Article
pubs.acs.org/JPCA

Site-Speciﬁc Electron-Relaxation Caused by Si:2p Core-Level
Photoionization: Comparison between F3SiCH2CH2Si(CH3)3 andCl3SiCH2CH2Si(CH3)3 Vapors by Means of Photoelectron Auger
Electron Coincidence Spectroscopy
Shin-ichi Nagaoka,*,† Takuhiro Kakiuchi,† Joji Ohshita,‡ Osamu Takahashi,§ and Yasumasa Hikosaka∥†

Department of Chemistry, Faculty of Science and Graduate School of Science and Engineering, Ehime University, Matsuyama
790-8577, Japan‡
Department of Applied Chemistry, Graduate School of Engineering and §Institute for Sustainable Sciences and Development,
Hiroshima University, Higashi-Hiroshima 739-8511, Japan∥
Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama 930-0194, JapanS Supporting Information*

ABSTRACT: Site-speciﬁc electron relaxations caused by Si:2p
core-level photoionizations in F 3SiCH2 CH2 Si(CH 3) 3 andCl3SiCH2CH2Si(CH3)3 vapors have been studied by means of
the photoelectron Auger electron coincidence spectroscopy.
F3SiCH2CH2Si(CH3)3 shows almost 100% site-speciﬁcity infragmentation caused by the Si:2p ionization. However,substitution of Cl for F of F3SiCH2CH2Si(CH3)3 considerably
reduces the site-speciﬁcity at the Si atom bonded to threehalogen atoms, with the site-speciﬁcity at the Si site bonded tothree methyl groups remaining largely unchanged. The sitespeciﬁcity reduction in Cl3SiCH2CH2Si(CH3)3 is considered totake place during the transient period between Si:L23VV Auger
electron emission and the subsequent fragmentation. The reason
for the reduction can be explained in terms of some diﬀerences between these two molecules in the L23VV Auger decay at the Sisite bonded to the three halogen atoms.

1. INTRODUCTION
Is there any communication between two atoms at either endof a molecule? The answer is often “No!” in fragmentation
caused by core-level photoionization.1−3 Because a coreelectron is localized very close to the nucleus of one particularatom, the core-level photoionization at the left-hand-side atom
inside the molecule frequently causes localized bond dissociation around the atom. Then, chemical bonding remains
unchanged around the right-hand-side atom with diﬀerent
chemical environment. Like this, in the core-level photoionization and the subsequent fragmentation, the right-handside atom inside the molecule, so to speak, does not know what
is being done at the left-hand-side atom, in contrast to
photoexcitation of a valence electron delocalized over the entire
molecule. Such fragmentation caused by the core-level
photoionization is called site-speciﬁc fragmentation and ispromising for controlling chemical reactions at an atomic
level.2,3 The site-speciﬁc fragmentation is expected to play arole of “molecular knife” that is an appropriate tool for “cutting”
a molecule, a molecular assembly, or a nanoscale device.3
Bridged (trihalosilyl)(trimethylsilyl) molecules (X3SiCnHmSi(CH3)3, X = F or Cl, n = 0−2, and m = 0−4) are useful for© 2016 American Chemical Society

