Spectral Interference and Molecular Conformation at Liquid Interface with Sum Frequency Generation Vibrational Spectroscopy (SFG-VS)

Abstract

In this report, we present an analysis of the interference effects in the SFG-VS spectra of the vapor/ethanol interfaces with normal and deuterated liquid ethanol. Crucial information on the ethanol molecular orientation can be obtained from analyzing the interference effects between the different SFG-VS spectral features. With the strength-coupled orientational functional dr(θ) of the different spectral features from the same or different molecular groups, and with the phase information contained in the SFG spectra measured under different polarization combinations, the detailed orientational conformation structure of the ethanol molecule at the vapor/ethanol interface can be obtained. The ethanol molecule is found to possess an orientational conformation with its CH 3 -group pointing away from the liquid phase at an orientational tilt angle of about 45°, its -CH 2 -group away from the liquid interface around 10°, and the C-C-O plane is near perpendicular to the plane of interface. Here, the conclusion that the -CH 2 -group has to tilt away from the liquid phase can only be reached with the knowledge of the spectral overlap and the analysis of the interference between the overlapping spectral features. The details of the molecular orientation angle and the orientational distribution width were discussed with novel approaches. These results suggested that studying coherent interference effects in the polarization analysis in SFG-VS can provide novel understanding of the interfacial molecular structure and interactions. The tools and the concepts of SFG-VS analysis developed in this study can be applied to more complex molecular interfaces and thin films.

Surface of room temperature ionic liquid [bmim][PF6] studied by polarization- and experimental configuration-dependent sum frequency generation vibrational spectroscopy

“…Since each susceptibility tensor has a coefficient that is a function of the incident angles of incoming laser beams; when the laser incident angles change, the interference among the four ppp susceptibility terms alters accordingly, producing different spectral line shapes [38,39]. On the other hand, only one second-order susceptibility tensor is involved in the ssp spectra.…”

“…Consequently, laser incident angles only affect the overall spectral intensities for ssp polarizations, and not the spectral line shapes. The different SFG-VS line shapes between ssp and ppp polarization combinations, as well as the dependence of the ppp spectra on different experimental geometries can be quite useful for resolving the structural details, assigning complex SFG-VS spectra, and identifying molecular orientations of the interfacial species [38,39]. The ssp and the ppp spectra from the four experimental configurations ( Figure 2) have been analyzed to investigate the possible [bmim] orientations at the air/[bmim][PF 6 ] interface.…”

Microscopic molecular optics theory of surface second harmonic generation and sum-frequency generation spectroscopy based on the discrete dipole lattice model

“…21,25 These development also provided more accurate data to test the SHG and SFG-VS theories at the detailed molecular level. 37,39,40,53,58 In this report we have taken the molecular optics theory approach and treated the molecular interface with more realistic discrete induced dipole lattice model. Based on this model, the following results are derived: a. the detailed expressions of the local field factors in the interface molecular layer; b. the detailed expressions for the far field radiation of the SHG as well as the SFG-VS process from the interface induced dipoles.…”

“…52,58,68 However, even though these values worked almost fine, the models and the reasonings behind it prevented the treatment of the molecular details of the molecular interface. With the development of the laser techniques and the measurement methodology in the surface SHG and SFG-VS, 21,25,39,40 now such details can be interrogated from the careful experiments. This shall provide new opportunities for the interface studies.…”

Interference effects in the sum frequency generation spectra of thin organic films. I. Theoretical modeling and simulation

“…[16][17][18][19][20] Since SFG is a coherent optical process, interference effects are inherently involved in the SFG observation. These include the interference between the resonant and nonresonant SFG signals, [21][22][23][24] interference between different vibrational modes, [25][26][27] and interference between SFG signals from different polarization combinations. 28,29 In particular, when the film thickness is comparable to the wavelength of the pumped or emitted light beams, the thickness interference of the light beams due to the thin film will significantly affect the observed SFG spectra.…”