Abstract
Broadband vibrational sum-frequency generation (BB-VSFG) spectroscopy has become a well-established surface analytical tool capable of identifying the orientation and structure of molecular layers. A straightforward way to boost the sensitivity of the technique could be to increase the laser repetition rate beyond that of standard BB-VSFG spectrometers, which rely on Ti:sapphire lasers operating at repetition rates of 1-5 kHz. Nevertheless, possible thermally induced artifacts in the vibrational spectra due to higher laser average powers are unexplored. Here, we discuss laser power induced temperature accumulation effects that distort the BB-VSFG spectra of 1,2-diacyl-sn-glycero-3-phosphocholine at an interface between two transparent phases at repetition rates of 5, 10, 50, and 100 kHz at constant pulse energy. No heat-induced distortions were found in the spectra, suggesting that the increase in the laser repetition rate provides a feasible route to an improved signal-to-noise ratio or shorter data acquisition times in BB-VSFG spectroscopy for thin films on transparent substrates. The results have implications for future BB-VSFG spectrometers pushing the detection limit for molecular layers with low surface coverage.
Highlights
A straightforward way to boost the sensitivity of the technique could be to increase the laser repetition rate beyond that of standard BB-VSFG spectrometers, which rely on Ti:sapphire lasers operating at repetition rates of 1-5 kHz
No heat-induced distortions were found in the spectra, suggesting that the increase in the laser repetition rate provides a feasible route to an improved signal-to-noise ratio or shorter data acquisition times in BB-VSFG spectroscopy for thin films on transparent substrates
We demonstrate here the feasibility of BB-VSFG experiments covering the range of vibrational resonances of the alkyl chains of phospholipids at repetition rates well above 1 kHz up to 100 kHz for the restrictive case, where the absorptive layer is between two transparent phases
Summary
As the investigation of lipids has gained increasing importance because of their broad biomedical applications, BB-VSFG spectra of lipid bilayers have been studied at different interfaces, e.g., solid/water, air/water, or solid/air interfaces, usually at laser repetition rates of 1 kHz. As a first step, we demonstrate here the feasibility of BB-VSFG experiments covering the range of vibrational resonances of the alkyl chains of phospholipids at repetition rates well above 1 kHz up to 100 kHz for the restrictive case, where the absorptive layer is between two transparent phases. We investigated the dependence of the vibrational spectra of lipid bilayers at a CaF2/air interface on the repetition rate. CaF2 is an ideal substrate material for such a study, as it is transparent both in the mid-infrared (MIR) and the visible (VIS) spectral range, and possible thermal effects are expected to arise due to the MIR laser energy absorbed solely in the lipid layer. We discuss characteristic fingerprints of phosphatidylcholine, spectral averaging and signal-to-noise ratio, the stability of the intensity of vibrational bands at varying repetition rate and under prolonged excitation, and the
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