Sum Frequency Generation Vibrational Spectroscopy
Sum frequency generation vibrational spectroscopy (SFGVS) is an in situ, non-destructive, and nonlinear laser spectroscopic method that provides unique opportunities to probe molecular interactions on surfaces and buried interfaces, owing to its high interface selectivity and sensitivity.1–3 Compared with conventional vibrational spectroscopic methods, such as FTIR, SFGVS is background-free and does not suffer from blackbody radiation under high temperature.
SFG has been applied to problems in many disciplines of science and technology. The use of SFG vibrational spectroscopy enables researchers to probe the reactions at the solid–gas, solid–liquid and solid–solid interfaces.4 Chemical composition, orientation and arrangement of molecules, and reaction mechanisms on surfaces or buried interfaces can be determined.1,5 The basic operational principle of SFGVS relies on a second-order nonlinear optical process in which infrared (ωIR) and visible light (ωVIS) are mixed to produce light at the sum of two frequencies (ωSF = ωIR + ωVIS). An energy level diagram and optical transition for SFGVS are shown in Figure 1.
A schematic representation of the experimental setup for SFGVS is shown in Figure 2a. The interfacial optical probing scheme in a reaction chamber is illustrated in Figure 2b. Two laser beams are spatially and temporally overlapped. To be SFG active, the vibrational mode of interest must be both IR and Raman active. Moreover, the SFG process is forbidden in media with inversion symmetry under the electric-dipole (ED) approximation, but is allowed at surfaces or interfaces where the inversion symmetry is necessarily broken. Hence, it is highly surface specific if the higher-order multiple contributions from the bulk can be neglected. This means SFGVS can probe the adsorbates at the gas-solid or liquid-solid interfaces. An SFG spectrum can be obtained by monitoring intensity of the SFG output while scanning the frequency of the IR light.
The advantages of SFGVS enable HyMARC researchers to gain a molecular-level understanding of the adsorption mechanism of hydrogen under high pressure or temperature in contact with a storage medium such as a metal hydride, graphite, or MOF. The use of SFGVS techniques, as surface characterization methods, will provide a better understanding of these materials and will enable development of improved materials.
On line and available for use in collaboration with HyMARC.
- J. W. Niemantsverdriet, Spectroscopy in Catalysis, Wiley Book (2007).
- G. Rupprechter, “A surface science approach to ambient pressure catalytic reactions,” Catalysis Today 126 (2007): 3–17.
- A. J. Foster and R. F. Lobo, “Identifying reaction intermediates and catalytic active sites through in situ characterization techniques,” Chem Soc Rev 39 (2010): 4783–4793.
- H. L. Han, G. Melaet, S. Alayoglu, and G. A. Somorjai, “In Situ Microscopy and Spectroscopy Applied to Surfaces at Work,” ChemCatChem 7 (2015): 3625–3638.
- Paul S. Cremer, Xingcai Su, Y. Ron Shen, A. Gabor, and A. Somorjai, “Hydrogenation and Dehydrogenation of Propylene on Pt(111) Studied by Sum Frequency Generation from UHV to Atmospheric Pressure,” The Journal of Physical Chemistry 100 (1996): 16302–16309.