A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43GHz) and at the high frequencies/fields (HF) 95, 190 and 285GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g‖ and g⊥, are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g⊥-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce3+ ions with tetragonal symmetry S4 in the PbWO4 scintillator.

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd(3+) impurities in PbWO(4) single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(parallel)-factor and the hyperfine constant A(parallel) of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S(4) site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.

Exploiting the high angular resolution of high field electron paramagnetic resonance measured at 95, 190, and 285 GHz we determine the rotational nonergodicity parameter of different probe molecules in the glass former o-terphenyl and polybutadiene in a model-independent way. Our results clearly show a characteristic change in the temperature of the nonergodicity parameter proving a rather sharp dynamic crossover in both systems, in contrast to previous results from other techniques.

. The reorientation of the paramagnetic molecule 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) dissolved in glassy polystyrene (PS) is studied by electron paramagnetic resonance (EPR) spectroscopy in a wide range of Larmor frequencies (9–285 GHz). High-field EPR reveals that TEMPO rotates by small angular steps with a distribution of correlation times and is able to detect the onset of fast motion of PS. Differently, X-band EPR shows that TEMPO undergoes a large-angle reorientation with virtually no distribution of correlation times and largely misses the fast dynamics of PS. A unified view is proposed.

The reorientation of guest paramagnetic molecules free radicals spin probes in the host polymers and glass formers is studied by High-Field High-Frequency Paramagnetic Resonance ((HFEPR)-E-2) spectroscopy at three different Larmour frequencies, 95, 190 and 285 GHz. The structural change of the host that occurs close to glass transition temperature changes the reorientation regime of the molecular spin probe. For the slow tumbling regime of the rotational motion, the line shape exhibits larger sensitivity of the free radicals spin probe to both the static and the dynamics of the environment. Change of the reorientation rate lead to considerable line shifts, whereas broadening effects are not dominant. Discussion is focused on the suitable choice of the spin probe used in the (HFEPR)-E-2 spectroscopy in the study of the polymeric and glass formers systems.

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron paramagnetic resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. In the temperature range of 180-220 K a large increase of the rotational mobility is observed with the widening of the distribution of correlation times which exhibits two components: (i) a deltalike, temperature-independent component representing the fraction of spin probes w which persist in the low-temperature dynamics; (ii) a strongly temperature-dependent component, to be described by a power distribution, representing the fraction of spin probes 1-w undergoing activated motion over an exponential distribution of barrier heights g(E). Above 180 K a steep decrease of w is evidenced. The shape and the width of g(E) do not differ from the reported ones for PS within the errors. For the first time the large increase of the rotational mobility of the spin probe at 180 K is ascribed to the onset of the fast dynamics detected by neutron scattering at T-f=175 +/- 25 K.

An electron paramagnetic resonance (EPR) powder spectrum investigation of rare earth trace impurities in the Gd(HBPz3)2 tropolonate complex (GdTrp) was carried out at 95, 190, and 285 GHz. Ce3+ ion impurities were identified by EPR transitions between levels of the two lowest-lying doublets of the ground J = 5/2 manifold. An important frequency and magnetic-field dependence of the g-factor was observed. The system was successfully described by a Hamiltonian appropriate for tetragonal symmetry without orthorhombic distortion. Compared to conventional microwave EPR spectroscopy, at high fields these effective g-values depend not only on the polar angle between the magnetic field B and the tetragonal axis, but also on the azimuthal angle φ between B and the binary axes of the molecular complex. These dependences include additional terms proportional to (B)2m(cos4φ)n, where m≥n are natural numbers. They show that in the present experiment, the Zeeman interaction is comparable with the zero-field splitting of the ground manifold. It is found that the second, and the third doublet are located at 13.98 ± 0.25 cm−1 and 9.32 ± 0.15 cm−1 above the first one, respectively.

High-field electron paramagnetic resonance (HF-EPR) spectroscopy was used to investigate the unusual temperature and frequency dependence of the powder spectrum of the Gd(HBPz3)2 tropolonate complex (GdTrp). A new type of H/T effect is evidenced. This effect is interpreted in terms of the formation of spin projection states delocalized and quasidelocalized along linear chains of Gd3+ ions in high magnetic fields due to the competition between the weak dipole and exchange spin-spin interaction and the particular structure of the molecular complex. The number of ions in the chain depends strongly on the orientation of the magnetic field and on the relaxation processes.

The reorientation of the paramagnetic molecule TEMPO dissolved in glassy polystyrene (PS) is studied by high-field Electron Paramagnetic Resonance spectroscopy. Two different regimes separated by a crossover region are evidenced. Below 180 K the rotational times are nearly temperature independent with no apparent distribution. TEMPO is trapped. In the temperature range 180–220 K a large increase of the rotational mobility is observed with widening of the distribution of correlation times which exhibits two components: i) a delta-like, temperature-independent component representing the fraction of TEMPO w still trapped; ii) a strongly temperature-dependent component representing the fraction of untrapped TEMPO 1 − w undergoing activated motion over an exponential distribution of barrier heights. Above 180 K a steep decrease of w is evidenced. The detrapping of TEMPO and the onset of its large increase of the rotational mobility at 180 K are interpreted as signatures of the onset of the fast motion detected by neutron scattering at Tf = 175 ± 25 K.

