Appendix
In a previous communication,3 we discussed site-specific fragmentation
following
Si:2p
core-level
photoionization
studied
by
the
PEPIPICO/AEPIPICO spectroscopy of FSMSE in the gas phase.
The
following reaction schemes show site-specific fragmentation processes
caused by the Si[Me]:2p and Si[F]:2p ionizations, respectively (Ref. 3 and its
EPAPS document):
Si[Me]:2p ionization:
FSMSE + h
 2e- (Si[Me]:2p INAT)
+ FSMSE2+ (12 holes in Si[Me]C2H4)
↳ F3SiCH2CH2+Si(CH3)3+
↳ SiF3+
+
(1)
↳ Si(CH3)2+
(2)
↳ Si(CH3)H2+
(3)
Si[F]:2p ionization:
FSMSE + h
 2e- (Si[F]:2p INAT)
+ FSMSE2+ (12 holes in Si[F]F and Si[F]C2H4)
↳ (Fragmentation localized at Si[F])
 SiF+-containing ion-pairs, SiF2+, or F+.
(4)
Here, INAT denotes ionization and normal Auger transition, and Si[Me]C2H4,
Si[F]F, and Si[F]C2H4 stand for molecular orbitals with Si[Me]C2H4, Si[F]F,
and Si[F]C2H4 bonding character, respectively.
In this work, again by means of PEPIPICO spectroscopy, we have
studied site-specific fragmentation caused by Si:1s core-level photoionization
in FSMSE vapor and have compared it with that caused by the Si:2p
photoionization.3
Site-specific fragmentation mechanisms in the Si:1s
ionization are given in the following two sections, although it may partly
repeat the text.
Site-specific fragmentation mechanism for ion production
The site-specific bond dissociation forming CH3+, SiCH3+, and F+ occurs
preferentially at the Si site where the 1s photoionization has taken place [Fig.
3(a)], as summarized in the following reaction schemes.
Si[Me]:1s ionization:
FSMSE + h
 4e- (INAAC)
+ FSMSE4+
↳ (Fragmentation localized at Si[Me])
 CH3+ and/or SiCH3+.
(5)
Si[F]:1s ionization:
FSMSE + h
 4e- (INAAC)
+ FSMSE4+
↳ (Fragmentation localized at Si[F])
 F+.
(6)
Here, INAAC denotes 1s ionization, KLL normal Auger transition, and
duplicated LVV Auger-cascade transitions.
The F+-production mechanism
through the Si[F]-localized dissociation [mechanism (6)] is similar to that
given in the Si[F]:2p ionization [mechanism (4)].
The Si[Me]-site-specificity
of SiCH3+ is higher in the Si:1s ionization [mechanism (5)] than that in the
Si:2p ionization (Fig. 3).
The SiCH3+ production by the Si[Me]-localized
dissociation is enhanced in the Si:1s ionization, owing to the violent
fragmentation mentioned in the text, compared with that in the Si:2p
ionization, where it is more likely to form larger fragments like Si(CH3)3+,
Si(CH3)2+, and Si(CH3)H2+ than SiCH3+ [mechanisms (1)(3)].
Therefore in
the Si:1s ionization the SiCH3+ yield from the Si[Me]-site is stronger than the
yield from the Si[F]-site.
Though SiF3+ is preferentially produced by the Si[Me]:1s ionization
[Figs. 1 and 3(a)], it cannot be produced directly by dissociation localized at
the initially core-ionized Si[Me] site.
However, the Si[Me]:1s ionization can
produce SiF3+ in the following two-step dissociation mechanism through
mechanism (5):
Si[Me]:1s ionization:
FSMSE4+ [mechanism (5)]
↳ F3SiCH2CH2m+Si(CH3)3n+
↳ SiF3+,
where m+n=4.
