1.2 Visible-Infrared SFG Vibrational Spectroscopy
5
2900
3000
12
10
8
6
4
0
2
3200 3400 3600 3800
3100 3300 3500 3700 3900
Frequency (cm -1 )
Frequency (cm -1 )
SF Intensity (a.u.)
Fig. 1.1 (Left panel) SFG spectrum of water surface in SSP configuration [16]. (Right panel)
Infrared absorption spectrum of water [2]. (Reprinted with permission from Ref. [16]. Copyright
2003 American Chemical Society.)
These features render the SFG spectroscopy a versatile tool of surface characterization, particularly to wet or soft interfaces [12]. Microscopic measurement of
wet interfaces has been limited due to scarcity of available experimental means,
compared to that of solid surfaces. The SFG spectroscopy is considered as a unique,
powerful technique to explore the wet or soft interfaces.
To exemplify the SFG spectroscopy, two typical visible-infrared SFG spectra are
illustrated, water and alkyl chains. Left panel of Fig. 1.1 displays the SFG spectrum
of liquid-vapor interface of water in the O-H stretching frequency region, 2900 ∼
3900 cm −1 . In the right panel, the conventional infrared (IR) absorption spectrum of
bulk water is shown for comparison. Comparing the two spectra of O-H stretching
vibrations in Fig. 1.1, qualitative difference in spectral structure is apparent. These
spectra essentially reflect the differences in structure of surface and bulk water. The
SFG spectrum is composed of a sharp band at about 3700 cm −1 and a broad, redshifted band at 3000 ∼ 3600 cm −1 . The former band of SFG finds no apparent
counterpart in the IR spectrum, and is attributed to the O-H moieties at the topmost
surface layer which are free from hydrogen bonds, called free O-H or dangling
O-H. The latter, broad band is assigned to hydrogen-bonding O-H moieties in a
few monolayer regions at the surface. The clear difference of SFG and IR spectra
in Fig. 1.1 demonstrates the surface sensitivity of the SFG spectroscopy to detect
structural features characteristic of surface water.
[Problem 1.3] Answer two frequencies of O-H stretching vibration of an isolated
water molecule (symmetric and anti-symmetric stretching). Where are these frequencies located in the spectra of Fig. 1.1?
Another example of SFG and IR is given in Fig. 1.2, C-H stretching vibrational
spectra of self-assembled monolayer of alkane thiol on gold substrate. Panel (a)
is the observed SFG spectrum which involves the strong background from gold
substrate, and panel (b) is the calibrated SFG spectrum of C-H stretching with
the gold background eliminated. Comparing these panels (a) or (b) with the
5
2900
3000
12
10
8
6
4
0
2
3200 3400 3600 3800
3100 3300 3500 3700 3900
Frequency (cm -1 )
Frequency (cm -1 )
SF Intensity (a.u.)
Fig. 1.1 (Left panel) SFG spectrum of water surface in SSP configuration [16]. (Right panel)
Infrared absorption spectrum of water [2]. (Reprinted with permission from Ref. [16]. Copyright
2003 American Chemical Society.)
These features render the SFG spectroscopy a versatile tool of surface characterization, particularly to wet or soft interfaces [12]. Microscopic measurement of
wet interfaces has been limited due to scarcity of available experimental means,
compared to that of solid surfaces. The SFG spectroscopy is considered as a unique,
powerful technique to explore the wet or soft interfaces.
To exemplify the SFG spectroscopy, two typical visible-infrared SFG spectra are
illustrated, water and alkyl chains. Left panel of Fig. 1.1 displays the SFG spectrum
of liquid-vapor interface of water in the O-H stretching frequency region, 2900 ∼
3900 cm −1 . In the right panel, the conventional infrared (IR) absorption spectrum of
bulk water is shown for comparison. Comparing the two spectra of O-H stretching
vibrations in Fig. 1.1, qualitative difference in spectral structure is apparent. These
spectra essentially reflect the differences in structure of surface and bulk water. The
SFG spectrum is composed of a sharp band at about 3700 cm −1 and a broad, redshifted band at 3000 ∼ 3600 cm −1 . The former band of SFG finds no apparent
counterpart in the IR spectrum, and is attributed to the O-H moieties at the topmost
surface layer which are free from hydrogen bonds, called free O-H or dangling
O-H. The latter, broad band is assigned to hydrogen-bonding O-H moieties in a
few monolayer regions at the surface. The clear difference of SFG and IR spectra
in Fig. 1.1 demonstrates the surface sensitivity of the SFG spectroscopy to detect
structural features characteristic of surface water.
[Problem 1.3] Answer two frequencies of O-H stretching vibration of an isolated
water molecule (symmetric and anti-symmetric stretching). Where are these frequencies located in the spectra of Fig. 1.1?
Another example of SFG and IR is given in Fig. 1.2, C-H stretching vibrational
spectra of self-assembled monolayer of alkane thiol on gold substrate. Panel (a)
is the observed SFG spectrum which involves the strong background from gold
substrate, and panel (b) is the calibrated SFG spectrum of C-H stretching with
the gold background eliminated. Comparing these panels (a) or (b) with the