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Barducci A.,CNR Institute of Applied Physics Nello Carrara
Journal of the Optical Society of America B: Optical Physics | Year: 2011

We describe a specific bandpass sampling procedure that provides high efficiency for interferogram sampling. This new approach is able to mitigate the important radiometric and noise disadvantages of Fourier transform spectrometry that recent theoretical investigations have pointed out. Proof of concept is given using simulations and measurements performed with a Sagnac triangular interferometer. Adopting an information-theoretic approach to spectrometry, we demonstrate the existence of important limitations to the radiometric efficiency achieved by any interferential or dispersive multiplex spectrometers. We find an extension to optics of the well-known data processing inequality, confirming that the Fellgett (multiplex) advantage is an inappropriate expectation. We give evidence of radiometric disadvantages implicit in the coded aperture architecture typical of compressive sensing. © 2011 Optical Society of America. Source

Muniz-Miranda M.,University of Florence | Muniz-Miranda M.,CNR Institute of Applied Physics Nello Carrara
Journal of Raman Spectroscopy | Year: 2013

The adsorption of 4-nitroanisole on silver colloidal nanoparticles was investigated by surface-enhanced Raman spectroscopy (SERS). Actually, the chemical binding with a metal substrate may play a role in changing the electronic structure of this molecule, which can be considered a push-pull chromophore, because an internal charge-transfer occurs between methoxy and nitrogroup. A SERS signal could be detected only in chloride-activated silver colloids, but the spectrum recorded with green-light excitation was not related to adsorbed 4-nitroanisole, but to its azoderivative, formed by photoreduction of the nitrogroup on the surface of the silver substrate. Copyright © 2013 John Wiley & Sons, Ltd. Source

Ceccherini S.,CNR Institute of Applied Physics Nello Carrara | Ridolfi M.,University of Bologna
Atmospheric Chemistry and Physics | Year: 2010

The variance-covariance matrix (VCM) and the averaging kernel matrix (AKM) are widely used tools to characterize atmospheric vertical profiles retrieved from remote sensing measurements. Accurate estimation of these quantities is essential for both the evaluation of the quality of the retrieved profiles and for the correct use of the profiles themselves in subsequent applications such as data comparison, data assimilation and data fusion. We propose a new method to estimate the VCM and AKM of vertical profiles retrieved using the Levenberg-Marquardt iterative technique. We apply the new method to the inversion of simulated limb emission measurements. Then we compare the obtained VCM and AKM with those resulting from other methods already published in the literature and with accurate estimates derived using statistical and numerical estimators. The proposed method accounts for all the iterations done in the inversion and provides the most accurate VCM and AKM. Furthermore, it correctly estimates the VCM and the AKM also if the retrieval iterations are stopped when a physically meaningful convergence criterion is fulfilled, i.e. before achievement of the numerical convergence at machine precision. The method can be easily implemented in any Levenberg-Marquardt iterative retrieval scheme, either constrained or unconstrained, without significant computational overhead. Source

Toci G.,CNR Institute of Applied Physics Nello Carrara
Applied Physics B: Lasers and Optics | Year: 2012

This paper reports a new theoretical model for the evaluation of the results of the so-called pinhole method for the measurement of the upper level lifetime in doped optical materials exhibiting radiation trapping effects due to resonant reabsorption of the emitted fluorescence. The model correctly predicts that the fluorescence decay has a double exponential behavior with two time constants, with the shorter one near the intrinsic decay time of the upper level, with a correction depending on the effect of the short range reabsorption. As a consequence, a new method is proposed for the data analysis and the interpretation of the results with respect to the previous literature. © Springer-Verlag 2011. Source

Chelli R.,University of Florence | Chelli R.,CNR Institute of Applied Physics Nello Carrara
Journal of Chemical Theory and Computation | Year: 2012

Configurational freezing (J. Chem. Theory Comput.2011, 7, 582) is a method devised for steered Monte Carlo simulations aimed at improving free energy estimates via nonequilibrium work theorems (see Jarzynski in Phys. Rev. Lett.1997, 78, 2690 and Crooks in J. Stat. Phys.1998, 90, 1481). The basic idea is to limit the sampling to particles located in the region of space where dissipation occurs, while leaving the other particles fixed. Therefore, the method is based on the reasonable assumption that dissipation is a local phenomenon in single-molecule nonequilibrium processes, a statement which holds for many processes including, for example, folding of biopolymers and protein-ligand binding/unbinding. In this article, the configurational freezing approach, based on the sampling of particles located around hot-spot sites encompassing the high dissipation domain, is supplemented by the possibility of selecting such particles (for trial Monte Carlo moves) dependent on their distance from the hot spots. This is accomplished by exploiting an extension of the Owickis preferential sampling (J. Am. Chem. Soc.1977, 99, 7413) in the original configurational freezing machinery. The combined strategy is shown to improve the accuracy of free energy estimates in physically sound cases: the calculation of the water to methane relative hydration free energy and the calculation of the potentials of mean force of two solvated methane molecules and two solvated benzene molecules along the direction connecting the centers of mass. © 2012 American Chemical Society. Source

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