Selection of dominant multi-exciton transitions in disordered linear J-aggregates

We show that the third-order optical response of disordered linear J-aggregates can be calculated by considering only a limited number of transitions between (multi-) exciton states. We calculate the pump-probe absorption spectrum resulting from the truncated set of transitions and show that, apart from the blue wing of the induced absorption peak, it agrees well with the exact spectrum.Comment: 8 pages, 2 figures, accepted to Journal of Luminescenc

Vibrational Spectra of a Mechanosensitive Channel

We report the simulated vibrational spectra of a mechanosensitive membrane channel in different gating states. Our results show that while linear absorption is insensitive to structural differences, linear dichroism and sum-frequency generation spectroscopies are sensitive to the orientation of the transmembrane helices, which is changing during the opening process. Linear dichroism cannot distinguish an intermediate structure from the closed structure, but sum-frequency generation can. In addition, we find that two-dimensional infrared spectroscopy can be used to distinguish all three investigated gating states of the mechanosensitive membrane channel.

Mapping Molecular Orientation with Phase Sensitive Vibrationally Resonant Sum-Frequency Generation Microscopy

We demonstrate a phase sensitive, vibrationally resonant sum-frequency generation (PSVR-SFG) microscope that combines high resolution, fast image acquisition speed, chemical selectivity, and phase sensitivity. Using the PSVR-SFG microscope, we generate amplitude and phase images of the second-order susceptibility of collagen I fibers in rat tail tendon tissue on resonance with the methylene vibrations of the protein. We find that the phase of the second-order susceptibility shows dependence on the effective polarity of the fibril bundles, revealing fibrous collagen domains of opposite orientations within the tissue. The presence of collagen microdomains in tendon tissue may have implications for the interpretation of the mechanical properties of the tissue. [Image: see text

HETERODYNE DETECTED SUM FREQUENCY GENERATION (SFG) AS A NOVEL TOOL TO MEASURE ADSORBANT CONCENTRATION AT INTERFACES

Author Institution: Department of Chemistry, Wayne State University, Detroit, MI, 48202 (email to I.S.:[email protected])The dynamics of molecular binding at interfaces is a central area in cell biology, toxicology, molecular electronics, and physical chemistry. However, detecting of low concentrations of untagged molecules at interfaces still remains a great challenge. In this talk we present a novel optical heterodyne-detected SFG technique sensitive to ultralow interface adsorbant concentrations. We demonstrate the superiority of our technique to the conventional homodyne SFG technique on octanol:deuturated octanol mixture at air/water interface system. In our technique the SFG signal is generated by interaction of the broad-band infrared laser pulse resonant with the adsorbant intramolecular vibrational modes and of the narrowband nonresonant optical laser pulse with the air/water interface. The SFG signal is heterodyne-detected and spectral interferograms are recorded with CCD equipped spectrometer. Spectral interferometry is used to recover the SFG signal spectrum. Improved signal to noise ratio and linear scaling of the recovered signal with the adsorbant concentration allowed us to detect concentrations as low as 1\% of the monolayer coverage while the conventional homodyne-detected SFG technique was effective only at the monolayer coverage above 25\%

Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy

The air-water interface is perhaps the most common liquid interface. It covers more than 70 per cent of the Earth's surface and strongly affects atmospheric, aerosol and environmental chemistry. The air-water interface has also attracted much interest as a model system that allows rigorous tests of theory, with one fundamental question being just how thin it is. Theoretical studies have suggested a surprisingly short 'healing length' of about 3 \ue5ngstr\uf6ms (1 \uc5 = 0.1 nm), with the bulk-phase properties of water recovered within the top few monolayers1-3. However, direct experimental evidence has been elusive owing to the difficulty of depth-profiling the liquid surface on the \ue5ngstr\uf6ms scale. Most physical, chemical and biological properties of water, such as viscosity, solvation, wetting and the hydrophobic effect, are determined by its hydrogen-bond network. This can be probed by observing the lineshape of the OH-stretch mode, the frequency shift of which is related to the hydrogen-bond strength4-5. Here we report a combined experimental and theoretical study of the air-water interface using surface-selective heterodyne-detected vibrational sum frequency spectroscopy to focus on the 'free OD' transition found only in the topmost water layer. By using deuterated water and isotopic dilution to reveal the vibrational coupling mechanism, we find that the free OD stretch is affected only by intramolecular coupling to the stretching of the other OD group on the same molecule. The other OD stretch frequency indicates the strength of one of the first hydrogen bonds encountered at the surface; this is the donor hydrogen bond of the water molecule straddling the interface, which we find to be only slightly weaker than bulk-phase water hydrogen bonds. We infer from this observation a remarkably fast onset of bulk-phase behaviour on crossing from the air into the water phase. \ua9 2011 Macmillan Publishers Limited. All rights reserved.Peer reviewed: YesNRC publication: Ye