Sub-1 cm−1 high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with significantly improved sensitivity and signal-to-noise ratio (SNR)

Sum-frequency generation vibrational spectroscopy (SFG-VS) has been well-established as a unique spectroscopic probe to interrogate the structure, interaction, and dynamics of molecular interfaces, with sub-monolayer sensitivity and broad applications. Sub-1 cm-1 High-Resolution Broadband SFG-VS (HR-BB-SFG-VS) has shown advantages with high spectral resolution and accurate spectral line shape. However, due to the lower peak intensity for the long picosecond pulse used in achieving sub-wavenumber resolution in the HR-BB-SFG-VS measurement, only molecular interfaces with relatively strong signal have been studied. To achieve detailed understanding and broader applications in molecular interfacial studies with HR-BB-SFG-VS, higher sensitivity and better signal-to-noise ratio (SNR) for HR-BB-SFG-VS is required. In this report, we present a systematic effort on the significant improvement of sensitivity and SNR for HR-BB-SFG-VS. Through optimization of laser pulse characteristics, automatic sample height control, and detection conditions, the sensitivity of HR-BB-SFG-VS was improved, reaching a level of 3 × 10-6 of the SFG signal from the α-quartz standard. The high SNR spectra of various molecular interfaces are thus obtained with exquisite line shapes and fine spectral features. To name a couple of examples, a new hydrogen-bonded water band around 3300 cm-1 can be explicitly identified in the air/neat-water interface spectra and pure chiral spectral peaks at the level of 1 × 10-5 of the quartz signal were measured at the air/Leucine aqueous solution interface etc. Such improvements in sensitivity and SNR in HR-BB-SFG-VS have brought and shall bring new opportunities and new discoveries with broad applications to molecular interface studies, in addition to the advantage of HR-BB-SFG-VS for its sub-wavenumber spectral resolution and the ability for intrinsic spectral line shape.