2 )Si? ? ?uu] 21 I* I*

→ → → → → → Scheme

(CH 3 ) 3 Si–I * (CH 3 ) 3 Si 21 1I * [(CH 3 )(H 2 )Si? ? ?uu] 21 CH 3 1H 1 1SiC 2 H 1 5 I1 I 21 3

In previous studies of core excitation of silane, de Souza et al. [31] proposed a two-step decay process for a valence resonance, namely a fast dissociation followed by the autoionization of the excited fragment, on the basis of the observation of Br(3d) core excitation of HBr [32].

4. Conclusions Specific fragmentation processes following the Si(2p), C(1s), and I(4d, 4p, 4s, 3d) core excitation / ionization of (CH 3 ) 3 SiI have been investigated in the range of 88–1000 eV by means of the PEPICO and PIPICO techniques coupled to synchrotron radiation. The giant resonance spanning 88 to about 160 eV is observed as a resonance owing to the diffusion of the outgoing photoelectrons by the angular momentum barrier in the 4d 9 ef outgoing channel. In addition to a structureless giant background resonance, we have observed significant discrete features owing to the Si(2p) and C(1s) core excitation. The another huge peak ranging from 160 to 620 eV may correspond to shake-off processes above the Si(2p) threshold, which led to the simultaneous ejection of two or more electrons ending up with fast relocation of energy and charge on the whole molecule, thereby yielding a variety of decomposition products with relatively high yield. In this shake-off process, siteselectivity in the fragmentation is not specifically observed. The H 1 , I 21 and I 1 ion intensities are dominant in the I(4d) threshold region, where the PIPICO chan-

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nels leading to the formation of H 1 –I 21 and H 1 –I 1 also prevail, indicating that those ions are the result of dissociative double and triple ionization of the molecule. Above the Si(2p) threshold, the ion intensities due to I 21 and I 1 are significantly diminished owing to the competition between the Si(2p) and I(4d) inner-shell processes. The dominant formation of the I 21 in the I(4d) region indicates that bond cleavage is faster than charge redistribution, and that local Auger process occurs leading to the formation of I 21 . The formation of the rearranged ionic products 1 1 1 such as H 1 (n#5) 2 , C 2 H n , SiH n , and SiCH n ratifies the stepwise or the sequential model in the specific dissociation events involving the valence ionization. The specific excitation and dissociation of molecules provides information on energy dissipation processes in conjunction with the relevant coreexcited states.

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation through the non-directed research program of 1998–1999. The staffs of the accelerator group of the Electrotechnical Laboratory in Tsukuba, Japan are greatly acknowledged for allowing the use of the synchrotron radiation facility.

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