PNNL

Abstract

In this report we show that the ability to measure the sub 1-cm-1 resolution phase-resolved and intensity high-resolution broadband sum frequency generation vibrational spectra (HR-BB-SFG-VS) of the –CN stretch vibration of the Langmuir-Blodgett (LB) monolayer of the 4-n-octyl-4’-cyanobiphenyl (8CB) on the z-cut a-quartz surface allows for the first time the direct comparison and understanding of the homogeneous and inhomogeneous broadenings in the imaginary and intensity SFG vibrational spectral lineshapes in detail. The difference of the full width at half maxima (FWHM) of the imaginary and intensity SFG-VS spectra of the same vibrational mode is the signature of the Voigt lineshape and it measures the relative contribution to the overall lineshape from the homogeneous and inhomogeneous broadenings in SFG vibrational spectra. From the phase-resolved and intensity spectra, we found that the FWHM of the 12238.00 0.02 cm-± peak in the phase-resolved imaginary and intensity spectra are -119.20.4 cm± and -121.6 cm, respectively, for the –CN group of the 8CB LB monolayer on the z-cut a-quartz crystal surface. The FWHM width difference of -12.4 cm agrees quantitatively with a Voigt lineshape with a homogeneous broadening half width of 15.290.08 cm-G=± and a inhomogeneous standard derivation width 1 5.42 0.07 cm?-?=±. These results shed new lights on the understanding and interpretation of the lineshapes of both the phase-resolved and the intensity SFG vibrational spectra, as well as other incoherent and coherent spectroscopic techniques in general.