Ekspla's Femtosecond broadband SFG (BB SFG) spectrometer allows fast SFG spectra acquisition since most vibrational modes can be resolved without scanning. The advantage of the broadband SFG system is that intense femtosecond pulses allow efficient sum frequency generation at low pulse energies thus reducing the possibility of sample modification. It is especially important for aqueous and biological samples.
Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) is powerful and versatile method for in-situ investigation of surfaces and interfaces. In SFG-VS experiment a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG is second order nonlinear process, which is allowed only in media without inversion symmetry. At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SF signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions.
Vibrational sum frequency generation (SFG) spectroscopy holds several important advantages over traditional spectroscopy methods for the molecular level analysis of interfaces, including (i) surface sensitivity, (ii) vibrational specificity, and (iii) the possibility to extract detailed information on the ordering and orientation of molecular groups at the interface by analysis of polarization-dependent SFG spectra.
Advantages
- Sensitive and selective to the orientation of molecules in the surface layer
- Intrinsically surface specific
- Selective to adsorbed species
- Sensitive to submonolayer of molecules
- Applicable to all interfaces accessible to light
- Nondestructive
- Capable of high spectral and spatial resolution
Applications
- Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes and other systems
- Studies of surface structure, chemical composition and molecular orientation
- Remote sensing in hostile environment
- Investigation of surface reactions under real atmosphere, catalysis, surface dynamics
- Studies of epitaxial growth, electrochemistry, material and environmental problems
Spectra examples: on the left: SFG spectra of monoolein surface, 1cm-1 step, 200 acquisitions per step. On the right: Water-air interface spectra, 200 acquisitions per step. (Courtesy of University of Michigan)
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Comparison of Narrowband and Broadband SFG Spectrometers
Narrowband picosecond scanning SFG spectrometer
In order to get SFG spectrum during measurement wavelength of narrowband mid-IR pulse is changed point-by-point throughout the range of interest. Narrowband SFG signal is recorded by the time-gated photomultiplier. Energy of each mid-IR, VIS and SFG pulse is measured. After the measurement, the SFG spectrum can be normalised according to IR and VIS energy. Spectral resolution is determined by the bandwidth of the mid-IR light source. The narrower mid-IR pulse bandwidth, the better the SFG spectral resolution. Separate vibrational modes are excited during the measurement.
Broadband femtosecond SFG spectrometer
A broadband mid-IR pulse is mixed with a narrowband VIS pulse. The result is broadband SFG spectrum which is recorded using a monochromator and a sensitive CCD camera. The full spectrum is acquired simultaneously by integrating signal over time. Spectral resolution is determined by the bandwidth of the VIS pulse and on the monochromator-camera combination. The narrower the bandwidth of VIS pulse, the better the SFG spectral resolution.