Sum-Frequency Spectroscopy of Molecular Adsorbates on Low-Index Ag Surfaces: Effects of Azimuthal Rotation

Abstract

Vibrational sum-frequency generation spectroscopy (SFG) lineshapes of p-cyanobenzenethiol on low-index Ag crystal surfaces are studied as a function of azimuthal rotation by angle phi. A broadband multiplex SFG method is used, with a new technique that variably suppresses the non-resonant (NR) background using time-asymmetric time-delayed picosecond laser pulses. When both resonant (R) and NR signals are present, the amplitude and phase of the R line shape can vary significantly with phi, leading to dramatic phi-dependent variations of the SFG spectrum. The CN-stretch transition of p-cyanobenzenethiol modified Ag(111) and Ag(110) surfaces has an SFG spectrum consisting of a single vibrational resonance R atop a NR background that originates from the metal surface. Using the NR suppression technique, it was found that the R amplitude of the CN-stretch was independent of phi on both Ag(111) and Ag(110), which proves that the CN dipole moment is parallel to the surface normal in both cases. We show that it is possible to accurately extract the phi-dependence of the R amplitude, the NR amplitude, and the phase difference from SFG spectra by suppressing the NR signal during sample roation, thereby proving that the R contribution from the CN-stretch transition evidence phi-invariant behavior on both Ag surfaces.

Vibrational Stark Effect of the Electric-Field Reporter 4-Mercaptobenzonitrile as a Tool for Investigating Electrostatics at Electrode/SAM/Solution Interfaces

“…, 24°–28° for thiophenol on Ag [27,28], and, most surprisingly, ca. 0° for MBN [29]. In view of this striking discrepancy between thiophenol and MBN on Ag, which is in sharp contrast to the findings for Au, one has to consider the reported value for Ag/MBN with caution.…”

Structure of the Cetyltrimethylammonium Surfactant at Fused Silica/Aqueous Interfaces Studied by Vibrational Sum Frequency Generation

“…13,45 The spectrum corresponding to high CTAC concentrations in Figure 2B is more challenging to interpret. The derivative line shape probably results from interference 46 between the C-H and water resonances. The dramatically higher contribution to the SFG signal from water relative to the C-H modes is explained by the net positive charge at the silica/aqueous interface resulting from adsorbed CTAC, which leads to a longrange array of aligned water dipoles that gives rise to a stronger resonant water signal than is observed in the low concentration spectrum (Figure 2A).…”

Understanding the Role of Nonresonant Sum-Frequency Generation from Polystyrene Thin Films

“…It has also been shown that delaying the visible pulse results in narrower and more symmetric line shapes . In many cases, particularly when T 2 is long, the resonant signal does not decrease substantially ,, and may increase . However, in cases of rapid dephasing due to intrinsically short lifetimes, fast molecular reorientation, − or energy transfer, the resonant signal will decrease with this delay.…”

“…This increase in NR-SFG occurs along with the decrease in the resonant signal, again suggesting that more of the material is “bulk-like” rather than “interface-like”. The NR signal leads to apparent shifting of the peak positions (Figure b), but this is merely a result of the interference. , These interference effects can also lead to the creation of “ghost peaks”, such as the apparent peak at ∼3105 cm −1 , which completely disappear when the NR signal is suppressed. If the NR signal is not properly accounted for, then peak positions and amplitudes could be improperly determined, leading to erroneous physical interpretations.…”