1,721,065 research outputs found
Metodi EPR impulsati per la caratterizzazione delle radiazioni ionizzanti
No full textIl rilascio di energia da parte di radiazioni ionizzanti all’ interno di materiali è associato alla formazione di centri paramagnetici con efficienze variabili. Questo processo nella scala dimensionale micrometrica non avviene in maniera omogenea, ma in maniera disomogenea, portando alla conseguente distribuzione di centri paramagnetici non uniformi. La distribuzione di tali centri dipende sia dal tipo di sorgente usata per l’irraggiamento sia dalla sua energia; ad esempio in materiali organici raggi γ emessi da un campione di 60Co portano ad una distribuzione di radicali quasi uniforme, mentre protoni con energia di 20 MeV producono distribuzioni di radicali allineati lungo delle direzioni (tracks) specifiche [1]. Le differenze fra i due tipi di irraggiamento non sono facilmente rivelate mediante normali tecniche di Electron Paramagnetic Resonance (EPR) in onda continua, anche se vari tipi di tentativi sono stati tentati [2].
Il modo più naturale di studiare queste diverse tipi di distribuzioni è quello di ricorrere alle tecniche impulsate.
La tecnica sicuramente più informative è il Double Electron-Electron Resonance (DEER), che permette la determinazione diretta della distribuzione di distanze fra due centri paramagnetici.
Informazioni sulle concentrazioni locali si possono ottenere mediante la misura della Diffusione Istantanea (ID), fenomeno che si manifesta durante il periodo di azione degli impulsi. La concentrazione locale che è misurata è funzione delle caratteristiche della radiazione ionizzante.
Una overview dello stato dell’ arte delle tecniche sarà affiancata ad una presentazione di casi-studio in cui l’ EPR impulsato viene utilizzato anche con finalità di valutazione retrospettiva del danno da radiazione
Identification of slow relaxing spin components by pulse EPR techniques in graphene-related materials
Full text linkElectron Paramagnetic Resonance (EPR) is a powerful technique that is suitable to study graphene-related materials. The challenging ability requested to the spectroscopy is its capability to resolve the variety of structures, relatively similar, that are obtained in materials produced through
different methods, but that also coexist inside a single sample. In general, because of the intrinsic inhomogeneity of the samples, the EPR spectra are therefore a superposition of spectra coming from different structures. We show that by pulse EPR techniques (echo-detected EPR, ESEEM and Mims
ENDOR) we can identify and characterize species with slow spin relaxing properties. These species are generally called molecular states, and are likely small pieces of graphenic structures of limited dimensions, thus conveniently described by a molecular approach. We have studied commercial reduced graphene oxide and chemically exfoliated graphite, which are characterized by different EPR spectra. Hyperfine spectroscopies enabled us to characterize the molecular components of the different materials, especially in terms of the interaction of the unpaired electrons with protons (number of protons and hyperfine coupling constants). We also obtained useful precious information about extent of delocalization of the molecular states
An EPR Study on Nanographites
No full textWe present the results of an electron paramagnetic resonance study (EPR) in the range of 4–290 K on samples of nanographites obtained by ball milling graphite for different times. With a careful simulation of the spectral line shapes, we disentangled the EPR bands, providing the spectral profiles and intensities of the components on varying the temperature, their g tensors, and the homogeneous line widths of the contributing spin packets. We have been able to follow the effect of decreasing progressively the size of the flakes on the EPR bands due to mobile electrons and on Lorentzian lines due to nonbonding electrons on the zigzag edges of the crystallites. The temperature dependence of the EPR intensities shows a common trend for the signals attributed to edge electrons and to mobile electrons, showing that they belong to the same bath
Pulse EPR of Paramagnetic Centers in Solid Phases
No full textWe present an overview of the most used Electron Spin Echo techniques and their applications to the study of structure and dynamics of paramagnetic centers in solid phases. A short theoretical section presents the tools necessary to understand the experiments. Three sections describe the experiments that are used to get information on the spin and spatial dynamics of the system, on the distribution of paramagnetic centers in the solid matrix, and on their local environment. Many examples of applications to different paramagnetic centers in various research fields are given
Pulsed EPR of paramagnetic centers in solid phase
No full textWe present an overview of the most used Electron Spin Echo techniques and their applications to the study of structure and dynamics of paramagnetic centers in solid phases. A short theoretical section presents the tools necessary to understand the experiments. Three sections describe the experiments that are used for measuring longitudinal relaxation, transverse relaxation and hyperfine interactions. Many examples of applications to different research fields are given
A stochastic model ffor dual fluorescence of DMABN in polar media
