Sum-Frequency Generation Spectroscopy
Sum-frequency generation (SFG) spectroscopy is used to assess the vibrational properties of surfaces and interfaces with monolayer sensitivity. Unlike Raman spectroscopy, which is sensitive to bulk properties, SFG spectroscopy is exquisitely sensitive to the physical and chemical properties of molecular layers at surfaces and interfaces where inversion symmetry is broken.
In SFG spectroscopy, a mid-IR beam and a visible beam overlap on the surface or interface of the sample. The specific properties of the resulting sum-frequency signal, such as polarization and intensity, provide information on dipole orientation and vibrational spectra at the surface. SFG is a second-order nonlinear process allowed only when inversion symmetry is broken, making it specifically surface-sensitive. In this type of spectroscopy, to obtain high spectral resolution and, subsequently, distinguish the vibrational fingerprints of the investigated molecule, one of the pulses is required to have a sufficiently narrow spectral bandwidth.
One of the main scientific targets of SFG spectroscopy is studying the dynamics of interfacial water by observing the OH stretching vibrations of normal water and OD stretching vibrations of heavy water at 2.6 – 3.5 μm and 3.5 – 4.5 μm, respectively. Other spectral ranges of interest include 4.5 – 5.5 μm, where the vibrations of metal carbonyls and nitriles occur, and 5.5 – 6.5 μm, where molecular vibrations of interfacial biomolecules take place. Moreover, going further into the mid-IR is also of high interest due to lower frequency biomolecular vibrations.
Broadband mid-IR sources such as ORPHEUS-MIR, pumped by PHAROS or CARBIDE femtosecond lasers, cover the spectral ranges of interest and address many vibrational levels at once, facilitating sum-frequency generation spectroscopy. An SHBC is used to double the laser frequency and narrow down the spectral bandwidth to meet the required specifications. In this configuration, the SFG signal is generated in the visible spectral range, eliminating the need for complex infrared detection.