(7)
Mechanism (7) is closely related to that given for the SiF3+
production from F3SiCH2CH2+ in the Si[Me]:2p ionization [mechanism (2)].3
The first step from FSMSE4+ in mechanism (7) is a dissociation localized at
the initially core-ionized Si[Me] site, and the second dissociation is due to
Coulomb explosion of F3SiCH2CH2m+ (m = 2 or 3) or due to a dissociation of
F3SiCH2CH2m+ (m = 1) with large excess energy.
The Si[Me]-site-specificity
of SiF3+ is higher in the Si:1s ionization [mechanism (7)] than in the Si:2p
ionization [mechanism (2)] (Fig. 3). Since the Si[F]-localized dissociation in
the Si:1s ionization [mechanism (6)] is much too violent for SiF3+ to survive,
its production is reduced in the Si[F]:1s ionization, compared with that in the
Si[F]:2p ionization. As a result, the ratio of the Si[Me]-site-specific SiF3+
production to the Si[F]-site-specific production is larger in the Si:1s
ionization than the ratio in the Si:2p ionization.
A two-step dissociation mechanism similar to mechanism (7) could also
account for the productions of SiF2+, C2H3+, and C2H2+ in the Si[Me]:1s
ionization.
Si[Me]:1s ionization:
F3SiCH2CH2m+ [mechanism (7)]
↳ SiF2+, C2H3+, and/or C2H2+.
(8)
Looking at Fig. 3, SiF2+ shows a very exceptional behavior.
SiF2+ is
preferentially produced by the Si[Me]:1s ionization [mechanism (8)] [Fig.
3(a)] and by the Si[F]:2p ionization [mechanism (4)] [Fig. 3(b)].3
This
counterintuitive result can be understood by looking at the details of the
production mechanisms.
Since F3SiCH2CH2+ formed in the Si[Me]:2p
ionization [mechanism (1)] does not show Coulomb explosion, F3SiCH2CH2+
is mostly stable and its SiF and SiC bonds can survive, and therefore the
Si[Me]:2p ionization is unlikely to form SiF2+.
Instead, the fragmentation
localized at the Si[F] site where the 2p photoionization has taken place
[mechanism (4)] produces SiF2+. In contrast, in the Si:1s ionization, the
two-step dissociation through F3SiCH2CH2m+ at the Si[Me] site [mechanism
(8)] produces SiF2+ rather than the Si[F]-localized bond dissociation like
mechanism (6), which is too violent for SiF2+ to survive.
In the Si:2p ionization, SiF+ is likely to be formed by the fragmentation
localized at the initially core-ionized Si[F] site [mechanism (4)] [Fig. 3(b)].3
However, in the Si:1s ionization, strong competition between
the
FSMSEF3SiCH2CH2m+SiF+ two-step dissociation at the Si[Me] site [like
mechanism (8)] and the fragmentation localized at the initially core-ionized
Si[F] site [like mechanism (6)] reduces the site-specificity of SiF+ [Fig. 3(a)].
Site-specific fragmentation mechanism for ion-pair formation
The Si[Me]:1s ionization increases the productions of CH3+SiCH3+,
CH3+SiF3+,
C2H3+SiF3+,
and
Si+SiF3+
[Fig.
4(a)].
The
Si[Me]-site-specificity of CH3+SiCH3+ in the Si:1s ionization is much higher
than that in the Si:2p ionization (Fig. 4) because the Si[Me]-site-specificity of
SiCH3+ is enhanced in the Si:1s ionization [mechanism (5)], compared with
that in the Si:2p ionization (Fig. 3).
In the Si[Me]:1s ionization,
CH3+SiCH3+ can be formed through the fragmentation localized at the
initially core-ionized Si[Me] site [mechanism (5)].
Si[Me]:1s ionization:
FSMSE4+ [mechanism (5)]
↳ (Fragmentation localized at Si[Me])
 CH3+SiCH3+.
(9)
As mentioned above, CH3+SiF3+, C2H3+SiF3+, and Si+SiF3+ are some
of the most Si[Me]-site-specific ionic-fragments in the Si:1s ionization [Fig.