studies of the site-speciﬁc fragmentation caused by the corelevel photoionization, because the chemical environment of theleft-hand-side Si atom bonded to three halogen atoms (heredenoted Si[X]) is very diﬀerent from that of the right-hand-side
Si atom bonded to three methyl groups (Si[Me]).3 In fact,
whereas the halogen atom is an electron acceptor with a large
electronegativity, the methyl group is an electron donor with asmall electronegativity.4 Furthermore, the site-speciﬁc fragmentation of Si-containing molecules such as X3SiCnHmSi(CH3)3
draws much attention, because Si materials are widely used for
fabrication of electronic devices and this fabrication has become
very precise in recent years.5
We previously studied the site-speciﬁc fragmentation of
X3SiCnHmSi(CH3)3 vapors by means of the energy-selectedphotoelectron photoion−photoion triple-coincidence spectroscopy (PEPIPICO) and photoelectron−photoion coincidence
spectroscopy (PEPICO).3,6,7 Upon Si:2p core-level photoionization, X3SiCnHmSi(CH3)3 showed two core photoelectron
Received: September 17, 2016
Revised: November 13, 2016
Published: November 28, 2016
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means of the repeated pump-thaw method prior to measurements.
On beamline BL4B at UVSOR synchrotron-radiation
facility,9 our experiments were performed during single bunch
operation, which provided light pulses with a repetition periodof 178 ns and a width of 110 ps. The synchrotron radiation
being emitted from a bending magnet of the storage ring was
monochromatized by a grazing-incidence monochromatorequipped with a varied-line-spacing plane-grating. We used
no optical ﬁlters for light order sorting.
All the photoelectron Auger electron coincidence measurements of FSMSE and CSMSE vapors were carried out with
Y.H.’s home-built magnetic-bottle electron spectrometer whose
design and capability had been described elsewhere.10 Brieﬂy,
photoelectrons and Auger electrons created by photoionizationof each sample gas were captured over a 4π solid angle bymagnetic ﬁeld lines formed inside the spectrometer and wereguided toward a microchannel-plate detector terminating a 1.5m ﬂight path. Electron signals from the detector were fed, after
ampliﬁcation and discrimination, into a time-to-digital converter (Roentdek TDC8) operated in the multistart commonstop mode. The converter was started by the ﬁrst-arrived
electron-signal, recording the arrival times of the following
electrons, and was stopped, after a 10 μs delay, by a masterclock signal for the storage ring operation. The time intervals
between the stop time and the arrival times of all the electrons
during the ∼10 μs time window were evaluated and converted
to seeming electron time-of-ﬂights with resetting the start timeto the relevant light-pulse incident-time before the ﬁrst arrivalof the electron signal. Because the light pulse was incident on
the sample vapor with a repetition period of 178 ns, theseeming electron-time-of-ﬂight from this start time is expressedby the sum of k × 178 ns (k ≥ 0) and the correct time-ofﬂight11 that the electron needed to travel the 1.5 m ﬂight path.
If k = 0, the electron creation coincided with the start time. If k
> 0, the electron was created k × 178 ns after the start time, andthe seeming time-of-ﬂight then obtained may induce a false
structure in the spectrum.
The conversion factor from the correct time-of-ﬂight to theelectron kinetic energy was calibrated by using Ar:2p
photoelectron spectra (PES)11 measured at various photon
energies. The kinetic-energy resolving power (E/ΔE) being
estimated from the photoelectron peaks gave a nearly constantvalue 35 over the whole kinetic-energy region in themeasurement, where E and ΔE denote the electron kinetic
energy and bandwidth, respectively. From coincidence yields
observed between the Ar:2p photoelectrons and the corresponding normal LMM Auger electrons,12 the electron
detection eﬃciency was estimated to be ∼50%.
An advantage of the photoelectron Auger electron
coincidence spectroscopy is that the coincidence data allow
us to identify the source of features in the Auger electron
spectrum (AES).13,14 The coincidence experiment provides anopportunity to ﬁnd the relationship between the PES and AES
peaks. Accordingly, even if the total AES is not clearly resolvedsite by site in each of FSMSE and CSMSE, it is possible toidentify Si[X]- and Si[Me]-site-speciﬁc Auger decay processes
that may contribute to the Auger peak intensity or modify thepeak shape. This coincidence spectroscopy is one of the few
means of separating out the relative importance of the two sitespeciﬁc processes in particular Auger peaks, and is suitable forour purpose. At one time, a disadvantage of this spectroscopy
was a long data-collection time, and Thurgate hoped that thepeaks reﬂecting the binding-energy diﬀerence between the
Si[X] and Si[Me] sites. In the subsequent normal Augertransition, a valence electron fell into the Si:2p core orbital andcreated a valence hole that spatially overlapped the initiallycore-ionized Si site (X3SiCnHmSi(CH3)3+). The electron falling
into the Si:2p core orbital gave its energy to another nearby
valence electron, which was emitted as a normal Auger electron(two-step model), creating another valence hole near by(X3SiCnHmSi(CH3)32+). Because these valence holes weakened
chemical bonds around the initially core-ionized Si site, the sitespeciﬁc fragmentation (X3SiCnHmSi(CH3)32+ → F1+ + F2+,
where F1+ and F2+ denote fragment ions) often occurred
around the Si site. In fact, the experimental results clearly
showed almost 100% site speciﬁcity in fragmentation caused by
the Si:2p photoionization of F3SiCnHmSi(CH3)3 with a longintersite bridge (−CnHm−), which was found to prevent
intramolecular electron migration between the two Si sites.
Such high site speciﬁcity in the fragmentation was observed atboth of the Si[F] and Si[Me] sites.
However, substitution of Cl for F in X3SiCnHmSi(CH3)3
reduced the maximum site speciﬁcity at the Si[X] site by no lessthan 50%, with the maximum site speciﬁcity at the Si[Me] siteremaining largely unchanged (Figures 7 and 8 of ref 3). Of
course the electronegativity of Cl is less than that of F andnearer to that of CH3,4 but 2p binding energies of the Si[Cl]and Si[Me] sites were clearly separated from each other evenafter the substitution (Figures S3 and S6 of ref 3). Nevertheless,why did the site speciﬁcity in the fragmentation caused by the
Si[X]:2p ionization drop much more than that by theSi[Me]:2p ionization? When did the site-speciﬁcity reduction
take place during relaxation after the Si[Cl]:2p ionization in
Cl3SiCnHmSi(CH3)3? The answers to these questions have notyet been worked out, although such works are essential not
only for full understanding of the site-speciﬁc fragmentation but
also for controlling chemical reactions at an atomic level andrealizing the molecular knife.
Accordingly, in the work presented here, we have attempted
to elucidate when and why the site-speciﬁcity reduction by the
F → Cl substitution occurs at the Si[X] site of X3SiCnHmSi(CH3)3. Answer candidates for the when-done-it are transientperiods among (1) core photoelectron emission, (2) Auger
electron emission, and (3) fragmentation. As mentioned above,
we already studied process 3 being site-selectively caused byprocess 1, which is to say correlation between processes 1 and
3, in X3SiCnHmSi(CH3)3 vapors by means of PEPIPICO andPEPICO.3,6,7 We also investigated correlation between
processes 2 and 3 by the Auger electron photoion−photoion
triple-coincidence spectroscopy (AEPIPICO) and Auger
electron photoion coincidence spectroscopy (AEPICO).
Here, to examine correlation between processes 1 and 2, wehave compared photoelectron Auger electron coincidence
spectra of 1-(triﬂuorosilyl)-2-(trimethylsilyl)ethane(F3SiCH2CH2Si(CH3)3, FSMSE) and 1-(trichlorosilyl)-2(trimethylsilyl)ethane (Cl3SiCH2CH2Si(CH3)3, CSMSE) vapors. The comparison has elucidated when and why the site
speciﬁcity is reduced in the fragmentation caused by the
Si[Cl]:2p ionization of Cl3SiCnHmSi(CH3)3. This is the ﬁrst
time that the site-resolved correlation between processes 1 and2 has been observed in X3SiCnHmSi(CH3)3 vapor.

