Classical- and Heterodyne-Detected
Vibrational Sum Frequency Generation
(VSFG) Spectroscopy and Its Application
to Soft Interfaces
Subhadip Roy, Subhamoy Saha, and Jahur Alam Mondal
Abstract Chemistry at interface, i.e., at the junction between two immiscible bulk
media is remarkably different from either of the bulk phases. This is primarily due
to an anisotropic environment at interface where reactants behave differently than
they do in isotropic bulk media. Interface-selective spectroscopy can provide valuable insight into such interface specific phenomena at the molecular level. Presently,
vibrational sum frequency generation (VSFG) is the best suited spectroscopic technique to unravel interfacial molecular properties, especially for the “soft interface”
such as liquid surface where linear optical spectroscopy and particle scattering based
techniques cannot be applied either due to the lack of interface-selectivity or due
to requirement of stringent experimental conditions. VSFG is inherently interfaceselective and probes molecules by their vibrational transitions, revealing site-specific
molecular properties. This Chapter provides a brief description of the theory of VSFG
followed by the origin of its interface-selectivity and detection of VSFG signal by
the conventional (classical-VSFG) and heterodyne-detection (HD-VSFG) methods.
Unlike classical-VSFG, HD-VSFG records unambiguous absorption spectra and
absolute orientation of interfacial molecules. Finally, we discuss the application of
narrowband classical-VSFG and broadband HD-VSFG techniques to aqueous interfaces of biological and environmental relevance. Specifically, the surface properties
of ions, molecules, lipids, pollutants, and the associated perturbation of H-bonding
and orientation of interfacial water.
Keywords Sum frequency generation (SFG) · Heterodyne detection · HD-VSFG ·
Aqueous interface · Perfluorinated-POP · Structure-making ion · TMAO · Lipid
1 Introduction
An interface is a few molecular layers thick junction between two immiscible
bulk media, where the inversion symmetries of bulk media are broken. Since
S. Roy · S. Saha · J. A. Mondal (B)
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Homi Bhabha National
Institute, Mumbai 400085, India
e-mail: mondal@barc.gov.in
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science
and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_4
87
Vibrational Sum Frequency Generation
(VSFG) Spectroscopy and Its Application
to Soft Interfaces
Subhadip Roy, Subhamoy Saha, and Jahur Alam Mondal
Abstract Chemistry at interface, i.e., at the junction between two immiscible bulk
media is remarkably different from either of the bulk phases. This is primarily due
to an anisotropic environment at interface where reactants behave differently than
they do in isotropic bulk media. Interface-selective spectroscopy can provide valuable insight into such interface specific phenomena at the molecular level. Presently,
vibrational sum frequency generation (VSFG) is the best suited spectroscopic technique to unravel interfacial molecular properties, especially for the “soft interface”
such as liquid surface where linear optical spectroscopy and particle scattering based
techniques cannot be applied either due to the lack of interface-selectivity or due
to requirement of stringent experimental conditions. VSFG is inherently interfaceselective and probes molecules by their vibrational transitions, revealing site-specific
molecular properties. This Chapter provides a brief description of the theory of VSFG
followed by the origin of its interface-selectivity and detection of VSFG signal by
the conventional (classical-VSFG) and heterodyne-detection (HD-VSFG) methods.
Unlike classical-VSFG, HD-VSFG records unambiguous absorption spectra and
absolute orientation of interfacial molecules. Finally, we discuss the application of
narrowband classical-VSFG and broadband HD-VSFG techniques to aqueous interfaces of biological and environmental relevance. Specifically, the surface properties
of ions, molecules, lipids, pollutants, and the associated perturbation of H-bonding
and orientation of interfacial water.
Keywords Sum frequency generation (SFG) · Heterodyne detection · HD-VSFG ·
Aqueous interface · Perfluorinated-POP · Structure-making ion · TMAO · Lipid
1 Introduction
An interface is a few molecular layers thick junction between two immiscible
bulk media, where the inversion symmetries of bulk media are broken. Since
S. Roy · S. Saha · J. A. Mondal (B)
Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Homi Bhabha National
Institute, Mumbai 400085, India
e-mail: mondal@barc.gov.in
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021
D. K. Singh et al. (eds.), Modern Techniques of Spectroscopy, Progress in Optical Science
and Photonics 13, https://doi.org/10.1007/978-981-33-6084-6_4
87