Micro-Refractometry and Local-Field Mapping with Single Molecules.

The refractive index n is one of the most important materials parameters of solids and, in recent years, has become the subject of significant interdisciplinary interest, especially in nanostructures and meta-materials. It is, in principle, a macroscopic quantity, so its meaning on a length scale of a few nanometers, i.e., well below the wavelength of light, is not clear a priori and is related to methods of its measurement on this length scale. Here we introduce a novel experimental approach for mapping the effective local value [Formula: see text] of the refractive index in solid films and the analysis of related local-field enhancement effects. The approach is based on the imaging and spectroscopy of single chromophore molecules at cryogenic temperatures. Since the fluorescence lifetime T1 of dye molecules in a transparent matrix depends on the refractive index due to the local density of the electromagnetic field (i.e., of the photon states), one can obtain the local [Formula: see text] values in the surroundings of individual chromophores simply by measuring their T1 times. Spatial mapping of the local [Formula: see text] values is accomplished by localizing the corresponding chromophores with nanometer accuracy. We demonstrate this approach for a polycrystalline n-hexadecane film doped with terrylene. Unexpectedly large fluctuations of local-field effects and effective [Formula: see text] values (the latter between 1.1 and 1.9) were found.

Effect of impurity molecules on the low-temperature vibrational dynamics of polyisobutylene: investigation by single-molecule spectroscopy.

The influence of impurity chromophore molecules--tetra-tert-butylterrylene (TBT) and dibenzo-anthanthrene (DBATT)--on the vibrational dynamics of the amorphous polymer polyisobutylene (PIB) has been studied via single-molecule spectroscopy. The measurements were performed in the temperature region of 7-30 K, where the interaction of the chromophores with quasilocalized low-frequency vibrational modes (LFMs) determines the observed spectral line broadening. The analysis of the individual temperature dependences of the linewidths for a large number of single probe molecules yielded effective frequency values of those LFMs which are located near the respective chromophores. In this way the distributions of the LFM frequencies were measured for the two systems, and they were found to be similar. Moreover, they are in good agreement with the vibrational density of states as measured in pure PIB by inelastic neutron scattering. This allows us to conclude that, at least in the case of PIB, doping with low concentrations of the nonpolar and neutral molecules TBT and DBATT does not affect the vibrational dynamics of the matrix markedly.

Does the standard model of low-temperature glass dynamics describe a real glass?

We present the first experimental single-molecule (SM) spectroscopy study of the local dynamics in molecular glasses (frozen toluene and deuterated toluene weakly doped with a substituted terrylene) at T=2-30 K. Surprisingly, the dynamics does not follow the standard model of low-temperature glasses. An extra contribution to the dynamics was detected causing drifts and irreproducible jumps of the SM spectra. The isotope effects in the SM linewidth distribution and the density of states (boson peak) show that the fast dynamics at T=2 K is due to excitations of tunneling nature, whereas at T>7 K it is related to vibrations; H/D atoms are directly involved in both types of excitations.

Experimental evidence of the local character of vibrations constituting the Boson peak in amorphous solids.

We report the first measurement of the density of states of low-energy vibrational excitations in a disordered solid via single-molecule (SM) spectroscopy. Optical spectra of many single tetra-tert-butylterrylene (TBT) molecules embedded to amorphous polyisobutylene (PIB) as spectral probes were recorded at low temperatures. The T dependences of SM spectral linewidths showed the broad distribution of local frequencies of vibrations under study. The obtained distribution was compared with the "Boson peak" in pure PIB measured in [R. Inoue, J. Chem. Phys. 95, 5332 (1991)] by neutron scattering. We found that embedding of a small amount of TBT into PIB does not influence markedly on the observed vibrational dynamics. These results prove the local character of low-energy vibrational excitations in glasses and the existence of relationship between these excitations and the Boson peak.

Characterization of the adsorption of Ru-bpy dyes on mesoporous TiO2 films with UV-Vis, Raman, and FTIR spectroscopies.