2. EXPERIMENTAL SECTION
Preparation methods of FSMSE and CSMSE were reported
previously.8 Each of FSMSE and CSMSE was degassed by
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time to produce a sensible spectrum would decrease to several
hours.13 In our measurements, this problem has been solved byadopting the magnetic-bottle electron-spectrometer with a highelectron-collection eﬃciency.10,15

3. COMPUTATIONAL METHOD AND PROCEDURE
To elucidate the site-speciﬁc fragmentation process in detail, we
deduced characters of doubly charged states formed after theabove-mentioned process (2) (Auger ﬁnal states) in FSMSE,
CSMSE, SiF4, and SiCl4 by using the ab initio molecular-orbital(MO) method. The geometries of FSMSE, CSMSE, SiF4, andSiCl4 were optimized out by using GAUSSIAN 09 program16 at
the MP2/cc-pVDZ level of theory to obtain their equilibrium
structures.
The basic computational method and procedure used in the
ab initio MO calculation of L23VV AES at a Si atom were
described in detail in previous papers.3,17 A set of MOs for theAuger ﬁnal states was selected out of MOs for the ground state,
which were a better choice than those for the Si:2p core−holestate. The energies of the Auger ﬁnal states were calculated byusing the conﬁguration interaction (CI) method within thevalence-two-hole space. The intensities of the Auger transition
rates were obtained from the electron population analysis.17
The probabilities of the Auger transitions to all the singlet andtriplet states were taken into account in this study.
A computational AES was constructed by Gaussianconvolution of the line spectrum thus obtained. The Gaussianfunctions used there had a full-width-at-half-maximum (fwhm)
being given as (E/80)+Δ, where E and Δ denote the Auger
electron kinetic energy and ﬁxed bandwidth, respectively. The
Δ’s were set to 1.5 and 2.0 eV for FSMSE and CSMSE,
respectively. The resultant computational AES was energyshifted so as to facilitate comparison with the corresponding
experimental AES.

Figure 1. Two-dimensional contour representation of correlation
between photoelectron and L23VV Auger electron emissions in Si:2p
core-level photoionization. Here, the k value deﬁned in section 2 iszero except for the false structure mentioned below. (a) FSMSE vapor.
This spectrum has been measured at a photon energy (hν) of 131.9
eV. (b) CSMSE vapor and hν = 119.5 eV. The island seen around acoordinate value of x = 45 eV and y = 16 eV is a false structure, where
the x- and y-axes are taken to be horizontal and vertical, respectively.
The k value for the false structure is one, and there Cl:2p core-level
photoionization due to the second-order light (hν = 239.0 eV) in
CSMSE has caused the photoelectron L23VV Auger electron
coincidence, whose correct two-dimensional contour representation
is given in the Supporting Information (Figure S1).

4. RESULTS AND DISCUSSION
4.1. Correlation between Photoelectron and L23VV
Auger Electron Emissions. Figure 1 shows two-dimensional
contour representation of correlation between photoelectronand L23VV Auger electron emissions caused by Si:2p core-level
photoionization in FSMSE and CSMSE vapors. The verticalaxis (y axis) indicates the kinetic energy of the slow electron of
the two emitted electrons, and the horizontal axis (x axis)
indicates that of the fast electron. Under our experimental
conditions, the photoelectron and L23VV Auger electron are
emitted as the slow and fast electrons, respectively. When aphotoelectron emission with a kinetic energy of y eV induces anAuger electron emission with that of x eV, the photoelectron
emission is correlated with (in coincidence with) the Auger
electron emission, and one count is added to the contour valuef(x,y) at the coordinate (x, y) in Figure 1. The two bars shownin each of Figure 1a,b correspond to two Si:2p-photoelectron
peaks reﬂecting the binding-energy diﬀerence between the
Si[X] and Si[Me] sites (X = F or Cl).
By summing up the contour values over x (g1(y) =∑x f(x,y)), we can extract a Si:2p photoelectron spectrum(PES) from each of Figure 1a,b (y vs g1(y) plot). Parts a and b
of Figure 2 respectively show the Si:2p PES of FSMSE andCSMSE vapors, and the Si[X] and Si[Me] sites of eachmolecule exhibit energy-resolved Si:2p-photoelectron peakgroups at lower and higher kinetic-energy sides. Each of
those peak groups is seen to be further split into two

