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S. Roy et al.
water is the most abundant liquid on earth, aqueous interfaces are wide-spread in
biotic and abiotic systems including ocean surface (air-water interface), mineral
surface (gas-solid or liquid-solid interface), cell and cellular organelles (lipid or
protein-water interface). Because of the anisotropy and rapidly varying number
density of water, most ions and molecules exhibit an orientational preference and
non-uniform distribution (adsorption/depletion) across the interface, giving rise to
interface-selective physicochemical processes. Molecular level understanding of
such interfacial processes requires selective probing of the interface with molecular
precision.
Conventional spectroscopies such as absorption, fluorescence and Ramanscattering are not interface-selective and hence, the inherently weak signal of interfacial molecules (due to extreme thinness of the interface, ~a few nm) gets buried into
the huge signal of the bulk phase. Particle scattering based surface-selective spectroscopies such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy as well as nano-scale microscopy imaging techniques including scanning
(or transmission) electron microscope are incompatible with most liquid surfaces
because of the dynamic nature of molecules on liquid surface and the need of ultrahigh vacuum for such techniques. It is important to note that though near ambient
pressure (~few mbar) XPS (NAP-XPS) overcomes the stringent requirement of ultrahigh vacuum to some extent, in general the photoelectron spectra are not sensitive to
the structural change of interfacial water. Therefore, the atmospherically and biologically relevant interfaces such as the marine boundary layer, aqueous aerosol droplet,
lipid membrane surface, which involve water as the liquid phase, broadly classified as
“soft interfaces”, needs special endeavor to achieve molecular level understanding.
Even-order nonlinear spectroscopy such as vibrational sum frequency generation
(VSFG), which is inherently interface-selective and applicable at ambient condition to any kind of interfaces accessible by light, can provide the much-needed
molecular spectra of an interface [1, 2]. Recent up-gradation of classical-VSFG to
phase-sensitive/heterodyne detected-VSFG, [3, 4] enables one to record the accurate
absorption spectra of interfacial molecules along with their absolute orientation.
Here, we provide a brief description of the theory of SFG and its interfaceselectivity followed by the instrumentation of the classical- and heterodyne-detection
(HD) methods. Applications of classical- and HD-VSFG techniques have been illustrated through the elucidation of surface prevalence and orientations of molecules,
ions and surfactants at the air-water interface and its effect on the structure and
orientation of the interfacial water.
2 Theory of SFG
SFG is one of the outcomes of nonlinear interaction of light with matter. In a material, the negatively charged electrons of its constituent atoms/molecules are bound
by electrostatic attraction with their positively charged nuclei. Interaction of light
(an oscillating electric field) with a material (molecule), induces an oscillation of its
electrons, generating an oscillating induced dipole moment
μ ind
. μ ind is proportional to the electric field of the light (E), μ ind = α E, where α is the proportionality
S. Roy et al.
water is the most abundant liquid on earth, aqueous interfaces are wide-spread in
biotic and abiotic systems including ocean surface (air-water interface), mineral
surface (gas-solid or liquid-solid interface), cell and cellular organelles (lipid or
protein-water interface). Because of the anisotropy and rapidly varying number
density of water, most ions and molecules exhibit an orientational preference and
non-uniform distribution (adsorption/depletion) across the interface, giving rise to
interface-selective physicochemical processes. Molecular level understanding of
such interfacial processes requires selective probing of the interface with molecular
precision.
Conventional spectroscopies such as absorption, fluorescence and Ramanscattering are not interface-selective and hence, the inherently weak signal of interfacial molecules (due to extreme thinness of the interface, ~a few nm) gets buried into
the huge signal of the bulk phase. Particle scattering based surface-selective spectroscopies such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy as well as nano-scale microscopy imaging techniques including scanning
(or transmission) electron microscope are incompatible with most liquid surfaces
because of the dynamic nature of molecules on liquid surface and the need of ultrahigh vacuum for such techniques. It is important to note that though near ambient
pressure (~few mbar) XPS (NAP-XPS) overcomes the stringent requirement of ultrahigh vacuum to some extent, in general the photoelectron spectra are not sensitive to
the structural change of interfacial water. Therefore, the atmospherically and biologically relevant interfaces such as the marine boundary layer, aqueous aerosol droplet,
lipid membrane surface, which involve water as the liquid phase, broadly classified as
“soft interfaces”, needs special endeavor to achieve molecular level understanding.
Even-order nonlinear spectroscopy such as vibrational sum frequency generation
(VSFG), which is inherently interface-selective and applicable at ambient condition to any kind of interfaces accessible by light, can provide the much-needed
molecular spectra of an interface [1, 2]. Recent up-gradation of classical-VSFG to
phase-sensitive/heterodyne detected-VSFG, [3, 4] enables one to record the accurate
absorption spectra of interfacial molecules along with their absolute orientation.
Here, we provide a brief description of the theory of SFG and its interfaceselectivity followed by the instrumentation of the classical- and heterodyne-detection
(HD) methods. Applications of classical- and HD-VSFG techniques have been illustrated through the elucidation of surface prevalence and orientations of molecules,
ions and surfactants at the air-water interface and its effect on the structure and
orientation of the interfacial water.
2 Theory of SFG
SFG is one of the outcomes of nonlinear interaction of light with matter. In a material, the negatively charged electrons of its constituent atoms/molecules are bound
by electrostatic attraction with their positively charged nuclei. Interaction of light
(an oscillating electric field) with a material (molecule), induces an oscillation of its
electrons, generating an oscillating induced dipole moment
μ ind
. μ ind is proportional to the electric field of the light (E), μ ind = α E, where α is the proportionality