Capturing inhomogeneous broadening of the –CN stretch vibration in a Langmuir monolayer with high-resolution spectra and ultrafast vibrational dynamics in sum-frequency generation vibrational spectroscopy (SFG-VS)

While in principle the frequency-domain and time-domain spectroscopic measurements should generate identical information for a given molecular system, the inhomogeneous character of surface vibrations in sum-frequency generation vibrational spectroscopy (SFG-VS) studies has only been studied with time-domain SFG-VS by mapping the decay of the vibrational polarization using ultrafast lasers, this due to the lack of SFG vibrational spectra with high enough spectral resolution and accurate enough lineshape. Here, with the recently developed high-resolution broadband SFG-VS (HR-BB-SFG-VS) technique, we show that the inhomogeneous lineshape can be obtained in the frequency-domain for the anchoring CN stretch of the 4-n-octyl-4^′-cyanobiphenyl (8CB) Langmuir monolayer at the air-water interface, and that an excellent agreement with the time-domain SFG free-induction-decay can be established. We found that the 8CB CN stretch spectrum consists of a single peak centered at 2234.00 ± 0.01 cm⁻¹ with a total linewidth of 10.9 ± 0.3 cm⁻¹ at half maximum. The Lorentzian contribution accounts only for 4.7 ± 0.4 cm⁻¹ to this width and the Gaussian (inhomogeneous) broadening for as much as 8.1 ± 0.2 cm⁻¹. Polarization analysis of the –CN spectra showed that the –CN group is tilted 57° ± 2° from the surface normal. The large heterogeneity in the –CN spectrum is tentatively attributed to the –CN group interactions with the interfacial water molecules penetrated/accommodated into the 8CB monolayer, a unique phenomenon for the nCB Langmuir monolayers reported previously.