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Probing Water and Biomolecules at the AirWater Interface with a Broad Bandwidth Vibrational Sum Frequency Generation Spectrometer from 3800 to 900 cm1

Volume 63, Number 5 (May 2009) Page 528-537

Ma, Gang; Liu, Jian; Fu, Li; Yan, Elsa C.Y.

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−1. 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−1), the C-H stretching region (3100-2700 cm−1), the C-D stretching region (2300-2000 cm−1), the C=O stretching region (1800-1700 cm−1), and the PO2 symmetric stretching region (1200-1000 cm−1). We also obtained the VSFG spectrum of neat water in the O-H stretching region (3800-3000 cm−1) and the VSFG spectrum of a protein, α-synuclein, in the amide I region (1700-1600 cm−1) at the air-water interface. The spectrometer can provide a VSFG spectrum in the O-H stretching region (3800-3000 cm−1) 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.