230
9 Applications: Aqueous Interfaces
by the electric double layer. However, the Im[χ (2) ] amplitude of NaI (red line)
in panel (b) shows a rather modest perturbation to the positive direction. The
difference between (b) and (b ) indicates that Eq. (5.27 ) or Eq. (3.41) overestimates
the positive perturbation on χ (2) . The deviation is obviously attributed to the cross
correlation among neighbor molecules (l = m) in Eq. (5.27). The cross correlation is
illustrated in Fig. 9.4c in the case of SSP polarization, where the z-component dipole
is relevant through the χ
(2)
yyz element (see Eq. (3.49)). In the NaI solution surface,
the induced z-component dipoles of different molecules tend to correlate antiparallel, which thereby suppresses the total amplitude of dipole. This correlation
effect becomes obvious in SFG spectra including surface-active and very polarizable
species, such as I − .
SPS polarization The SPS spectra of aqueous systems could provide complementary information to the SSP spectra. However, the SPS spectra have been less
explored than SSP, because the signal intensity is generally weak and the analysis
is less intuitive. The SPS spectra are associated to the χ
(2)
yzy element, which involves
the y-component dipole. Since the relevant dipole is parallel to the interface, the
mechanism of SPS spectra is not interpreted in terms of up/down dipole orientation
of surface species. To understand the SPS spectra even in qualitative sense, the MD
analysis is often required.
In the SPS polarization, the correlation effect discussed above has an opposite
influence on the χ (2) amplitude, as illustrated in Fig. 9.4c. In contrast to the zcomponent dipole in SSP, the induced y-component dipoles in SPS tend to correlate
parallel in the electric double layer and thus enhance the total χ (2) amplitude
for the SPS polarization. The constructive effect of dipole correlation in the SPS
case has been demonstrated by MD calculation and consistently elucidated the
experimental spectrum [28]. The SPS spectrum of NaI solution provides another
decisive evidence for the electric double layer formation by Na + and I − .
9.3.2 Buried Ions: F − , SO 2−
4
In contrast to the ions in the preceding subsection, some other ions are repelled from
the water surface and buried, in accord with the traditional picture of interfacial ions.
MD simulation predicts that F − and SO
2−
4 are typical examples of such buried ions.
One may expect that such buried electrolytes little perturb the surface structure,
since these ions do not penetrate into the topmost layer. Yet the SFG spectroscopy
can report perturbed SFG spectra for some of these electrolyte solutions, which
implies the water surface is still perturbed by the ions. The mechanism of spectral
perturbation and surface structure are elucidated with the help of MD analysis.
Figure 9.6a shows the computational [23] and experimental [17, 49] SFG spectra
of NaF and Na 2 SO 4 solutions. We find that the NaF (blue) and Na 2 SO 4 (red)
9 Applications: Aqueous Interfaces
by the electric double layer. However, the Im[χ (2) ] amplitude of NaI (red line)
in panel (b) shows a rather modest perturbation to the positive direction. The
difference between (b) and (b ) indicates that Eq. (5.27 ) or Eq. (3.41) overestimates
the positive perturbation on χ (2) . The deviation is obviously attributed to the cross
correlation among neighbor molecules (l = m) in Eq. (5.27). The cross correlation is
illustrated in Fig. 9.4c in the case of SSP polarization, where the z-component dipole
is relevant through the χ
(2)
yyz element (see Eq. (3.49)). In the NaI solution surface,
the induced z-component dipoles of different molecules tend to correlate antiparallel, which thereby suppresses the total amplitude of dipole. This correlation
effect becomes obvious in SFG spectra including surface-active and very polarizable
species, such as I − .
SPS polarization The SPS spectra of aqueous systems could provide complementary information to the SSP spectra. However, the SPS spectra have been less
explored than SSP, because the signal intensity is generally weak and the analysis
is less intuitive. The SPS spectra are associated to the χ
(2)
yzy element, which involves
the y-component dipole. Since the relevant dipole is parallel to the interface, the
mechanism of SPS spectra is not interpreted in terms of up/down dipole orientation
of surface species. To understand the SPS spectra even in qualitative sense, the MD
analysis is often required.
In the SPS polarization, the correlation effect discussed above has an opposite
influence on the χ (2) amplitude, as illustrated in Fig. 9.4c. In contrast to the zcomponent dipole in SSP, the induced y-component dipoles in SPS tend to correlate
parallel in the electric double layer and thus enhance the total χ (2) amplitude
for the SPS polarization. The constructive effect of dipole correlation in the SPS
case has been demonstrated by MD calculation and consistently elucidated the
experimental spectrum [28]. The SPS spectrum of NaI solution provides another
decisive evidence for the electric double layer formation by Na + and I − .
9.3.2 Buried Ions: F − , SO 2−
4
In contrast to the ions in the preceding subsection, some other ions are repelled from
the water surface and buried, in accord with the traditional picture of interfacial ions.
MD simulation predicts that F − and SO
2−
4 are typical examples of such buried ions.
One may expect that such buried electrolytes little perturb the surface structure,
since these ions do not penetrate into the topmost layer. Yet the SFG spectroscopy
can report perturbed SFG spectra for some of these electrolyte solutions, which
implies the water surface is still perturbed by the ions. The mechanism of spectral
perturbation and surface structure are elucidated with the help of MD analysis.
Figure 9.6a shows the computational [23] and experimental [17, 49] SFG spectra
of NaF and Na 2 SO 4 solutions. We find that the NaF (blue) and Na 2 SO 4 (red)