Vibrational spectrum of the spin crossover complex [Fe(phen)(2)(NCS)(2)] studied by IR and Raman spectroscopy, nuclear inelastic scattering and DFT calculations

The vibrational modes of the low-spin and high-spin isomers of the spin crossover complex [Fe(phen)(2)(NCS)(2)] (phen = 1,10-phenanthroline) have been measured by IR and Raman spectroscopy and by nuclear inelastic scattering. The vibrational frequencies and normal modes and the IR and Raman intensities have been calculated by density functional methods. The vibrational entropy difference between the two isomers, DeltaS(vib), which is--together with the electronic entropy difference DeltaS(el)--the driving force for the spin-transition, has been determined from the measured and from the calculated frequencies. The calculated difference (DeltaS(vib) = 57-70 J mol(-1) K(-1), depending on the method) is in qualitative agreement with experimental values (20-36 J mol(-1) K(-1)). Only the low energy vibrational modes (20% of the 147 modes of the free molecule) contribute to the entropy difference and about three quarters of the vibrational entropy difference are due to the 15 modes of the central FeN(6) octahedron.     

Electronic, vibrational, and structural properties of a spin-crossover catecholato-iron system in the solid state: theoretical study of the electronic nature of the doublet and sextet states

As a functional model of the catechol dioxygenases, [(TPA)Fe(Cat)]BPh4 (TPA = tris(2-pyridylmethyl)amine and Cat = catecholate dianion) exhibits the purple-blue coloration indicative of some charge transfer within the ground state. In contrast to a number of high-spin bioinspired systems, it was previously shown that, in the solid state, [(TPA)Fe(Cat)]BPh4 undergoes a two-step S = 1/2 = S = 5/2 spin-crossover. Therefore, the electronic and vibrational characteristics of this compound were investigated in the solid state by UV/Vis absorption and resonance Raman spectroscopies over the temperature range of the transition. This allowed the charge-transfer transitions of the low-spin (LS) form to be identified. In addition, the vibrational progression observed in the NIR absorption of the LS form was assigned to a five-membered chelate ring mode. The X-ray crystal structure solved at two different temperatures, shows the presence of highly distorted pseudo-octahedral ferric complexes that occupy two nonequivalent crystalline sites. The variation of the molecular parameters as a function of temperature strongly suggests that the two-step transition proceeds by a successive transition of the species in the two nonequivalent sites. The thermal dependence of the high-spin fraction of metal ions determined by M?ssbauer experiments is consistent with the magnetic data, except for slight deviations in the high temperature range. The optimized geometries, the electronic transitions, vibrational frequencies, and thermodynamic functions were calculated with the B3LYP density functional method for the doublet and the sextet states. The finding of a ground state that possesses a significant mixture of Fe(III)-catecholate and FeII-semiquinonate configurations is discussed with regard to the set of experimental and theoretical data.     

EFFINO

The program EFFINO (Environment For FItting Nuclear Optics) evaluates Mössbauer absorption and time spectra both in nuclear forward scattering and in grazing incidence reflection geometry. Timeintegral prompt and delayed angular scan spectra are also treated. The time spectra are calculated by Fourier transformation from frequency to time domain. The electric quadrupole and magnetic dipole fields at the nuclear sites are considered static at present. The specimen in both forward scattering and grazing incidence is assumed to be a multilayer, with individual thickness and interface roughness (the latter only for the grazing incidence case at present) and electronic index of refraction. Up to eight different layers plus eight repetition periods of those layers are treated. Each layer may contain zero to eight nuclear sites (zero in all layers being prompt Xray reflectivity), with their own effective thickness or (for grazing incidence) their own complex nuclear index of refraction. From the forward scattering amplitude, a differential 4 × 4 propagation matrix is constructed for each layer. Several experimental spectra of the same or different type(s) can be fitted simultaneously. Correlations between parameters of the same or of different spectra can be introduced.     

Coarsening of antiferromagnetic domains in multilayers: the key role of magnetocrystalline anisotropy

The domain structure of an antiferromagnetic superlattice is studied. Synchrotron M?ssbauer and polarized neutron reflectometric maps show micrometer-size primary domain formation as the external field decreases from saturation to remanence. A secondary domain state consisting mainly of at least 1 order of magnitude larger domains is created when a small field along the layer magnetizations induces a bulk-spin-flop transition. The domain-size distribution is reproducibly dependent on the magnetic prehistory. The condition for domain coarsening is shown to be the equilibrium of the external field energy with the anisotropy energy.     

