An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

Abstract

Knowledge of the ratios between different polarizability betai'j'k' tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the betai'j'k' tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.

Orientation of Methylguanidinium Ions at the Water–Air Interface

“…For instance, β aca /β ccc was found to be 0.4 for methanol and 4.5 for ethanol. 44 Hence, we will use this ratio as a fit parameter S (=β aca /β ccc ) in describing the experimental spectra.…”

Nonlinear optical response of a gold surface in the visible range: A study by two-color sum-frequency generation spectroscopy. I. Experimental determination

“…25,32,40,41 Furthermore, for the r A/S ratio, existing controversy regarding the accuracy of the various methods 41 leads to an even wider range between 0.4 and 4.5. 34,36,40,41 In the present study, we do not have sufficient information on the structure of our films to resolve the debate on these values nor to estimate the tilt angle for the CH 3 group. We therefore rely on a general analysis that takes into account broad distributions for the values assigned to r a/c and θ CH 3 .…”

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

“…Therefore, the computed SFG spectra are usually not as good as in agreement with the SFG experimental spectra, in comparison to the cases with the IR or 2D-IR spectra. In comparison, empirical treatment of the SFG signal assisted with the molecular group symmetry and the Raman depolarization data has been more reliable in analyzing the SFG-VS data [21,80,81,[127][128][129][130].…”