9.2 Ice Surface
223
The two sub-bands at 3200 and 3400 cm −1 are widely seen in the O–H bands
of a variety of aqueous systems, though they exhibits varying spectral lineshapes
and relative intensities. These O–H band shapes of various aqueous systems are
likely influenced by inhomogeneous species as well as vibrational couplings.
Previous time-resolved SFG measurements confirmed that the O–H band includes
comparable homogeneous and inhomogeneous widths[8, 25, 51]. Comprehensive
understanding of those band shapes is one of the principal aims of the further SFG
analysis of water surface.
Bend vibration Most SFG studies on water surface have dealt with the O–H
stretching vibrations. The O–H stretching vibration is easy to measure and provides
rich information on hydrogen-bonding environment, whereas the spectral analysis is
complicated. There are some attempts to analyze other modes of the SFG spectrum,
such as bend and libration [46, 60, 64, 75]. Here we discuss the bend mode in the
SFG spectrum of water.
The SFG measurement of the bend band was first carried out by Benderskii
and co-workers [108], and subsequently the heterodyne measurement of the bend
band was reported by Kundu et al. [47]. The measured Im[χ (2) ] spectrum revealed
a positive band over the bend frequency region, though a previous simulation
by Nagata et al. had predicted a bipolar shape of the Im[χ (2) ] band [60]. The
qualitative disagreement between the simulation and experiment was elucidated
by further theoretical analysis [47], concluding that the bend band of SFG is
actually dominated by the χ IQB term of bulk quadrupole origin discussed in Chap. 7.
The sign of Im[χ (2) ] in the bend region is therefore insensitive to the molecular
orientation at surface.
We note in passing that the role of quadrupole contribution on SFG spectra was
also found in benzene [45, 50], as discussed in Sect. 10.2. Systematic investigation
of the quadrupole effect on various systems is desired.
9.2 Ice Surface
The structure of ice surface has been drawing wide attention for more than a
century. Faraday suggested premelting of ice surface [15] below the freezing
temperature, and subsequently a number of experiments using optical, magnetic
or electrical means confirmed that the premelting layer is developed in several tens
of nanometers at about T −10 ◦ C [79]. The SFG spectroscopy is powerful to
selectively probe the microscopic hydrogen bonding environment at the ice surface.
The experimental SFG spectrum of ice Ih basal surface is shown in Fig. 9.2. We
notice the remarkably intense band at about 3200 cm −1 in the ice spectrum, much
stronger than that of the water spectrum. The intensity further augments with
lowering temperature [19, 113]. The origin of this remarkable band is understood
with the help of MD analysis.
223
The two sub-bands at 3200 and 3400 cm −1 are widely seen in the O–H bands
of a variety of aqueous systems, though they exhibits varying spectral lineshapes
and relative intensities. These O–H band shapes of various aqueous systems are
likely influenced by inhomogeneous species as well as vibrational couplings.
Previous time-resolved SFG measurements confirmed that the O–H band includes
comparable homogeneous and inhomogeneous widths[8, 25, 51]. Comprehensive
understanding of those band shapes is one of the principal aims of the further SFG
analysis of water surface.
Bend vibration Most SFG studies on water surface have dealt with the O–H
stretching vibrations. The O–H stretching vibration is easy to measure and provides
rich information on hydrogen-bonding environment, whereas the spectral analysis is
complicated. There are some attempts to analyze other modes of the SFG spectrum,
such as bend and libration [46, 60, 64, 75]. Here we discuss the bend mode in the
SFG spectrum of water.
The SFG measurement of the bend band was first carried out by Benderskii
and co-workers [108], and subsequently the heterodyne measurement of the bend
band was reported by Kundu et al. [47]. The measured Im[χ (2) ] spectrum revealed
a positive band over the bend frequency region, though a previous simulation
by Nagata et al. had predicted a bipolar shape of the Im[χ (2) ] band [60]. The
qualitative disagreement between the simulation and experiment was elucidated
by further theoretical analysis [47], concluding that the bend band of SFG is
actually dominated by the χ IQB term of bulk quadrupole origin discussed in Chap. 7.
The sign of Im[χ (2) ] in the bend region is therefore insensitive to the molecular
orientation at surface.
We note in passing that the role of quadrupole contribution on SFG spectra was
also found in benzene [45, 50], as discussed in Sect. 10.2. Systematic investigation
of the quadrupole effect on various systems is desired.
9.2 Ice Surface
The structure of ice surface has been drawing wide attention for more than a
century. Faraday suggested premelting of ice surface [15] below the freezing
temperature, and subsequently a number of experiments using optical, magnetic
or electrical means confirmed that the premelting layer is developed in several tens
of nanometers at about T −10 ◦ C [79]. The SFG spectroscopy is powerful to
selectively probe the microscopic hydrogen bonding environment at the ice surface.
The experimental SFG spectrum of ice Ih basal surface is shown in Fig. 9.2. We
notice the remarkably intense band at about 3200 cm −1 in the ice spectrum, much
stronger than that of the water spectrum. The intensity further augments with
lowering temperature [19, 113]. The origin of this remarkable band is understood
with the help of MD analysis.