Dietmar Möbius

Enhanced light reflection by dye monolayers at the air–water interface

The reflection of light from an air–water interface is enhanced by dye containing monolayers in the spectral region of the dye absorption. The intensity of the reflected light depends linearly on the density of chromophores in the mixed monolayer. From the measurement of both absorption and reflection spectra of a dye containing monolayer the damping constant of coherent light scattering can be estimated. The value of the reflection measurement for the investigation of adsorption processes and the control of monolayer organization is demonstrated.

Local surface potentials and electric dipole moments of lipid monolayers: Contributions of the water/lipid and the lipid/air interfaces

Abstract The water/air interface is split up into two interfaces by the formation of a lipid monolayer, i.e., the water/lipid and the lipid/air interface. The absolute contribution of the lipid/air interface to the total surface potential is determined by partial dipole compensation in a specially designed molecular assembly presented in this paper. The dipole moment per terminal CH3-group of aligned hydrocarbon chains within a close-packed monolayer is +0.35 D, directed from the monolayer (−) to the air (+). The local and effective dipole moments of hydrated lipid head groups can now be calculated from surface potential measurements since the contribution of the lipid/air interface is known. Such data are given for various charged, zwitterionic, and unchanged lipids.

Reflection and transmission of light by dye monolayers

Using a point‐dipole model, we discuss short‐ and long‐range interactions in a 2D array representing a monolayer. Long‐range forces may be treated exactly if the film is homogeneous at the wavelength scale and if the effective polarizability including short‐range forces is known. Reflection and transmission amplitudes for polarized light are calculated for two monolayer models taking the subphase into account for arbitrary incidence angle and reflection strength. Experimental results are then compared to the theory. The good agreement shows that oblique incidence reflection and transmission spectra are valuable tools for investigating orientation and organization of chromophores in monolayers. Although in most of the studied cases—including J aggregates—the dye chromophore lies flat on the water surface, two instances are presented of molecular aggregates whose transition moments are vertical.

Nonradiative decay of excited molecules near a metal surface

Abstract Nonradiative energy transfer from excited dye molecules near a metal surface to plasmon surface polaritons has been investigated experimentally. By using the Langmuir-Blodgett technique, the molecule-metal separation was varied and the coupling efficiency measured with the inverse attenuated-total-reflection technique. For an orientation of the transition dipole moment of the dye parallel to a silver surface and nonresonant interaction, maximum energy transfer to plasmon surface polaritons was observed for a molecule-silver spacing of about 18 nm. Reducing the spacing results in a monotonic decrease of the coupling efficiency: this decay channel becomes unimportant and eventually vanishes if the molecule is placed very close to the metal surface. The experimental results are discussed in connection with theoretical predictions and the surface-enhanced Raman scattering observed for some adsorbates at metal surfaces.

Hydrated Polar Groups in Lipid Monolayers - Effective Local Dipole-Moments and Dielectric-Properties

Abstract The effective local dipole moments of polar groups at the water-lipid interface have been calculated from surface potential measurements. The contribution of the lipid-air interface was previously determined experimentally via a partial dipole compensation in a specially designed monolayer assembly. Hydration phenomena and the local dielectric properties at the water-lipid interface are discussed.

Energy transfer in monolayers with cyanine dye Sheibe aggregates

A monolayer of an oxacyanine dye organized in Scheibe aggregates is doped with a thiacyanine dye (e.g., one molecule of the guest per 104 molecules of the host). Strong energy transfer from the excited host to the guest takes place at room temperature, the efficiency decreasing with lowering temperature T. The rate of energy transfer is proportional to T. A simple model of the excited state is discussed based on a coherent exciton extending over a certain domain. The exciton moves over the aggregate, occasionally reaching the vicinity of an acceptor molecule. The domain size of the exciton is inversely proportional to the absolute temperature T. As a consequence, the lifetime of the exciton is proportional to T, and therefore the energy transfer efficiency is proportional to T.

Non-linear optical properties of hemicyanine monolayers and the protonation effect.

Second- and third-harmonic generation have been observed from organized monolayers of the hemicyanine dye H3C-(C5N+H4)-CHCH-(C6H4)-N(C18H37)2. A second-order polarizability β=2.5×10−27 esu at 1.06 μm and 6.3×10−27 esu for resonant excitation at 0.915 μm was found. For molecules with the same chromophore, a stronger charge transfer leads to a larger β. Protonation of the molecules, on the other hand, reduces the charge transfer and hence β.

REFLECTOMETRY AT THE BREWSTER-ANGLE AND BREWSTER-ANGLE MICROSCOPY AT THE AIR-WATER-INTERFACE

Abstract No reflection occurs if a p-polarized light beam is incident on a water subphase under the Brewster angle. Formation of a monolayer at this air-water interface leads to a measurable reflectivity depending on various film properties. A microscope based on this principle had been constructed. This paper gives examples of the potential of reflectometry and microscopy at the Brewster angle.

Exciton–surface plasmon coupling: An experimental investigation

Langmuir–Blodgett monolayer assemblies, which contained dye molecules, have been deposited on silver films. Exciton–surface plasmon interactions have been studied with attenuated total reflection (ATR) spectroscopy. Reflectivity and dispersion curves for plasmon surface polaritons (PSP’s) at the metal interface are reported for both angular and wavelength scans. In agreement with theory dispersion curves from angle scans exhibit a double ‘‘back bending’’ at the transverse exciton frequency ωT (due to PSP interaction with the in plane component of the dye transition dipole moment) and at the longitudinal frequency ωL (due to PSP interaction with a perpendicular component). Correspondingly, dispersion curves from wavelength scans break into separate branches at these frequencies.

Three-phase contact line motion in the deposition of spread monolayers

Abstract A monolayer of a water-insoluble surfactant can be transferred with some maximum rate vmax from a water surface to the surface of a solid being taken out. This rate was measured by observing the motion of the three-phase contact line (detecting fluorescence quenching or change of light reflection by water entrainment). The rate vmax was found to be determined by the reactivity between the monolayer head groups and the solid surface. vmax is particularly large in the case of opposite sign of the charges of the immobile groups on the solid surface and in the monolayer. In the case of equal sign vmax is strongly increased by the presence of divalent counterions in the solution. The hydration of the head groups decreases vmax, and therefore optimal transfer is found when the charged head groups of the monolayer are laterally separated from each other by uncharged head groups. Particularly large values of vmax are observed when the monolayer head groups and the solid surface interact by zipper-like hydrogen bonding between undissociated carboxyl groups.