Quantum Mechanical Hysteresis and the Electron Transfer Problem

We study a simple quantum mechanical symmetric donor-acceptor model for electron transfer (ET) with coupling to internal deformations. The model contains several basic properties found in biological ET in enzymes and photosynthetic centers; it produces tunnelling with hysteresis thus providing a simple explanation for the slowness of the reversed rate and the near 100% efficiency of ET in many biological systems. The model also provides a conceptual framework for the development of molecular electronics memory elements based on electrostatic architectures.Comment: Accepted Physica

On the Experimental Estimation of Surface Enhanced Raman Scattering (SERS) Cross Sections by Vibrational Pumping

We present an in-depth analysis of the experimental estimation of cross sections in Surface Enhanced Raman Scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented.Comment: To be submitted to J. Phys. Chem.

Enhancement factor distribution around a single SERS Hot-spot and its relation to Single Molecule detection

We provide the theoretical framework to understand the phenomenology and statistics of single-molecule (SM) signals arising in Surface-Enhanced Raman Scattering (SERS) under the presence of so-called electromagnetic hot-spots (HS's). We show that most characteristics of the SM-SERS phenomenon can be tracked down to the presence of tail-like (power law) distribution of enhancements and we propose a specific model for it. We analyze, in the light of this, the phenomenology of SM-SERS and show how the different experimental manifestations of the effect reported in the literature can be analyzed and understood under a unified ``universal'' framework with a minimum set of parameters.Comment: 13 pages, 4 figures, submitted to J. Chem. Phy

Efecto de una corriente continua en los espectros Raman de dos magnones en Bi₂Sr₂Ca₁ Cu₂O₈₊∗

Es conocido que los óxidos superconductores de alta temperatura tienen como característica principal el orden antiferromagnético de los planos de CuO₂. Esto permite detectar, con técnicas comunes a espectroscopia Raman, scattering inelástico de pares de magnones como una banda ancha centrada alrededor de 2500 cm-¹. En esta comunicación presentamos medidas preliminares de dichas excitaciones en pastillas super-conductoras de Bi-2212 nominalmente puras y semiconductoras dopada con Fe bajo una corriente DC a temperaturas inferiores a Tc. Encontramos que bajo estas condiciones aumenta el ancho medio a media altura del perfil aproximadamente simétrico de la banda de dos magnones de la pastilla pura y que del lado de más altas frecuencias hay un significativo incremento en su intensidad. Estas medidas sugieren una explicación según la cual los portadores involucrados en la corriente continua afectarían la interacción de intercambio a través de una interacción electrón magnón.Facultad de Ciencias Exacta

Optical detection of single non-absorbing molecules using the surface plasmon of a gold nanorod

Current optical detection schemes for single molecules require light absorption, either to produce fluorescence or direct absorption signals. This severely limits the range of molecules that can be detected, because most molecules are purely refractive. Metal nanoparticles or dielectric resonators detect non-absorbing molecules by a resonance shift in response to a local perturbation of the refractive index, but neither has reached single-protein sensitivity. The most sensitive plasmon sensors to date detect single molecules only when the plasmon shift is amplified by a highly polarizable label or by a localized precipitation reaction on the particle's surface. Without amplification, the sensitivity only allows for the statistical detection of single molecules. Here we demonstrate plasmonic detection of single molecules in realtime, without the need for labeling or amplification. We monitor the plasmon resonance of a single gold nanorod with a sensitive photothermal assay and achieve a ~ 700-fold increase in sensitivity compared to state-of-the-art plasmon sensors. We find that the sensitivity of the sensor is intrinsically limited due to spectral diffusion of the SPR. We believe this is the first optical technique that detects single molecules purely by their refractive index, without any need for photon absorption by the molecule. The small size, bio-compatibility and straightforward surface chemistry of gold nanorods may open the way to the selective and local detection of purely refractive proteins in live cells