We have built a broad bandwidth vibrational sum frequency generation (VSFG) spectrometer that can provide high-quality spectra over the range of 3800 to 900 cm<sup>−1</sup>. The spectrometer contains a commercial Ti:sapphire based 6 W regenerative amplifier as the master light source, a home-built pulse shaper to produce a narrow bandwidth 800 nm beam, a commercial optical parametric amplifier to generate a broad bandwidth femtosecond infrared (IR) pulse, and a detection system with a monochromator and a charge-coupled device (CCD). We applied this spectrometer to obtain VSFG spectra of a lipid monolayer at the air–water interface in the O–H stretching region (3800–3000 cm<sup>−1</sup>), the C–H stretching region (3100–2700 cm<sup>−1</sup>), the C–D stretching region (2300–2000 cm<sup>−1</sup>), the C=O stretching region (1800–1700 cm<sup>−1</sup>), and the PO<sub>2</sub><sup>−</sup> symmetric stretching region (1200–1000 cm<sup>−1</sup>). We also obtained the VSFG spectrum of neat water in the O–H stretching region (3800–3000 cm<sup>−1</sup>) and the VSFG spectrum of a protein, α-synuclein, in the amide I region (1700–1600 cm<sup>−1</sup>) at the air–water interface. The spectrometer can provide a VSFG spectrum in the O–H stretching region (3800–3000 cm<sup>−1</sup>) without scanning the IR frequency. This feature will be useful in probing water dynamics at interfaces because the free OH and H-bonded OH can be investigated simultaneously. We have also provided instrumental details and discussed further improvements that should be beneficial to other researchers interested in setting up VSFG instrumentation.
Vol. 4, Iss. 7 Virtual Journal for Biomedical Optics
Gang Ma, Jian Liu, Li Fu, and Elsa C. Y. Yan, "Probing Water and Biomolecules at the Air–Water Interface with a Broad Bandwidth Vibrational Sum Frequency Generation Spectrometer from 3800 to 900 cm−1," Appl. Spectrosc. 63, 528-537 (2009)