components, which come from spin−orbit coupling. The
binding energies of Si[X]: and Si[Me]:2p electrons in FSMSE
and CSMSE vapors have already been reported (ref 18 andTable S1 of ref 3).
By summing up the contour values over y (g2(x) = ∑y f(x,y)),a normal Si:L23VV AES can be extracted from each of Figure 1aand Figure 1b (x vs g2(x) plot, Figures 3a and 4a). In contrast
to the Si:2p PES shown in Figure 2, the Si:L23VV AES is notclearly resolved site by site. The normal Auger transitions
produce various doubly charged states (Auger ﬁnal states). As
the Auger electron kinetic energy decreases in Figures 3 and 4,
the Auger decay leads to a higher-lying Auger ﬁnal state with
one or two inner valence holes as shown in Figure 5.19 To
examine site-speciﬁc electron relaxation in detail, we have toobserve Si[XR]:L23VV Auger emission caused by Si[XR]:2p
photoelectron emission (XR = F, Cl, or Me).
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Figure 3. Si:L23VV AES of FSMSE, Si(CH3)4, and SiF4 vapors. The
experimental AES (black curves) of FSMSE have been extracted from
Figure 1a. Each computational spectrum is drawn in color except
black. (a) Si:L23VV AES obtained by summing up the contour values
along the vertical axis (y axis) in Figure 1a. Summation of Si[F]:L23VV
and Si[Me]:L23VV AES of FSMSE. (b) Si[Me]:L23VV Auger emission
caused by Si[Me]:2p photoelectron emission in FSMSE. The
experimental spectrum has been obtained by summing up the contour
values over y = 24.2−26.5 eV in Figure 1a. (c) Si[F]:L23VV Auger
emission caused by Si[F]:2p photoelectron emission in FSMSE. The
experimental spectrum has been obtained by summing up the contour
values over y = 20.6−23.1 eV in Figure 1a. (d) Si(CH3)4.21 (e) SiF4.20

consistent with the experimental spectra. The spectra of
FSMSE vapor (Figure 3a−c) are similar to those obtained forcondensed F3SiCD2CH2Si(CH3)3.23
4.2. Peak Assignments of Site-Speciﬁc AES. 4.2.1. Si[Me]:L23VV AES of FSMSE and CSMSE. Because the Si[Me]:L23VV AES of FSMSE and CSMSE (Figures 3b and 4b)
are close to each other and similar to that of Si(CH3)4 vapor
except the spectral resolution (Figures 3d and 4d),21,24 we canassign the Si[Me]:L23VV Auger peaks seen in both FSMSE andCSMSE according to the data of Si(CH3)4.24 The Si[Me]:L23VV Auger peaks in 67−83 eV (Figures 3b and 4b)
are assigned to Auger ﬁnal states with two valence holesdelocalized over the four Si[Me]−C bonds. Note that theSi[Me]−C bonding orbitals are polarized, if anything, toward
the Si[Me] atom, because each of the alkyl groups bonded toSi[Me] is an electron donor with a small electronegativity.4
Accordingly, the holes created by the Si[Me]:L23VV Auger
decay appear with their principal density around the Si[Me]atom.
4.2.2. Si[F]:L23VV AES of FSMSE. In Figure 3c, FSMSE showsa Si[F]:L23VV Auger peak at 73.4 eV, which comes from holecreation in higher-lying Si[F]−CH2(bridge) bonding orbitals25
and appears as a shoulder in the computational Si[F]:L23VV
AES. Except for the peak at 73.4 eV, the Si[F]:L23VV AES of
FSMSE (Figure 3c) is similar to the Si:L23VV AES of SiF4
(Figure 3e).20,26 Furthermore, the Si atom of SiF4 can be
regarded as a model of the Si[F] site in FSMSE.27 Thus, wehave assigned the ﬁve Si[F]:L23VV Auger peaks of FSMSE seenin 30−71 eV (Figure 3c) according to an assignment result for
SiF4 by Aksela et al.26 and a somewhat-revised assignment
result based on ab initio MO calculations of SiF4 by TarantelliFigure 2. Si:2p PES (dots) obtained by summing up the contour
values along the horizontal axis in Figure 1. The red, blue, and green
thin proﬁles indicate results of Gaussian ﬁtting, and the black thickcurve is the summation of the thin proﬁles. (a) FSMSE vapor.
Calibration of the binding-energy scale has been performed using ahigh-resolution Si:2p PES reported previously.18 The intensity ratios
between the 2p3/2 and 2p1/2 photoelectrons seen in this ﬁgure are
2.0:1.0 and 2.0:1.5 at the Si[Me] and Si[F] sites, respectively. (b)CSMSE vapor. Calibration of the binding-energy scale has been
performed using a low-resolution Si:2p PES reported previously (seeTable S1 of ref 3). The binding energies of the Si[Me]:2p3/2,
Si[Me]:2p1/2, Si[Cl]:2p3/2, and Si[Cl]:2p1/2 electrons have been
determined to be 106.1, 106.7, 108.9, and 109.5 eV, respectively.
The intensity ratios between the 2p3/2 and 2p1/2 photoelectrons seenin this ﬁgure are 2.0:1.3 at both of the Si[Me] and Si[F] sites. The
spectral resolution of this PES is higher than that in Figure 2a.

