Dynamics of ferrocene in molecular sieves probed by Mossbauer spectroscopy and nuclear resonant scattering

A detailed study on the slow dynamics of ferrocene in the unidimensional channels of the molecular sieves SSZ-24 and AlPO4-5 has been carried out, using Mössbauer spectroscopy (MS), nuclear forward scattering (NFS) and synchrotron radiation-based perturbed angular correlations (SRPAC). In both host systems, anisotropic rotational dynamics is observed above 100 K. For SSZ-24, this anisotropy persists even above the bulk melting temperature of ferrocene. Various theoretical models are exploited for the study of anisotropic discrete jump rotations for the first time. The experimental data can be described fairly well by a jump model that involves reorientations of the molecular axis on a cone mantle with an opening angle dependant on temperature

Nuclear forward scattering in particulate matter: dependence of lineshape on particle size distribution

In synchrotron Moessbauer spectroscopy, the nuclear exciton polariton manifests itself in the lineshape of the spectra of nuclear forward scattering (NFS) Fourier-transformed from time domain to frequency domain. This lineshape is generally described by the convolution of two intensity factors. One of them is Lorentzian related to free decay. We derived the expressions for the second factor related to Frenkel exciton polariton effects at propagation of synchrotron radiation in Moessbauer media. Parameters of this Frenkelian shape depend on the spatial configuration of Moessbauer media. In a layer of uniform thickness, this factor is found to be a simple hypergeometric function. Next, we consider the particles spread over a 2D surface or diluted in non-Moessbauer media to exclude an overlap of ray shadows by different particles. Deconvolving the purely polaritonic component of linewidths is suggested as a simple procedure sharpening the experimental NFS spectra in frequency domain. The lineshapes in these sharpened spectra are theoretically expressed via the parameters of the particle size distributions (PSD). Then, these parameters are determined through least-squares fitting of the line shapes.Comment: 13 pages, 12 figure

Enhancement and speed up in nuclear resonant diffraction

The interaction of Mössbauer radiation with the nuclei in a single crystal provides the unique possibility to enhance the coherent channel in nuclear resonance scattering by means of a properly phased excitation of the scattering centers. When a primary beam is incident in the exact Bragg direction, all nuclei are excited in phase. The resonance parameters of such a collective nuclear excitation of a perfect single crystal (γ-exciton) are entirely different from those of an individual nuclear excitation. In Bragg geometry diffraction, the resonance lines are shifted and broadened (enhancement effect), the lifetime of the collective excited state is shortened (speedup effect) and the reflectivity becomes total (suppression effect). Recent experiments arc reviewed, where these effects were studied in the resonant diffraction of Mössbauer and of synchrotron radiation

Hybrid forms of beat phenomena in nuclear forward scattering of synchrotron radiation

In nuclear forward scattering (NFS) of synchrotron radiation, inter-resonance interferenceleads to a quantum beat (QB), and intra-resonance interference to a dynamical beat (DB).In general both interference processes determine the time evolution of NFS. Only in thecase of far distant resonances the resulting interference pattern can be interpreted as awell distinguishable combination of QB and DB. Multiple scattering by near neighbouringresonances, by contrast, leads to a hybridisation of QB and DB. In particular, asymmetricalcontinuous distributions of resonances make QB and DB blend into a fast hybrid beat withthickness dependent period and distribution sensitive modulatio

Synchrotron-radiation

We report the observation of γ-γ-correlations from 119Sn using nuclear resonant scattering of synchrotron radiation, extending nuclear resonant spectroscopy with 119Sn to vanishing recoilless fractions and new applications. The 23.87 keV M1 (+E2) Ig : 1/2 $\rightarrow$ Ie : 3/2 Mössbauer transition was excited from the ground state, and the time differential correlations between the incident and the scattered photons were recorded for different angles in the plane perpendicular to the incident beam. The experiments were performed on samples of tributyltin-fluoride, which has a very low Lamb-Mössbauer factor at ambient temperature. In the time spectra we observed quantum beats from the static perturbation through electric quadrupole interaction

ENHANCED NUCLEAR RESONANCE SCATTERING IN DYNAMICAL DIFFRACTION OF GAMMA-RAYS

The angular and energy dependence of the dynamical diffraction of gamma-rays in Bragg geometry was studied experimentally. Mössbauer spectra of gamma-quanta diffracted by a pure nuclear Bragg reflecion of a nearly perfect 57FeBO3 single crystal were measured with high angular resolution. In the spectrum taken at exact Bragg position very broad hyperfine resonance peaks were observed. In the spectra just above and below the Bragg angle the reflection peaks were shifted to energy positions above and below resonance. The results are interpreted by means of the dynamical theory and compared with computer calculations