Anomalous pressure dependence of the Lamb-Mössbauerf-factor in the spin crossover system [Fe(2-pic-ND2)3]Cl2·EtOD

The pressure dependence of the Lamb-Mössbauer factor of the spin crossover compound [Fe(2-pic-ND₂)₃]Cl₂·EtOD(2-pic-ND₂=2-picolylamine, deuterated at the^?NH₂ group) has been measured at pressures up to 1500 bar and at three temperatures (115.7, 146.7, 185.6 K) around the transition temperatureT _t=135 K. The temperature dependence of the unit cell volume has been determined by X-ray diffraction. In the transition region (T=146.7 K), the pressure dependence of thef-factor shows an anomalous increase as compared to the dependence below and above the transition. The behaviour off(p, T), its anomaly and absolute value, can be consistently explained within a theoretical model which treats the compound as an isotropic elastic medium. The Poisson ratio of σ=0.4, the bulk modulusK (185.6 K)=1.26·10¹⁰ Nm^?2, and the Grüneisen constant γ^G=3.1 were determined.     

Polarisation effects in Mössbauer absorption by powder samples in external magnetic fields

The polarisation dependence of the absorption cross section must generally be taken into account in calculating Mössbauer absorption spectra of powder samples if they are partly polarised by an external magnetic field. The dependence of the intensity of the absorption lines on the aboorber thickness with an external field ofH _a=50 kG is shown in the case of the paramagnetic specimen Fe(ClO₄)₂·6H₂O. The calculation procedure is described and the results are compared with the usual thickness corrections neglecting polarisation effects.     

TheA-scaling of the multiplicity distributions ofπ ′,K − and\bar P interactions with nuclei at 40 GeV/c

The multiplicity distributions of charged particles produced from π^?,K ^? and \(\bar p\) interactions with the nuclei Li, C, S, Cu, CsI, Pb at 40 GeV/c were studied. It was found that the linear relationD=a〈n〉+b is satisfied for the distributions of different kinds of secondary particles including knocked out protons. Consequently, the use of the scaling variablez′=(an+b)/(a〈n〉+b) ensures the scaling of the distributions with respect to the mass numberA at least up to the second moment.     

Neutral strange particle production in high transverse momentum π− nucleus interactions at ∼40 GeV/c

Measurements ofK _s ⁰ , Λ and \(\bar \Lambda \) production in π^? nucleus (C, Cu, Pb) interactions are presented. The experiment was carried out with the streamer chamber spectrometer RISK using a π^? beam of ～40 GeV/c and a trigger requiring a secondary charged particle with transverse momentum above 1.1 GeV/c. Production cross sections, relative production rates and distributions of Feynmanx and transverse momentum squared as well as correlations between theV ⁰ and the trigger particle are presented. The results are compared and found to be in agreement withK _s ⁰ , Λ and \(\bar \Lambda \) data from untriggered π^? p and π^? C interactions, except for the relative production rate of antilambdas which is about to times larger in high-p _t collisions. Our results can be well interpreted within the dual topological unitarization model.     

Search for the electron-positron decay of an axionlike particle from a 590 MeV proton beam dump

A search was made for the decay of a light neutral particle into ane ⁺ e ^?-pair behind the dump of the PSI (formerly SIN) 590 MeV proton beam. The detector consisted of two counter triggered optical spark chambers mounted two meters apart. A decay signal would have been detected within a cone of about 10° around the direction to the beam dump. For 300 coulomb of protons we found no excess of decay events over the backgroud. Consequently we can exclude the existence of a standard axion forX-parameters between 13.6 and 300, equivalent to masses between 2m _e and 20 MeV. For the decay modea ⁰→e ⁺ e ^? the standard axion model predicts a decay rate Г _ee ≈2×10⁶ m _a ^?1 MeVs^?1 whereas our analysis yields a limit Г _ee <36m _a ^?1 MeVs^?1 with 90% confidence. In the general case, e.g. for an axionlike boson χ⁰ we exclude values for the decay constantf _x <10⁴ GeV with 90% confidence for masses between 2m _e and 20 MeV.