9.3 Electrolyte Solution Surfaces
225
3200 cm −1 is substantially augmented by the charge transfer, and accordingly the
classical model that omits the charge transfer effect could not sufficiently reproduce
the remarkable intensity. The band intensity is sensitive to the local disorder of the
tetrahedral ice structure as well as the charge transfer. The ordered ice structure
facilitates delocalized O–H vibrations, which couple with the charge transfer and
augment the band intensity. The large temperature dependence of the band intensity
is elucidated with the sensitivity to the local disorder.
The intense band is mainly attributed to the bilayer-stitching O–H vibrations
between the first (B1) and second (B2) bilayers of the ice surface in Fig. 9.2b.
By looking at the panel (b), one may think that the signals from the upward O–
H (green) and downward O–H (blue) vibrations should cancel each other in the
ideal ice lattice. However, the MD analysis showed that the upward and downward
O–H bonds between B1 and B2 layers are actually inequivalent because of more
structural disorder in the B1 bilayer than that in B2, and the broken symmetry near
the surface causes the strong SFG signal [71]. The larger disorder in the B1 layer is
indicative of the surface premelting in the atomic level, and the premelting develops
toward deeper bilayers with increasing temperature [92].
The above interpretation of the intense SFG band should be examined in
comparison with experimental measurement of the Im[χ (2) ] spectrum. However,
experimentally reported lineshapes of the Im[χ (2) ] spectrum are under serious
controversy at present [68, 71, 97], though those of the SFG intensity spectrum
agree. Otsuki et al. [71] and Smit et al. [97] reported a negative main band
of the Im[χ (2) ] spectrum, which is in accord with previous theoretical studies
[11, 31, 35, 112], whereas Nojima et al. [68] experimentally reported a positive band.
This problem of Im[χ (2) ] spectrum should be resolved to establish the spectrum.
There remain some important issues to be elucidated in relation to the SFG spectrum
of ice, including the proton ordering near the ice surface [68, 102, 112] and the
contribution of bulk signal in SFG [95, 112].
9.3 Electrolyte Solution Surfaces
Understanding of electrolyte aqueous solution surfaces has been remarkably
advanced in this century [43, 89, 115]. In early days of the twentieth century,
people believed that water surface is generally void of ions, on the basis of surface
tension measurements [21, 84] and the theory of dielectrics [70]. This picture
appears to be consistent to intuitive idea that ions prefer to be strongly hydrated
in the interior of bulk rather than to expose themselves to the air. This intuitive
picture was challenged in 2001 with MD simulation by Jungwirth and Tobias
[42]. They predicted that some anions, such as I − or Br − , rather prefer to be
exposed to the air as shown in Fig. 9.3. This prediction stimulated experimental
studies of electrolyte solution surfaces by various means, including SFG, SHG and
photoelectron spectroscopies [43, 72, 77]. The SFG spectroscopy played one of
225
3200 cm −1 is substantially augmented by the charge transfer, and accordingly the
classical model that omits the charge transfer effect could not sufficiently reproduce
the remarkable intensity. The band intensity is sensitive to the local disorder of the
tetrahedral ice structure as well as the charge transfer. The ordered ice structure
facilitates delocalized O–H vibrations, which couple with the charge transfer and
augment the band intensity. The large temperature dependence of the band intensity
is elucidated with the sensitivity to the local disorder.
The intense band is mainly attributed to the bilayer-stitching O–H vibrations
between the first (B1) and second (B2) bilayers of the ice surface in Fig. 9.2b.
By looking at the panel (b), one may think that the signals from the upward O–
H (green) and downward O–H (blue) vibrations should cancel each other in the
ideal ice lattice. However, the MD analysis showed that the upward and downward
O–H bonds between B1 and B2 layers are actually inequivalent because of more
structural disorder in the B1 bilayer than that in B2, and the broken symmetry near
the surface causes the strong SFG signal [71]. The larger disorder in the B1 layer is
indicative of the surface premelting in the atomic level, and the premelting develops
toward deeper bilayers with increasing temperature [92].
The above interpretation of the intense SFG band should be examined in
comparison with experimental measurement of the Im[χ (2) ] spectrum. However,
experimentally reported lineshapes of the Im[χ (2) ] spectrum are under serious
controversy at present [68, 71, 97], though those of the SFG intensity spectrum
agree. Otsuki et al. [71] and Smit et al. [97] reported a negative main band
of the Im[χ (2) ] spectrum, which is in accord with previous theoretical studies
[11, 31, 35, 112], whereas Nojima et al. [68] experimentally reported a positive band.
This problem of Im[χ (2) ] spectrum should be resolved to establish the spectrum.
There remain some important issues to be elucidated in relation to the SFG spectrum
of ice, including the proton ordering near the ice surface [68, 102, 112] and the
contribution of bulk signal in SFG [95, 112].
9.3 Electrolyte Solution Surfaces
Understanding of electrolyte aqueous solution surfaces has been remarkably
advanced in this century [43, 89, 115]. In early days of the twentieth century,
people believed that water surface is generally void of ions, on the basis of surface
tension measurements [21, 84] and the theory of dielectrics [70]. This picture
appears to be consistent to intuitive idea that ions prefer to be strongly hydrated
in the interior of bulk rather than to expose themselves to the air. This intuitive
picture was challenged in 2001 with MD simulation by Jungwirth and Tobias
[42]. They predicted that some anions, such as I − or Br − , rather prefer to be
exposed to the air as shown in Fig. 9.3. This prediction stimulated experimental
studies of electrolyte solution surfaces by various means, including SFG, SHG and
photoelectron spectroscopies [43, 72, 77]. The SFG spectroscopy played one of