The reorientation of one small paramagnetic molecule (spin probe) in glassy polystyrene (PS) is studied by high-field electron spin resonance spectroscopy at two different Larmor frequencies (190 and 285 GHz). The exponential distribution of the energy barriers for the rotational motion of the spin probe is unambiguously evidenced at both 240 and 270 K. The same shape for the distribution of the energy barriers of PS was evidenced by the master curves provided by previous mechanical and light scattering studies. The breadth of the energy barrier distribution of the spin probe is in the range of the estimates of the breadth of the PS energy barrier distribution. The evidence that the deep structure of the energy landscape of PS exhibits the exponential shape of the energy barrier distribution agrees with the results from extreme-value statistics (Bouchaud and Mezard 1997 J Phys. A: Math. Gen. 30 7997) and the trap model by Bouchaud and co-workers (1996 J. Phys. A: Math. Gen. 29 3847, 2001 Phys. Rev. B 64 104417).

The phenomenon of the micromechanically detected magnetic resonance is interpreted in terms of nonlinear processes at magnetic resonance. A close analysis in the frequency domain of the irradiation and detection scheme shows that the technique corresponds to a multiple irradiation with one or more couple of frequencies separated by omega(c) and to the detection of the longitudinal component of magnetization oscillating at omega(c). The study of longitudinal detection of magnetic resonance allows the direct measurement of the spin-lattice relaxation time of samples. Samples of Mn2+:MgO prepared in order to obtain a mixture of spin systems with very different relaxation processes were studied by electron-spin-resonance experiments with micromechanical detection: measurements evidence a very strong rejection of the system with lower longitudinal relaxation time. Direct confirmation of the theoretical interpretation is obtained; in addition the microscopy technique increases its "contrast" capability, adding the possibility of determining maps of samples based on the distribution of both concentration and longitudinal relaxation times of spin systems.

The line shape measurements of the high-field high-frequency electron paramagnetic resonance transitions for paramagnetic Pb3+ in calcite performed at four frequencies between 95 and 285 GHz as function of temperature are reported. The linewidth analysis is based on a recent theory of relaxation for multilevel spin systems at high temperatures and is used to identify the spin lattice relaxation (SLR) mechanisms at such high microwave frequencies. In comparison with previous results obtained at lower microwave frequencies, this analysis emphasizes for a multilevel spin system two relevant new features: (a) the direct SLR process by modulation of the hyperfine interaction significantly contributes to the linewidths even at room temperature and (b) the SLR time of some simple transitions measured from the linewidth under conditions of negligible saturation could be very different from that measured by the continuous saturation method. The difference between the perpendicular-detected transitions and the parallel-detected transition concerning the contributions of the different SLR processes to the SLR times measured by the two methods mentioned above comes out to be very significant.

A vectorial analysis of magnetic resonance spectrometers, based on traveling wave resonators and including the reference arm and the automatic control of frequency, has been developed. The proposed model, valid also for stationary wave resonators, gives the response function of the spectrometer for any working condition, including scalar detectors with arbitrary response law and arbitrary excitation frequency. The purely dispersive and purely absorptive linear responses are discussed in detail for different scalar detectors. The developed approach allows for optimizing the performances of the spectrometer and for obtaining the intrinsic lineshape of the sample in a very broad range of working conditions. More complex setups can be modeled following the proposed scheme.

The experimental problems linked to the use of the ultra-wide band high field-high frequency Electron Paramagnetic Resonance (HF2-EPR) spectrometers are illustrated with some key examples. The single pass technique is critically analyzed and compared with other techniques. Propagation and magnetic field dependent phenomena are illustrated with the aid of three key examples and discussed. The possible actions to be taken in order to minimize and possibly eliminate these extrinsic effects affecting the spectra obtained with the single pass technique are proposed. Future instrumental developments are discussed in the light of the experience acquired in this study.

A dynamical model giving the infrared plasma reflection in a semiconductor is used to reproduce the transient reflectivity at 10.6 µm due to an intense photo-plasma. In this way we have derived the `cubed' coefficient of the Auger recombination in indium arsenide and gallium antimonide from the stationary peak reflectivity produced by a ns Nd laser. We have derived the cubic coefficients 12(4)×10-27 cm6 s-1 and 9(3)×10-28 cm6 s-1 for InAs and GaSb respectively.

Performing high-field and high-frequency electron paramagnetic resonance (HFHFEPR) experiments on a one-dimensional (1-D) magnetic system, we put in evidence a number of effects that can be attributed to extrinsic properties of the sample and have to be distinguished from the intrinsic physical properties of the investigated system. We have studied a 1-D molecular-based magnetic system made up by trivalent gadolinium ions (Gd3+) and the nitronyl-nitroxide radicals (NIT-Et = 2-ethyl-4,4,5,5,-tetramethyl-4,5-dihydro-1-H-imidazolyl-1-oxyl-3-oxide). In addition to showing the effects of short-range magnetic correlation on HFHFEPR spectra of this compound, this paper is also intended to be a starting point of an investigation of the methods according to which HFHFEPR spectra should be performed in order to estimate the importance, minimize and possibly correct the effects of orientation, propagation and demagnetization phenomena when dealing with magnetic materials.

In order to reproduce the transient IR-FIR reflectivity and transmission due to an intense photo-plasma generated by a fast laser pulse at a frequency above the band gap, we have used a dynamical model of plasma evolution. In this way we have derived both the `quadratic' and `cubed' coefficients of the Auger recombination in indium antimonide, by analysing the transient reflectivity at 10.6 µ and 119 µ induced by a fast Nd pulse. In particular we have derived a cubic coefficient of about 7±3×10-26 cm6 s-1, a result larger than those derived in previous experimental works but quite in agreement with the theory.