In the present work the adsorption of a new dye, [Ru(dcbpyH(2))(2)(bpy-TPA(2))](PF(6))(2), and the well-known (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)] complex on mesoporous anatase films were investigated to clarify the role of the carboxylate groups in the anchoring process of the dyes on the semiconductor surface. For this purpose UV-vis, Raman, resonance Raman, and ATR-FTIR spectroscopies have been used. The results of the Raman experiments at different excitation wavelengths demonstrate that photoinduced charge-transfer processes take place efficiently between the adsorbate and the substrate. Moreover, this is the first time that the Raman spectrum of a Ru-bpy dye (in this case, the dye N719) adsorbed on TiO(2) has been obtained without the resonance condition, only by means of SERS enhancement. The coordination of both complexes on the TiO(2) paste films is proposed to occur via bidentate or bridging linkage.

Influence of the solvent on the surface-enhanced raman spectra of ruthenium(II) bipyridyl complexes.

In the present work a new dye, [Ru(dcbpyH2)2(bpy-TPA2)](PF6)2, and the well-known (Bu4N)2[Ru(dcbpyH)2(NCS)2] complex were investigated. The electronic transitions of both dyes showed solvatochromic shifts due to specific interactions of the ligands with the solvent molecules. The surface-enhanced Raman (SER) spectra of the dyes dissolved in water, ethanol, and acetonitrile were measured in silver and gold colloidal solutions. The results demonstrate that the dyes were adsorbed on the metallic nanoparticles in different ways for different solvents. It was also found that in the gold colloid, the aqueous solutions of both dyes did not produce any SERS signal, whereas in ethanolic solution the SERS effect was very weak. Deprotonation, H-bonding, and donor-acceptor interactions seem to determine these different behaviors. Our results indicate the important role of the charge-transfer mechanism in SERS.

Relation between pressure shift and electric-field shift of single-molecule lines in a polymer glass.

The pressure shifts and the electric-field shifts of individual chromophores in an amporphous matrix are--due to strong disorder--subject to broad distributions. By means of single-molecule spectroscopy we measured both the pressure and the electric-field shift of about 800 tetra-tert-butylterrylene molecules in polyisobutylene. We found a significant correlation of 0.52 (Kendall's correlation coefficient) between the two observables. Analytical calculations and Monte Carlo simulations based on a model by Laird and Skinner predict a nonzero, yet, distinctly smaller correlation. The Monte Carlo simulations showed that the usual assumptions of a spherical shape and isotropic polarizability of the chromophores in glassy systems is an oversimplification of the complex nanoscopic structure and cannot reproduce our experimental results. By taking the molecular anisotropy into account, we obtain agreement of the simulated and the measured correlation between pressure shift and electric-field shift parameter.

Lévy statistics for random single-molecule line shapes in a glass.

We demonstrate that the statistical behavior of random line shapes of single tetra-tert-butylterrylene chromophores embedded in an amorphous polyisobutylene matrix at T=2 K is described by Lévy statistics as predicted theoretically by Barkai, Silbey, and Zumofen [Phys. Rev. Lett. 84, 5339 (2000)]. This behavior is a manifestation of the long-range interaction between two-level systems in the glass and the single molecule. A universal amplitude ratio is investigated, which shows that the standard tunneling model assumptions are compatible with the experimental data.

Three-dimensional materials analysis by confocal Raman microspectroscopy.

Two- and three-dimensional spatial analysis of various composite materials was performed with a scanning confocal Raman microspectrometer. Samples include TiO(2) microparticles, mixtures of polymers, and the surface of an older Eprom computer chip. In the last case both structural and compositional information was obtained by means of comparing the signal intensity of the Rayleigh line with that of the silicon Raman line at 520 cm(-1). The spatial compositions of a pain-relief medicine and a pharmaceutical salt mixture could be visualized from characteristic Raman lines of the components.

Microscopic analysis of currency and stock exchange markets.

Recently it was shown that distributions of short-term price fluctuations in foreign-currency exchange exhibit striking similarities to those of velocity differences in turbulent flows. Similar profiles represent the spectral-diffusion behavior of impurity molecules in disordered solids at low temperatures. It is demonstrated that a microscopic statistical theory of the spectroscopic line shapes can be applied to the other two phenomena. The theory interprets the financial data in terms of information which becomes available to the traders and their reactions as a function of time. The analysis shows that there is no characteristic time scale in financial markets, but that instead stretched-exponential or algebraic memory functions yield good agreement with the price data. For an algebraic function, the theory yields truncated Lévy distributions which are often observed in stock exchange markets.