No full textA stochastic model for interpreting the dual fluorescence emission of dimethyl-aminobenzonitrile is presented, which takes into account the conformational dynamics of the singlet state S1, the emission dynamics to the ground state S0 and the relaxation of the polar solvent. The probability density for the population of the excited state is determined by solving numerically a bidimensional diffusive operator for a torsional variable of the molecule coupled to a solvent coordinate, with source and sink terms. The resulting diffusion equation is the continuous analogue of the well known Grabowski kinetic scheme. The fluorescence emission is evaluated by integrating over the microscopic coordinates at each emission frequency
Intramolecular electron-transfer reaction in dimethylaminobenzonitrile
No full textThe fluorescence emission of dimethylaminobenzonitrile in polar solvents is analysed in terms of a model which is a microscopic generalization of the Grabowski two-level kinetic scheme. The ground and the lowest excited state are described by adiabatic potential surfaces, defined in terms of an internal torsional coordinate and a solvent polarization coordinate. The potential energy functions are given by the energy of the isolated molecule plus a contribution due to the electrostatic stabilization in the polar solvent. The interconversion process between the two minima of the potential surface corresponding to the excited singlet state, i.e. a planar state with a low dipole moment and a strongly polar charge transfer state, is described by a stochastic operator. The emission features are interpreted by a minimal set of parameters, both for radiative and non-radiative channels
First determination of the spin relaxation properties of a nitronyl nitroxide in solution by Electron Spin Echoes at X-band. A comparison with Tempone
No full textWe studied by electron spin echo pulse methods the spin relaxation properties of a phenyl nitronyl nitroxide radical (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, PTIO) at X-band in fluid toluene solution in a wide temperature range, and in a water/glycerol 1:1 mixture near room temperature. The relaxation properties of PTIO have been compared with that of Tempone, as a widely used nitroxide. By a new procedure, based on experimental results on the temperature dependences of the relaxation times T1 and T2, and on the approximation of an isotropic Brownian rotational diffusion, we separated non-secular, spin rotational and residual terms from the transverse relaxation rate to isolate secular and pseudosecular contributions. By comparing the results for the two radicals we found the differences in the magnetic properties that give rise to slower transverse (T2) and longitudinal (T1) electron spin relaxation for PTIO in the whole temperature range explored in this work
Rotational disorder of bis(mesitylene)vanadium in channels of a triazine inclusion compound
No full textElectron paramagnetic resonance (EPR) was applied to study rotational disorder of bis(mesitylene)vanadium (VBM, S = 1⁄2) in channels of 2,4,6-tris(4-Br-phenoxy)-1,3,5-triazine. To ensure a magnetically dilute system, decamethylbis(cyclopentadienyl)ruthenium was used as a main guest to fill the channels. X-ray diffraction confirmed inclusion formation, resulting in an incommensurate host–guest system. EPR spectra were measured using prismatic single crystals oriented to have Bo either parallel to the channel axis c or at any angle in between c and the perpendicular direction. Temperature-dependent spectra were recorded down to 80 K. The analysis revealed two types of paramagnetic guest entities, i.e. VBM (I) and VBM (II). Both showed dynamical effects due to a reorientational motion. VBM (I) is confined to show a molecular axis z distributed around c by a small angle (± 1.25°), whereas for VBM (II) the distribution is completely disordered. The reorientational motions are frozen at low temperature giving for VBM (II) a powder-like EPR spectrum. Analysing EPR intensities allowed us to conclude that the ratio of (I)/(II) species is of the order of 1.5
Stochastic model for solvent-assisted intramolecular charge transfer
No full text(N,N-Dimethylamino)benzonitrile (DMABN) and related aromatic donor-acceptor compounds show dual fluorescence emission in polar solvents. The static and dynamic features of the spectra are strongly affected by the polarity and viscosity of the medium. A successful model was first proposed by Grabowski et al., by using a phenomenologic kinetic scheme. According to this interpretation, the excited singlet state undergoes an adiabatic intramolecular electron transfer. Two metastable states are assumed to interconvert by a torsional motion, which provides a natural reaction coordinate for the electron transfer (ET) process. In this work we discuss a stochastic model which extends the simple kinetic picture to a continuous description. The dynamics of interconversion is described as a diffusional process coupled to a solvent polarization coordinate. Decay to the ground state is included in the form of a sink term depending upon instantaneous conformation. The model provides a satisfactory description of all static and dynamic fluorescence spectral features available from experiments. The Grabowski scheme is derived from the continuous model in the case of a relatively high barrier between interconverting metastable states. Agreement between theoretical simulations and observed experimental spectra supports the original hypothesis based on intramolecular electron-transfer involving distinct conformers
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