Classical- and Heterodyne-Detected Vibrational Sum …
103
followed by a positive band around 3200 cm
−1 . Morita and coworkers [26, 27]
simulated the Imχ
(2) spectra reproducing the experimental Imχ
(2) including the
positive band around 3200 cm
−1 . The positive sign of the 3700 cm
−1 represents that
the dangling OH is indeed pointed towards the air (i.e. away from the aqueous phase)
while the liquid like water has a net H-down orientation (Fig. 7d). Nevertheless, the
origin of the 3200 cm
−1 band was debated—unlike Shen and coworkers, Morita
as well as Tahara and Bonn suggested that the 3200 cm
−1 positive band is due to
strongly coupled water pairs at interface [28]. Very recently, Tahara and Yamaguchi
[29, 30] observed that the 3200 cm
−1 positive band is nearly non-existent in the Imχ
(2)
spectra measured with HD-VSFG spectrometer of precisely controlled optical path
(using a laser displacement sensor) and air-D 2 O interface (instead of z-cut quartz)
as the reference. In our laboratory, we used mathematical phase-correction of the
experimental χ
(2) spectra by internal referencing of zero Imχ
(2) signal in the region
of non-resonance (say around 2700 cm
−1 for the OH stretch band of water) [31,
32]. The positive band around 3200 cm
−1 is not observed in our phase–corrected
spectra. Recently, we have also measured the air-water interface with high resolution
(~100 nm) optical displacement sensor [33, 34] and the Imχ
(2) does not show any
distinct positive band around 3200 cm
−1 (Fig. 7b). Very recently, Tian and coworkers
[35–37] as well as other groups [38, 39] reported no distinguishable band around
3200 cm
−1 for the air-H 2 O interface. Thus, the emerging picture of water structure
at the pristine air-water interface reconcile with the fact that the interfacial water has
a continuous “liquid-like” structure with a net H-down orientation in the H-bonded
region of the OH stretch band (3000-3600 cm
−1 ); while in the non-H-bonded region
(>3600 cm
−1 ), the dangling OH is pointed towards the air (H-up), which originates
from the top-most water layer (Fig. 7d).
Along with Imχ
(2) the HD-VSFG measurement provides the Reχ
(2) spectrum
of the air-water interface (Fig. 7b). The Reχ
(2) spectrum shows a dispersive band
shape along with a negative non-resonant back ground signal embedded into it. In
the region below 3000 cm
−1 , where the Imχ
(2) signal is zero, the Reχ
(2) spectrum
is negative in sign and appears as a constant due to the non-resonant background.
The
χ
(2)
2 spectrum (Fig. 7c) deduced from the experimentally measured Imχ
(2)
and Reχ
(2) spectra is in qualitative agreement with the
χ
(2)
2 measured by narrow
band classical VSFG spectrometer (Fig. 7a). Unlike the Imχ
(2) spectrum, the sign
of the squared-χ
(2) spectrum is positive throughout the OH stretch region due to
squaring of χ
(2) . In summary, the HD-VSFG measurement is clearly an advantage
and more informative in elucidating the structure and orientation of molecules at a
soft interface relative to that of its classical-analogue.
103
followed by a positive band around 3200 cm
−1 . Morita and coworkers [26, 27]
simulated the Imχ
(2) spectra reproducing the experimental Imχ
(2) including the
positive band around 3200 cm
−1 . The positive sign of the 3700 cm
−1 represents that
the dangling OH is indeed pointed towards the air (i.e. away from the aqueous phase)
while the liquid like water has a net H-down orientation (Fig. 7d). Nevertheless, the
origin of the 3200 cm
−1 band was debated—unlike Shen and coworkers, Morita
as well as Tahara and Bonn suggested that the 3200 cm
−1 positive band is due to
strongly coupled water pairs at interface [28]. Very recently, Tahara and Yamaguchi
[29, 30] observed that the 3200 cm
−1 positive band is nearly non-existent in the Imχ
(2)
spectra measured with HD-VSFG spectrometer of precisely controlled optical path
(using a laser displacement sensor) and air-D 2 O interface (instead of z-cut quartz)
as the reference. In our laboratory, we used mathematical phase-correction of the
experimental χ
(2) spectra by internal referencing of zero Imχ
(2) signal in the region
of non-resonance (say around 2700 cm
−1 for the OH stretch band of water) [31,
32]. The positive band around 3200 cm
−1 is not observed in our phase–corrected
spectra. Recently, we have also measured the air-water interface with high resolution
(~100 nm) optical displacement sensor [33, 34] and the Imχ
(2) does not show any
distinct positive band around 3200 cm
−1 (Fig. 7b). Very recently, Tian and coworkers
[35–37] as well as other groups [38, 39] reported no distinguishable band around
3200 cm
−1 for the air-H 2 O interface. Thus, the emerging picture of water structure
at the pristine air-water interface reconcile with the fact that the interfacial water has
a continuous “liquid-like” structure with a net H-down orientation in the H-bonded
region of the OH stretch band (3000-3600 cm
−1 ); while in the non-H-bonded region
(>3600 cm
−1 ), the dangling OH is pointed towards the air (H-up), which originates
from the top-most water layer (Fig. 7d).
Along with Imχ
(2) the HD-VSFG measurement provides the Reχ
(2) spectrum
of the air-water interface (Fig. 7b). The Reχ
(2) spectrum shows a dispersive band
shape along with a negative non-resonant back ground signal embedded into it. In
the region below 3000 cm
−1 , where the Imχ
(2) signal is zero, the Reχ
(2) spectrum
is negative in sign and appears as a constant due to the non-resonant background.
The
χ
(2)
2 spectrum (Fig. 7c) deduced from the experimentally measured Imχ
(2)
and Reχ
(2) spectra is in qualitative agreement with the
χ
(2)
2 measured by narrow
band classical VSFG spectrometer (Fig. 7a). Unlike the Imχ
(2) spectrum, the sign
of the squared-χ
(2) spectrum is positive throughout the OH stretch region due to
squaring of χ
(2) . In summary, the HD-VSFG measurement is clearly an advantage
and more informative in elucidating the structure and orientation of molecules at a
soft interface relative to that of its classical-analogue.