In the case that the summation ∑y f(x,y) is made within oneof the two bars seen in each of the two-dimensional contour
plots in Figure 1, a Si[XR]:2p photoelectron Si[XR]:L23VV
Auger electron coincidence spectrum can be extracted. The
coincidence spectrum represents the Si[XR]:L23VV Auger
emission caused by the Si[XR]:2p photoelectron emission.
Figures 3b,c and 4b,c show those coincidence spectra of
FSMSE and CSMSE vapors, together with the computational
spectra. The summation of the computational Si[X]:L23VV andSi[Me]:L23VV Auger electron spectra (AES) in each molecule
is given in Figures 3a and 4a. For comparison, Figures 3 and 4
also represent Si:L23VV AES of Si(CH3)4, SiF4, and SiCl4
vapors obtained previously.20−22 The computational spectraof FSMSE and CSMSE given in Figures 3 and 4 are fairly
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density around the F atoms, although the Auger decay islocalized at the Si atom as explained for X3SiCnHmSi(CH3)3 in
the Introduction. By performing ab initio Green’s-function
calculations, Tarantelli et al. showed that pronounced hole
localization on one or two of four Si−F bonds takes place in all
the Si:L23VV Auger ﬁnal states of SiF4.28,29 The holelocalization phenomena cannot be described within a framework in the absence of two-hole conﬁguration mixing.30 From
the above-mentioned similarity between Figure 3c and Figure
3e, it is considered that similar hole-localization phenomena
take place also in the Si[F]:L23VV Auger decay of FSMSE.
Our assignment for FSMSE is schematically summarized in
Figure 6a. The Si[F]:L23VV Auger peak-groups of FSMSE seenin 55−71 and 30−50 eV (Figure 3c) are respectively assigned
to Auger ﬁnal states with two outer valence holes located in one
or two Si−F bonds and to those with one outer hole and one
inner hole located in the Si−F bonds. Each of those two peakgroups (55−71 and 30−50 eV) is further split into two or threecomponents. The peaks at 68.6 and 47.4 eV are assigned toAuger ﬁnal states with one hole located in a Si−F bond and one
hole in another Si−F bond, and the peaks at 60.6 and in 30−44
eV are assigned to those with two holes residing in one of thethree Si−F bonds. Strong electrostatic interaction between thetwo holes conﬁned on the same Si−F bond increases the Auger
ﬁnal state energy and decreases the Auger electron kinetic
energy. The Auger ﬁnal states in 30−44 eV are further split into
the triplet and singlet states at 42.0 and 33.0 eV, respectively.
Strong localization of the two holes in the same small region of
space produces the singlet−triplet splitting in 30−44 eV. As in
SiF4,29 two-additional moderate peak groups would also appear
below an Auger electron kinetic energy of 30 eV (lower limit in
our measurement) in FSMSE. Those peak groups could be
assigned to Auger ﬁnal states with two inner valence holes
located on diﬀerent Si−F bonds and to those with two innerholes conﬁned on the same Si−F bond.
4.2.3. Si[Cl]:L23VV AES of CSMSE. Because the Si[Cl]:L23VV
AES of CSMSE (Figure 4c) is similar to the Si:L23VV AES of
SiCl4 (Figure 4e),22,32 we have assigned the Si[Cl]:L23VV
Auger peaks of CSMSE according to the result of our ab initio
MO calculation for SiCl4 (Supporting Information). Note thatthe hole-localization phenomena seen in SiF4 were also
reported in SiCl4.32 The assignment for CSMSE is schematically summarized in Figure 6b. The Si[Cl]:L23VV peaks at 75.8
and 67.8 eV (Figure 4c) are assigned to Auger ﬁnal states with
two outer valence holes located on diﬀerent Si−Cl bonds andto those with two outer holes conﬁned on the same Si−Cl
bond, respectively. The Si[Cl]:L23VV Auger peak at 75.8 eV in
CSMSE would contain an unresolved component coming fromhole creation in higher-lying Si[Cl]−CH2(bridge) bonding
orbitals, just as the Si[F]:L23VV Auger peak at 73.4 eV in
FSMSE (Figure 3c) comes from hole creation in higher-lying
Si[F]−CH2(bridge) bonding orbitals.25 The Si[Cl]:L23VV
Auger hump at 59.4 eV in CSMSE (Figure 4c) is assigned toAuger ﬁnal states with one outer valence hole and one innerhole located on diﬀerent Si−Cl bonds, though the computational Si[Cl]:L23VV Auger peak around 60 eV in CSMSE isstronger than the experimental one. Any Si[Cl]:L23VV Auger
decay producing an Auger electron with a kinetic energy of <55
eV is negligible in CSMSE.
4.3. Transient Period during Which Site Speciﬁcity Is
Reduced. As mentioned in the Introduction, the F → Cl
substitution in X3SiCnHmSi(CH3)3 (X = F or Cl) reduces thesite speciﬁcity in the fragmentation caused by the Si[X]:2p