4(a)].
They cannot be produced immediately by dissociation localized at the
initially core-ionized Si[Me] site.
Instead, the Si[Me]:1s ionization can
produce these ion-pairs in the following two-step dissociation mechanism
through mechanism (5).
Si[Me]:1s ionization:
FSMSE4+ [mechanism (5)]
↳ (Fragmentation localized at Si[Me])
 F3SiCH2CH2k+CH3+ and/or Si+
↳ SiF3+
↳ C2H3+SiF3+,
(10)
where k  3.
The Si[F]:1s ionization increases the productions of H+F+, F+Si+, and
C2H2+SiF+ [Fig. 4(a)].
F+Si+ can be produced immediately by
fragmentation localized at the initially 1s core-ionized Si[F] site.
Si[F]:1s ionization:
FSMSE4+ [mechanism (6)]
↳ (Fragmentation localized at Si[F])
 F+Si+.
(11)
However, H+F+ and C2H2+SiF+ are formed by Si[F]-localized fragmentation
followed by further bond dissociations.
Si[F]:1s ionization:
FSMSE4+ [mechanism (6)]
↳ (Fragmentation localized at Si[F])
 (Further bond dissociations)
 H+F+ or C2H2+SiF+.
(12)
Asymmetry
The site-specific fragmentation pattern is clearly seen in the diagram
showing the corresponding asymmetry, which was given in previous papers
(Ref. 12 and EPAPS document of Ref. 3).
asymmetry = (a  b)/(a + b),
a = A/AN,
b = B/BN.
(13)
A and B denote the numbers of mass-selected ions detected in coincidence
with the core-photoelectrons from the Si[F] and Si[Me] sites, respectively.
AN and BN stand for the numbers of core-photoelectrons from the Si[F] and
Si[Me] sites, respectively. This definition of the asymmetry ensures that if
the core ionizations at the Si[F] and Si[Me] sites lead to the same pattern of
fragmentation, the asymmetry of all ionic fragments is zero.
If ions of a
specific mass are produced only by the core ionization at the Si[F] site, the
asymmetry is +1, and if ions of another specific mass are produced only by
the core ionization at the Si[Me] site, the asymmetry is 1.
Figure 7 shows the asymmetries in the mass spectra of ions detected in
coincidence with the Si:1s and 2p photoelectrons in FSMSE vapor.
Figure 8
shows the asymmetries in the ion-pair formations measured by PEPIPICO
spectroscopy for the Si:1s and 2p ionizations of FSMSE vapor.
In the
measurements we find more photoelectron-peak overlap in the 1s case (Fig.
1) than in the 2p case.3 The reason is purely technical: we had to reduce the
energy resolutions of the monochromator and the electron energy analyzer to
obtain reasonable count rates in the 1s case. However, we can attribute
the observed decrease in 1s asymmetry only partially to the lower energy
resolutions, for example, when we compare the asymmetry of F+ in the 1s
case with the corresponding asymmetry in the 2p case (Fig. 7). Figure 3
clearly shows that there is a significant yield of F+ not only from the overlap
region of the Si[Me] and Si[F] peaks but also from the full binding-energy
interval covered by the Si[Me] peak.
So the observed reduction of the
asymmetry is a property of the 1s ionization and no experimental artifact.
FIG. 7.
Asymmetry, a measure exhibiting site-specificity [Eq. (13)], in ion
production measured in FSMSE vapor by PEPICO spectroscopy.
ionization.
(a) Si:1 s
(b) Si:2p ionization (from Nagaoka et al. [Ref. 3]).
FIG.
8.
Asymmetry, a measure exhibiting site-specificity [Eq. (13)], in ion-pair
formation measured in FSMSE vapor by PEPIPICO spectroscopy.
ionization.
(b) Si:2p ionization (from Nagaoka et al. [Ref. 3]).
(a) Si:1s