Classical- and Heterodyne-Detected Vibrational Sum …
113
phosphate group of DPPC in endothelial membrane surface, perturbing the subsequent receptor-ligand electrostatic interaction that regulated deposition/removal of
fat on the inner wall of artery.
6 Summary
Vibrational sum frequency generation (VSFG) spectroscopy is an inherently
interface-selective technique applicable at ambient condition to variety of surfaces
and interfaces that are accessible to light. VSFG measurements provided a deeper
insight into the molecular level structure and orientation of water at the air-water interface - the simplest soft interface, albeit sufficiently complex to capture the molecular
information by using conventional methods such as linear optical spectroscopy and
scattering based techniques. Despite the interface-selectivity, classical-VSFG spectroscopy as it detects the SFG-intensity i.e. square modulus of χ
(2) , does not directly
provide the absolute orientation of interfacial molecules. Moreover, for relatively
weak signal the spectral band-shape may deform form the true absorption spectrum. Heterodyne-detection of VSFG signal (HD-VSFG) overcomes the shortcomings of classical-VSFG and utilize the full potential of sum frequency generation by
independently providing the Im- and Reχ
(2) spectra. The Imχ
(2) spectrum reveals
the accurate absorption characteristics of interfacial molecules and its sign shows
the absolute orientation of interfacial molecules or molecular groups. HD-VSFG
through its unique advantages of the retention of sign of Imχ
(2) and signal amplification reveals the perturbation of aqueous interface by ions and small molecules,
especially by the structure making anions and osmolytes which are repelled form the
topmost water layer are uniquely captured by HD-VSFG measurement. Narrow band
classical-VSFG, because of its readily achievable high spectral resolution (~6 cm
−1 ),
reveal the alkyl chain conformation of adsorbed surfactant by monitoring its sharp
CH-stretch bands. The combined response of CH and OH stretch region from the
POP-water, surfactant-water and surfactant-POP-water interfaces shed light into
the surface prevalence and mutual interaction of POP at atmospherically relevant
interfaces.
References
1. J.H. Hunt, P. Guyot-Sionnest, Y.R. Shen, Chem. Phys. Lett. 133, 189 (1987)
2. Y.R. Shen, V. Ostroverkhov, Chem. Rev. 106, 1140 (2006)
3. C.S. Tian, Y.R. Shen, J. Am. Chem. Soc. 131, 2790 (2009)
4. S. Nihonyanagi, S. Yamaguchi, T. Tahara, J. Chem. Phys. 130, 204704 (2009)
5. R.W. Boyd, Nonlinear Optics, 3rd edn. (Academic Press, Inc., 2008)
6. A.G. Lambert, P.B. Davies, D.J. Neivandt, Appl. Spectrosc. Rev. 40, 103 (2005)
7. J. Löbau, K. Wolfrum, J. Opt. Soc. Am. B 14, 2505 (1997)
8. X. Zhuang, P.B. Miranda, D. Kim, Y.R. Shen, Phys. Rev. B 59, 12632 (1999)
113
phosphate group of DPPC in endothelial membrane surface, perturbing the subsequent receptor-ligand electrostatic interaction that regulated deposition/removal of
fat on the inner wall of artery.
6 Summary
Vibrational sum frequency generation (VSFG) spectroscopy is an inherently
interface-selective technique applicable at ambient condition to variety of surfaces
and interfaces that are accessible to light. VSFG measurements provided a deeper
insight into the molecular level structure and orientation of water at the air-water interface - the simplest soft interface, albeit sufficiently complex to capture the molecular
information by using conventional methods such as linear optical spectroscopy and
scattering based techniques. Despite the interface-selectivity, classical-VSFG spectroscopy as it detects the SFG-intensity i.e. square modulus of χ
(2) , does not directly
provide the absolute orientation of interfacial molecules. Moreover, for relatively
weak signal the spectral band-shape may deform form the true absorption spectrum. Heterodyne-detection of VSFG signal (HD-VSFG) overcomes the shortcomings of classical-VSFG and utilize the full potential of sum frequency generation by
independently providing the Im- and Reχ
(2) spectra. The Imχ
(2) spectrum reveals
the accurate absorption characteristics of interfacial molecules and its sign shows
the absolute orientation of interfacial molecules or molecular groups. HD-VSFG
through its unique advantages of the retention of sign of Imχ
(2) and signal amplification reveals the perturbation of aqueous interface by ions and small molecules,
especially by the structure making anions and osmolytes which are repelled form the
topmost water layer are uniquely captured by HD-VSFG measurement. Narrow band
classical-VSFG, because of its readily achievable high spectral resolution (~6 cm
−1 ),
reveal the alkyl chain conformation of adsorbed surfactant by monitoring its sharp
CH-stretch bands. The combined response of CH and OH stretch region from the
POP-water, surfactant-water and surfactant-POP-water interfaces shed light into
the surface prevalence and mutual interaction of POP at atmospherically relevant
interfaces.
References
1. J.H. Hunt, P. Guyot-Sionnest, Y.R. Shen, Chem. Phys. Lett. 133, 189 (1987)
2. Y.R. Shen, V. Ostroverkhov, Chem. Rev. 106, 1140 (2006)
3. C.S. Tian, Y.R. Shen, J. Am. Chem. Soc. 131, 2790 (2009)
4. S. Nihonyanagi, S. Yamaguchi, T. Tahara, J. Chem. Phys. 130, 204704 (2009)
5. R.W. Boyd, Nonlinear Optics, 3rd edn. (Academic Press, Inc., 2008)
6. A.G. Lambert, P.B. Davies, D.J. Neivandt, Appl. Spectrosc. Rev. 40, 103 (2005)
7. J. Löbau, K. Wolfrum, J. Opt. Soc. Am. B 14, 2505 (1997)
8. X. Zhuang, P.B. Miranda, D. Kim, Y.R. Shen, Phys. Rev. B 59, 12632 (1999)