Figure 4. Si:L23VV AES of CSMSE, Si(CH3)4, and SiCl4 vapors. The
experimental AES (black curves) of CSMSE have been extracted from
Figure 1b. Each computational spectrum is drawn in color except
black. (a) Si:L23VV AES obtained by summing up the contour values
along the vertical axis (y axis) in Figure 1b. Summation ofSi[Cl]:L23VV and Si[Me]:L23VV AES of CSMSE. (b) Si[Me]:L23VV
Auger emission caused by Si[Me]:2p photoelectron emission in
CSMSE. The experimental spectrum has been obtained by summingup the contour values over y = 12.3−14.0 eV in Figure 1b. (c)Si[Cl]:L23VV Auger emission caused by Si[Cl]:2p photoelectron
emission in CSMSE. The experimental spectrum has been obtained bysumming up the contour values over y = 9.4−11.2 eV in Figure 1b. (d)Si(CH3)4.21 (e) SiCl4.22

Figure 5. Energy level diagram of some MOs and doubly charged
states (Auger ﬁnal states) related to normal AES. The solid arrows
show an outer valence orbital → core−hole transition and thesubsequent Auger electron emission from the outer valence orbital.
The broken arrows correspond to a case that an inner valence orbital
participates in the Auger decay.

et al.28,29 A similar calculation of FSMSE is very diﬃcult
because FSMSE is much larger in molecular size than SiF4.30,31
Note that Si−F bonding orbitals in SiF4 are polarized toward
the F atom in contrast to the Si[Me]−C orbitals mentioned
above, because F is an electron acceptor with a large
electronegativity.4 Accordingly, valence holes created by the
Si:L23VV Auger decay in SiF4 appear with their principal
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in Figures 3a and 4a are not clearly resolved site by site incontrast to the PES shown in Figures 2a and 2b.
As shown in Figures 3 and 4, the Si[X]: and Si[Me]:L23VV
AES are very diﬀerent from each other in each of FSMSE andCSMSE, and so some site speciﬁcity in electron relaxation
would be maintained during the transient period between thephotoelectron and Auger electron emissions. Thus, the sitespeciﬁcity reduction at the Si[Cl] site of CSMSE is considered
to take place during the transient period between the
Si[Cl]:L23VV Auger electron emission and the subsequentfragmentation. If a part or the whole of the site-speciﬁcfragmentation came from ultrafast dissociation,20,33 somespeciﬁcity reduction by the F → Cl substitution should occur
during the transient period between the photoelectron andAuger electron emissions. Accordingly, the ultrafast dissociationis unlikely to play a great role in the site-speciﬁc fragmentation.
This conclusion is consistent with the computational result thatlittle molecular deformation is induced by the Si:2p core−holeformation in FSMSE.6
In Figures 3 and 4, the Si[Me]:L23VV AES of FSMSE andCSMSE are close to each other, and similar to that of Si(CH3)4
vapor.21,24 However, the Si[X]:L23VV AES of FSMSE andCSMSE are totally diﬀerent from each other. Furthermore,those are similar to the Si:L23VV AES of SiF4 and SiCl4
vapors,20,22,26,32 as mentioned in sections 4.2.2 and 4.2.3.
These observations of the Si[XR]:L23VV AES of FSMSE andCSMSE (XR = F, Cl, or Me) would also show that the site
speciﬁcity is maintained up to the Auger electron emission evenin CSMSE.
4.4. Reason for Site-Speciﬁcity Reduction. In a previous
paper,3 we noted, “further investigations are needed to clarify
the reason for the greater decrease in the Si[X] site speciﬁcity(due to the F → Cl substitution in X3SiCnHmSi(CH3)3 (X = F
or Cl)).” In the present paper, we have elucidated the reason by
adopting the photoelectron Auger electron coincidence
technique and by linking the data to the previously obtained
results3,6 in the following way.
F+ production is the most prominent site-speciﬁc fragmentation caused by the Si[F]:2p ionization in FSMSE,3,6,7 and highsite speciﬁcity appears in the F+ production (almost 100% in
EPAPS document of ref 6). Because the F+ production iscaused by some violent fragmentation at the Si[F] site, it would
take place eﬃciently in higher-lying Auger ﬁnal states, to which
the Si[F]:L23VV Auger decays of FSMSE considerably lead
(Figures 3c and 6a). Furthermore, ionic Si−F bond rupture andthe subsequent F+ production are expected to occur more
eﬀectively in an Auger ﬁnal state with two holes conﬁned on
the same Si−F bond than in an Auger ﬁnal state with two holes
located on diﬀerent Si−F bonds. The Si[F]:L23VV Auger
decays of FSMSE lead substantially to the Auger ﬁnal stateswith two holes conﬁned on the same Si−F bond, and theeﬃcient site-speciﬁc F+ production actually takes place in these
Auger ﬁnal states (Si[F]:L23VV Auger peak at 60.6 eV).3,6 In
addition, as mentioned in section 4.2.2, the Si[F]:L23VV Auger
decays of FSMSE would lead moderately to still higher-lying
Auger ﬁnal states (Auger electron kinetic energy of <30 eV)with two inner valence holes often residing in one of the three
Si−F bonds. These Auger decays could also contribute to theeﬃcient site-speciﬁc F+ production. For all these reasons, thesite-speciﬁc F+ production in FSMSE is relatively abundant.
In contrast to FSMSE, the Si[Cl]:L23VV Auger decays of
CSMSE lead mainly to lower-lying Auger ﬁnal states (Figures
4c and 6b), in which any violent fragmentation producing Cl+

