The polarized absorption spectra of crystalline pentacene are obtained for excitation normal to the ab herringbone plane by measuring transmitted light in ultrathin crystals. The spectral line shapes for excitation polarized along b and orthogonal to b are analyzed theoretically using a Holstein-like Hamiltonian which includes both Frenkel and charge transfer (CT) excitons represented in a multiparticle basis set. The model agrees with prior estimates regarding the strong CT contribution (≈45%) of the exciton responsible for the b-polarized lower Davydov component.The polarization resolution allows one to also establish the nature of the upper Davydov component, which is found to contain far less CT content (≈15%), as well as the natures of the higher-energy vibronic excitons, which are found to consist of a complex mixture of Frenkel one-and two-particle states and CT excitons. Generally, the spectrum polarized along b displays J-aggregatelike vibronic signatures while the spectrum polarized orthogonal to b displays H-aggregate-like vibronic signatures. The assignment is entirely consistent with the calculated exciton band dispersions which agree well with the measured ones.

Small aluminum nanoparticles have the potential to exhibit localized surface plasmon resonances in the deep ultraviolet region of the electromagnetic spectrum, however technical and scientific challenges make it difficult to attain this limit. We report the fabrication of arrays of Al/Al2O3 core/shell nanoparticles with a metallic-core diameter between 12 and 25 nm that display sharp plasmonic resonances at very high energies, up to 5.8 eV (down to λ = 215 nm). The arrays were fabricated by means of a straightforward self-organization approach. The experimental spectra were compared with theoretical calculations that allow the correlation of each feature to the corresponding plasmon modes.

The demonstration of a nonlinear optical technique for directly monitoring adsorption of surfactants on the surface of microparticles in colloids is reported. In this approach, dye molecules with strong hyperpolarizability are first adsorbed on the particle surface to give detectable second-harmonic generation. The surfactant is then added to the colloidal solution in competition with dye for adsorption on the surface. The displacement of the dye molecules on the surface results in a decrease of the second-harmonic signal, indicating the adsorption of the surfactant molecules. A continuous flow/titration system in combination with a high-repetition-rate femtosecond laser allows the adsorption to be monitored in real time. This approach was first demonstrated on a methacrylate polymeric surfactant on latex and talc particles in an aqueous solution. The adsorption free energy and surface density of this surfactant on these particles have been determined.

It is shown that the nonlinear optical phenomenon known as second-harmonic generation can be used for label-free, time-resolved study of the transport of molecules through living cell membranes. The adsorption and transport of a 300-Da molecular-mass hydrophobic ion at the Escherichia coli membrane is observed. Remarkably, at low ion concentrations, the second-harmonic generation technique clearly exposes a multistep molecular transport process: Transport of the molecular ion across the outer and cytoplasmic membranes of the Gram-negative bacteria is recorded, in sequence, in time. Fitting of the data to a multiprocess kinematic model reveals that the transport of this hydrophobic ion through the outer membrane is much faster than through the cytoplasmic membrane, likely reflecting the effectiveness of ion transport porins. The observations illustrate an experimental means for studying the interactions of small molecules with cell membranes.

The dynamic and spectroscopic behavior of CO adsorption on a stepped Cu(100) surface is investigated using transient IR diode laser-reflection absorption spectroscopy. Disproportionate intensity behavior, defying Beer–Lambert’s law, is observed which makes it impossible to use spectral intensity for determining either the total or site-specific concentrations. A theoretical model, based on the Persson–Ryberg treatment of mixed isotope studies of CO at fixed coverage, is used here, with modifications added to allow for coverage dependence to account for dynamic dipole coupling between CO molecules and simulate the IR absorption spectra. This enables the spectral intensities and positions to be analyzed and the extraction of previously unattainable information on site-specific molecular spectroscopic parameters and concentrations on this CO/Cu(100) system. The CO stretching frequencies indicate that the Cu–CO bond is formed by transfer of the CO antibonding 5σ electron to copper and that the binding energy of CO at terrace sites decreases with increasing coverage. The model calculation shows that, as a result of dynamical dipole coupling, a 7% step-CO concentration, with a vibrational polarizability of 0.2 Å3, causes a 3 times larger IR absorption peak than the remaining 93% of CO at terrace sites. CO adsorption on this Cu(100) surface was found to be repulsive correlated with the order parameter determined as n=3/2. Concentrations determined from the dynamical coupling calculation show that CO occupies step and on-top terrace sites at all coverages at 90 K, with the more tightly bound step sites saturated at lower coverage. A simple model is devised to describe the equilibrium between the step and terrace CO populations and provide an estimate of the dynamical parameters governing CO motion between the step and terrace sites.

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Second harmonic generation from small gold metallic particles: From the dipolar to the quadrupolar response