220
9 Applications: Aqueous Interfaces
spectral assignment. Just as an example, two equivalent O–H vibrations of a single
isolated water molecule are coupled and split into symmetric and antisymmetric
vibrational modes, which have orthogonal transition dipole moments. Therefore,
the single water molecule exhibits two different directions of transition moments.
We could not determine the orientation of the molecule from the observed direction
of transition moment unless we know the character of the vibrational mode. In
condensed phase, the water O–H moieties are coupled by the hydrogen bonds,
and consequently the vibrational modes are delocalized among intermolecular O–H
moieties. Such delocalized vibrations are often beyond our intuitive understanding.
To properly assign the observed O–H vibrations in relation to the interface structure,
theoretical analysis is particularly required.
Here we summarize the current status of the theoretical analysis of SFG spectra
of water and aqueous interfaces. We discuss the surfaces of liquid water, ice, and
electrolyte solutions. Besides the air-aqueous interfaces, liquid/liquid (water/oil)
and water/monolayer interfaces are also treated. The knowledge described below
should be regarded as fundamentals for future progress. In this chapter, we discuss
the SFG spectra of O–H stretching region in the SSP polarization, the most
commonly employed geometry, unless otherwise noted.
9.1 Water Surface
The vibrational spectra of water surface have been computationally analyzed by
a number of researchers [4, 10, 13, 29, 52, 56, 57, 62, 73, 74, 80, 81, 103, 110].
Here we briefly show some basic information of water surface derived from the
computational analysis of the SFG spectra.
Historical perspective The SFG spectrum of water surface in the O–H stretching
region was first reported by Shen and co-workers [14], and subsequently studied
by other groups including Shultz and Richmond [88, 94] in the pioneer stage of
SFG spectroscopy. Since then the water surface is one of the most intensively
studied surfaces by SFG [9, 93]. The spectral shape has been shown in Fig. 1.1
in Chap. 1, which apparently consists of two bands, a sharp band at 3700 cm −1 and
a broad one at 3000∼3600 cm −1 . We note that the spectral lineshapes reported in
early period [14, 88, 94] were noticeably different from each other. This problem
of disagreement has been resolved by the progress of spectroscopy, and Fig. 1.1 is
currently considered to be the established intensity spectrum of liquid water in the
O–H stretching region.
First MD calculation of the water SFG spectrum was carried out by Morita and
Hynes on the basis of the energy representation in Sect. 4.1 [57]. The calculated
SFG spectrum of water surface reproduced the two-band structure in the O–H
stretching region. The computation predicted the Im[χ (2) ] spectrum that the sharp
band at 3700 cm −1 has a positive amplitude of Im[χ (2) ] while the broad band at
3000∼3600 cm −1 has a negative sign (see Figs. 8.2a and 9.1). As we have argued in
9 Applications: Aqueous Interfaces
spectral assignment. Just as an example, two equivalent O–H vibrations of a single
isolated water molecule are coupled and split into symmetric and antisymmetric
vibrational modes, which have orthogonal transition dipole moments. Therefore,
the single water molecule exhibits two different directions of transition moments.
We could not determine the orientation of the molecule from the observed direction
of transition moment unless we know the character of the vibrational mode. In
condensed phase, the water O–H moieties are coupled by the hydrogen bonds,
and consequently the vibrational modes are delocalized among intermolecular O–H
moieties. Such delocalized vibrations are often beyond our intuitive understanding.
To properly assign the observed O–H vibrations in relation to the interface structure,
theoretical analysis is particularly required.
Here we summarize the current status of the theoretical analysis of SFG spectra
of water and aqueous interfaces. We discuss the surfaces of liquid water, ice, and
electrolyte solutions. Besides the air-aqueous interfaces, liquid/liquid (water/oil)
and water/monolayer interfaces are also treated. The knowledge described below
should be regarded as fundamentals for future progress. In this chapter, we discuss
the SFG spectra of O–H stretching region in the SSP polarization, the most
commonly employed geometry, unless otherwise noted.
9.1 Water Surface
The vibrational spectra of water surface have been computationally analyzed by
a number of researchers [4, 10, 13, 29, 52, 56, 57, 62, 73, 74, 80, 81, 103, 110].
Here we briefly show some basic information of water surface derived from the
computational analysis of the SFG spectra.
Historical perspective The SFG spectrum of water surface in the O–H stretching
region was first reported by Shen and co-workers [14], and subsequently studied
by other groups including Shultz and Richmond [88, 94] in the pioneer stage of
SFG spectroscopy. Since then the water surface is one of the most intensively
studied surfaces by SFG [9, 93]. The spectral shape has been shown in Fig. 1.1
in Chap. 1, which apparently consists of two bands, a sharp band at 3700 cm −1 and
a broad one at 3000∼3600 cm −1 . We note that the spectral lineshapes reported in
early period [14, 88, 94] were noticeably different from each other. This problem
of disagreement has been resolved by the progress of spectroscopy, and Fig. 1.1 is
currently considered to be the established intensity spectrum of liquid water in the
O–H stretching region.
First MD calculation of the water SFG spectrum was carried out by Morita and
Hynes on the basis of the energy representation in Sect. 4.1 [57]. The calculated
SFG spectrum of water surface reproduced the two-band structure in the O–H
stretching region. The computation predicted the Im[χ (2) ] spectrum that the sharp
band at 3700 cm −1 has a positive amplitude of Im[χ (2) ] while the broad band at
3000∼3600 cm −1 has a negative sign (see Figs. 8.2a and 9.1). As we have argued in