Figure 6. Energy level diagram of singly and doubly charged states
related to normal Si[X]:L23VV Auger decays. The straight arrows showmajor decays. The present AES measurements have been made in an
Auger electron kinetic-energy region of >30 eV. (a) FSMSE vapor. As
explained in section 4.2.2, some Auger ﬁnal states would be locatedalso below an Auger electron kinetic energy of 30 eV (see the boldbroken line). A previous AEPICO measurement, which was made only
above an Auger electron kinetic energy of 54 eV, showed that F+ ionswere produced in the Auger ﬁnal states at 60.6 eV (see the wavy
arrow).3,6 (b) CSMSE vapor.

ionization. In the present work, by means of the photoelectron
Auger electron coincidence spectroscopy, we have attempted toelucidate the transient period during which the site-speciﬁcity
reduction occurs. Only from the AEPIPICO and AEPICO
measurements besides the PEPIPICO and PEPICO ones3,6
would it be diﬃcult to elucidate it, because the total AES shown
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The Journal of Physical Chemistry A

fragments. Like this, these daughter fragments are produced
through dissociation around the Si[Me] site after the
Si[Cl]:L23VV Auger decay, and then the Si[Me] site may, as
it were, know what the Si[Cl] site is doing. The daughter
fragments would be the same as some minor products formed
after the Si[Me]:L23VV Auger decay and the subsequentfragmentation. As a result, we could not distinguish the Si[Cl]site-speciﬁc fragments from the Si[Me]-site-speciﬁc ones, andthe whole Si[Cl] site speciﬁcity would be reduced again in
CSMSE.

at the Si[Cl] site would be inactive. Furthermore, because theresultant two holes are not eﬀectively conﬁned on the same Si−
Cl bond, ionic Si−Cl bond rupture and the subsequent Cl+
production would not be active. In addition, the Si[X]:L23VV
Auger decay is localized at the Si[X] site as explained in the
Introduction, and the Si−Cl bond is much longer than Si−F.34
Accordingly, valence hole creation on the distant Cl atoms (Cl+production) after the localized Si[Cl]:L23VV Auger decay in
CSMSE would be less eﬃcient than that on the nearby F atoms(F+ production) after the similarly localized Si[F]:L23VV Auger
decay in FSMSE. For all these reasons, site-speciﬁc Cl+
production in CSMSE is less eﬃcient than the abovementioned site-speciﬁc F+ production in FSMSE.
On the contrary, site speciﬁcity in fragmentation caused by
the Si[Me]:2p ionization in CSMSE is similar to, or a little
lower than, that in FSMSE,3,6,7 and Si[Me]-site-speciﬁc large
fragments containing halogen atoms are produced in both
FSMSE and CSMSE. After that, some minor disintegration
forming a halogen cation from the Si[Me]-site-speciﬁc large
fragments seems to occur more in CSMSE than in FSMSE,
because the Si−Cl bond is much easier to dissociate than Si−F
and the resultant Cl atom is much easier to be ionized than F.35
In the fragmentation after the Si[Me]:2p ionization of CSMSE,
there may be some communication between the Si[Me] andSi[Cl] sites at either end of the molecule, because the Si[Me]site-speciﬁc fragmentation may induce the Si[Cl]−Cl dissociation, leading to the Cl+ production. Note that the minor
disintegration forming the halogen cation from the Si[Me]-sitespeciﬁc large fragments is not likely to contribute to the
maximum Si[Me] site speciﬁcity as suggested previously.3
Like this, the site-speciﬁc Cl+ production caused by the
Si[Cl]:2p ionization in CSMSE is less active than the sitespeciﬁc F+ production in FSMSE, whereas the Cl+ formation
through the minor disintegration after the Si[Me]:2p ionization
in CSMSE is more active than the corresponding F+ formation
in FSMSE. As a result, the site speciﬁcity for the Cl+ production
caused by the Si[Cl]:2p ionization of CSMSE (only ∼50% in
Figure S4 of ref 3) is much lower than that for the F+
production by the Si[F]:2p ionization of FSMSE (almost
100% in EPAPS document of ref 6).
In FSMSE, high site speciﬁcity by the Si[F]:2p ionization is
also seen in production of SiF+ and SiF2+ ions and SiF+containing ion pairs (EPAPS document of ref 6). In contrast,site speciﬁcity for production of SiCl+, SiCl2+, and SiCl+containing ion pairs is low in the Si[Cl]:2p ionization of
CSMSE (Figures S4 and S5 of ref 3). Some minor
disintegration forming SiCl+, SiCl2+, and SiCl+-containing ion
pairs from the Si[Me]-site-speciﬁc large fragments would again
reduce the whole Si[Cl] site speciﬁcity of CSMSE.
As mentioned above, the localized Si[Cl]:L23VV Auger decay
in CSMSE would not eﬃciently cause the valence-hole creationon the distant Cl atoms. Instead, the corresponding hole would
be created on the Si[Cl] atom itself or the nearby C atom
bonded to Si[Cl].34 The hole creation on the Si[Cl] atom may
induce Si[Cl]−CH2(bridge) dissociation more eﬀectively thanit does Si−Cl dissociation, because the Si−C bond energy isless than the Si−Cl one.35,36 The Si[Cl]−CH2(bridge)
dissociation after the Si[Cl]:L23VV Auger decay would produce
some ionic daughter fragments coming from the intersite bridge(−CH2CH2−), something containing mono- to trivalent
Si[Me], or methyl groups around Si[Me], which are mucheasier to be ionized than Cl.35,37 The hole creation on the C
atom bonded to Si[Cl] would also form similar daughter

