4
1 Introduction
the SFG emission is convenient to experimental detection of its weak signal. This is
a consequence of interference among oscillating polarizations in spatially different
regions, which is analogous to the Bragg’s law of diffraction [11].
The coherent nature also implicates that the SFG signals emitted from different
oscillators (vibrating atomic groups) can interfere each other, which may complicate
the assignment of experimental SFG spectra. In the case that an observed SFG
spectrum originates from different oscillators in the same frequency region, the
interference of the overlapping source signals may enhance or suppress the intensity
of the SFG signal owing to the phase relation. Therefore, the observed SFG intensity
is not amenable to straightforward decomposition into the source oscillators. (In
other words, weak SFG signal does not necessarily mean that there is no or weak
SFG source. The weak intensity could be a consequence of cancelling interference.)
The interpretation of the SFG spectroscopy has to take account of the interference
effects, as discussed later.
1.2 Visible-Infrared SFG Vibrational Spectroscopy
The most common application of SFG to surface nonlinear spectroscopy employs
the combination of incident visible and infrared lights, and presents the SFG signal
as a function of infrared frequency ω 2 with the visible frequency ω 1 fixed. Such
measurement is considered as a vibrational spectroscopy of the interface. Some
features of the vibrational SFG spectroscopy are briefly summarized below.
1. It is possible to observe molecular species at the interface selectively, even though
overwhelming amount of the same species exists in the isotropic bulk region. The
sensitivity of detection is fairly high, and a submonolayer amount of the surface
species can be easily detected.
2. The vibrational spectra provide highly specific information to the molecular
species at the interface. The frequency shift also offers useful information on
local environment of the interface. Molecular orientation at the interface can
be measured, by choosing proper combinations of light polarizations of visible,
infrared and SFG.
3. Since the interface selectivity is entirely attributed to the symmetry reasons, no
vacuum condition is necessary in principle. The optical measurement is suitable
to in-situ detection of a variety of interfaces. This technique is also applicable to
buried interfaces, such as liquid-liquid or liquid-solid, as long as the interfaces
are accessible by light.
4. This technique is suitable to study ultrafast dynamics at the interfaces, by making
use of high temporal resolution of pulse lasers.
1 Introduction
the SFG emission is convenient to experimental detection of its weak signal. This is
a consequence of interference among oscillating polarizations in spatially different
regions, which is analogous to the Bragg’s law of diffraction [11].
The coherent nature also implicates that the SFG signals emitted from different
oscillators (vibrating atomic groups) can interfere each other, which may complicate
the assignment of experimental SFG spectra. In the case that an observed SFG
spectrum originates from different oscillators in the same frequency region, the
interference of the overlapping source signals may enhance or suppress the intensity
of the SFG signal owing to the phase relation. Therefore, the observed SFG intensity
is not amenable to straightforward decomposition into the source oscillators. (In
other words, weak SFG signal does not necessarily mean that there is no or weak
SFG source. The weak intensity could be a consequence of cancelling interference.)
The interpretation of the SFG spectroscopy has to take account of the interference
effects, as discussed later.
1.2 Visible-Infrared SFG Vibrational Spectroscopy
The most common application of SFG to surface nonlinear spectroscopy employs
the combination of incident visible and infrared lights, and presents the SFG signal
as a function of infrared frequency ω 2 with the visible frequency ω 1 fixed. Such
measurement is considered as a vibrational spectroscopy of the interface. Some
features of the vibrational SFG spectroscopy are briefly summarized below.
1. It is possible to observe molecular species at the interface selectively, even though
overwhelming amount of the same species exists in the isotropic bulk region. The
sensitivity of detection is fairly high, and a submonolayer amount of the surface
species can be easily detected.
2. The vibrational spectra provide highly specific information to the molecular
species at the interface. The frequency shift also offers useful information on
local environment of the interface. Molecular orientation at the interface can
be measured, by choosing proper combinations of light polarizations of visible,
infrared and SFG.
3. Since the interface selectivity is entirely attributed to the symmetry reasons, no
vacuum condition is necessary in principle. The optical measurement is suitable
to in-situ detection of a variety of interfaces. This technique is also applicable to
buried interfaces, such as liquid-liquid or liquid-solid, as long as the interfaces
are accessible by light.
4. This technique is suitable to study ultrafast dynamics at the interfaces, by making
use of high temporal resolution of pulse lasers.