5. SUMMARY
In our previous site-speciﬁc studies performed for X3SiCnHmSi(CH3)3 by means of the photoelectron, Auger electron, andtheir ion-coincidence spectroscopies,3,6,25 the experiments andtheory alike relied on some models and assumptions to extracta mechanism (e.g., wavy arrow in Figure 6a) from the raw
experimental data. Hence, in spite of the previous successes of
those spectroscopies, there was a need for studying thephotoelectron Auger electron coincidence spectroscopy capableof yielding observables that are more likely linked to correlation
between the photoelectron and Auger electron emissions.14
Accordingly, we have studied the site-speciﬁc electron
relaxations caused by the Si:2p core-level photoionizations in
FSMSE and CSMSE vapors by means of the photoelectron
Auger electron coincidence spectroscopy, and linked the data to
the results obtained in our previous works.3,6,25
Now we have the answers to the questions asked in the
Introduction. When does the site-speciﬁcity reduction takeplace during the relaxation after the Si[Cl]:2p ionization in
Cl3SiCnHmSi(CH3)3? The site-speciﬁcity reduction at the
Si[Cl] site of Cl3SiCnHmSi(CH3)3 is considered to take place
during the transient period between the Si[Cl]:L23VV Auger
electron emission and the subsequent fragmentation. Why does
the site speciﬁcity in the fragmentation caused by the Si[X]:2p
ionization drop much more than that by the Si[Me]:2p
ionization in the case that Cl is substituted for F in
X3SiCnHmSi(CH3)3? The reason can be explained in terms ofsome diﬀerences in the experimental results of the Si[X]:L23VV
Auger decay between FSMSE and CSMSE. The Si[F]:L23VV
Auger decays of FSMSE lead considerably to the higher-lying
Auger ﬁnal states with one or two inner valence holes and to
those with two holes conﬁned on the same Si−F bond; incontrast, the Si[Cl]:L23VV Auger decays of CSMSE lead mainly
to the lower-lying Auger ﬁnal states with outer valence holes,
which are located mostly on diﬀerent Si−Cl bonds.
The present work would have brought the goal of controllingchemical reactions at an atomic level and other applications one
step closer to realization. The site-speciﬁc fragmentation is on
the trail of the Holy Grail.

■

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.6b09399.
Complete refs 3, 6, 12, and 16; correct Cl:2p
photoelectron L23VV Auger electron coincidence spectrum of CSMSE (Figure S1) (PDF)
Assignments of AES peaks based on our ab initio MO
calculations for SiF4 (XLSX)Assignments of AES peaks based on our ab initio MO
calculations for SiCl4 (XLSX)
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AUTHOR INFORMATION

Corresponding Author*S. Nagaoka. Phone: 81-89-927-9592. Fax: 81-89-927-9590. Email: nagaoka@ehime-u.ac.jp.
ORCID

Shin-ichi Nagaoka: 0000-0003-1564-7328
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS■

REFERENCES

We express our sincere thanks to Professor Kazuhiko Mase of
Institute of Materials Structure Science and to Ms. Megumi
Hino of Ehime University for their preliminary measurementsof Auger photoelectron coincidence spectra of condensed
CSMSE in the early stage of this work. We are grateful to the
UVSOR staﬀ for the stable operation of the storage ring and to
Dr. Isao H. Suzuki of Institute of Materials Structure Sciencefor his valuable discussion on AES of SiF4 and SiCl4. We thankthe Information Media Center at Hiroshima University for theuse of a grid with high-performance PCs, Research Center for
Computational Science, Okazaki, Japan, for the use of Fujitsu
PRIMERGY, and the Research Institute for Information
Technology, Fukuoka, Japan, for the use of Hitachi
HA8000tc/HT210. This work was partly supported by JSPS
KAKENHI Grant Number 15K04755 and by Shimadzu Science
Foundation and Daiko Foundation.

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