Modulation sidebands spectra within inhomogeneous CW-EPR line detected by double modulation EPR spectroscopy.

Conventional modulation spectrum, MS, in continuous wave electron paramagnetic resonance, CW-EPR, is produced by applying longitudinal radiofrequency (RF) fields with the frequencies, ωrf, which exceeds the linewidth, 2πδ, of a single spectral line given in frequency units. The second longitudinal RF field with frequency, ωrf2, is employed to produce double modulation spectrum, DMS. In this work DMS are presented as a specific type of MS which can be produced from an ordinary homogenous line. The numerical simulations of DMS in the limit of low power saturation relied on the recently introduced multi-photon transitions formalism which includes one microwave photon in combination with an arbitrary number of radiofrequency photons. It is shown that DMS of an inhomogeneous line exhibits similar basic structure as MS and exhibits sideband peaks at multiples of basic radiofrequencies. Linewidths of these peaks are significantly narrower (cca. two - three orders of magnitude) than the inhomogeneous linewidth and can be correlated with the underlying homogeneous linewidth components on the basis of characteristic spin-lattice, T1, and spin-spin, T2, relaxation times. The capability to extract T1 and T2 from DMS was tested on the well-known E' defect in irradiated vitreous SiO2. The obtained results revealed the impact of "rapid passage" effect on DMS in improving the detection sensitivity of DMS in the study of paramagnetic centers with long relaxation times. Therefore, double modulation method can be considered as a complementary method for studying inhomegeneous broadening in the EPR spectra.

Enhanced accuracy of the microwave field strength measurement in a CW-EPR by pulsed modulation technique.

The microwave magnetic field strength, B1, in the cavity of a conventional continuous wave electron paramagnetic resonance, CW-EPR, spectrometer was measured by employing modulation sidebands, MS, in the EPR spectrum. MS spectrum in CW-EPR is produced by applying the modulation frequency, ωrf, which exceeds the linewidth, δB, given in frequency units. An amplitude-modulated CW-EPR, AM-CW-EPR, was selected as detection method. Theoretical description of AM-CW-EPR spectrum was modified by adding Bloch-Siegert-like shift obtained by taking into account the cumulative effect of the non-resonant interactions between the driving fields and the spin system. This approach enables to enhance the precision of B1 measurement. In order to increase the sensitivity of the method when saturation effects, due to higher intensity of B1, decrease the resolution of AM-CW-EPR spectrum, detection at the second harmonic of CW-EPR has been employed.

Evidence for disorder in L-alanine lattice detected by Pulsed-EPR spectroscopy at cryogenic temperatures.

The unusual behavior of lattice dynamics of L-alanine has been assigned to intermolecular dynamics and localization of vibrational energy. Recent heat capacity and Pulsed-EPR measurements support presence of thermally activated dynamic orientational disorder in the L-alanine lattice below 20 K. In the present study, the additional evidence for possible thermally activated disordered behavior of L-alanine lattice have been obtained by investigating dependences of longitudinal relaxation time of first stable L-alanine radical, SAR1, on sample cooling rates for the same low temperature interval. The obtained relaxation time by Pulsed-EPR shows clear dependence on cooling rates and this behavior can be explained within two types of suggested spin-lattice relaxation mechanisms for the paramagnetic centers in the hydrogen-bonded organic crystal.

Thermal stability of radiation-induced free radicals in gamma-irradiated L-alanine single crystals.

Decay of the radiation-induced stable free radicals in l-alanine single crystals and powders at the temperatures from 379 to 476 K was examined by electron paramagnetic resonance. For single crystals, the calculated activation energy of the radical decay is 104.3+/-1.7 kJ/mol (i.e. 12 538+/-202 K) and the frequency factor lnv (O) is 24.1+/-0.4 min(-1). The lifetime of the radical in single crystals at 296 K is 162 years. The results confirm the long-term stability of the radicals, but the decay was found to be faster in large crystals than in powders.

Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures.

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.

An EPR method for probing surface magnetic fields, dipolar distances, and magnetization fluctuations in single molecule magnets.

We outline a spectroscopic method for probing the effective magnetic field B on the surface of crystals of the single molecule magnet (SMM) [(C6H15N3)6Fe8(mu3-O)2(mu2-OH)12]Br7(H2O)Br.H2O, (Fe8Br8). This technique utilizes the line shape changes in the EPR spectra of the organic radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) (g = 2.0036, single peak) adsorbed onto the sample. The temperature dependence of the EPR line shifts scale with the sample's magnetization as measured by a SQUID magnetometer. Analysis of the line shape in terms of dipolar coupling between the DPPH and the SMM molecules on the surface, yields their average dipolar distance. The method's potential for measuring the magnetization fluctuation dynamics is briefly pointed out using the SMM [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-acetate).

Probing magnetic fields on crystals of the nanomagnet Mn12-acetate by electron paramagnetic resonance.

We report on electron paramagnetic resonance (EPR) probing of magnetic fields and magnetic field gradients near the surface of a single crystal of the nanomagnet [Mn12O12(CH3COO)16(H2O)4].2CH3COOH.4H2O (Mn12-Ac). As the EPR probe, we utilized a 0.7 mm x 30 microm x 30 microm fibrous needle of the organic conductor N-methylphenazinium-tetracyanoquinodimethane (NMP-TCNQ), which yields an exceptionally sharp peak, with a 0.2 G (approximately 20 microT) width. In the presence of Mn12-Ac, the probe's peak exhibits splitting on temperature lowering, which depends on the orientation of the Zeeman field relative to the axis of easy magnetization of the employed Mn12-Ac crystal. The shifted peaks yield the magnitude of the magnetic field from Mn12-Ac crystal to which the various fibers of the probe are subjected. In conjunction with electron microscopy, the shifts yield the field gradient at the crystal surface and its change with temperature. For Mn12-Ac at 10 K, the surface magnetic field was measured to be in the mT range and its gradient on the order of 50 T/m.

A pulse EPR study of longitudinal relaxation of the stable radical in gamma-irradiated L-alanine.

The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature interval (5-290 K). The complex nonexponential recovery of the longitudinal magnetization in this temperature interval has been described with two characteristic relaxation times, 1/T*(1a) as the faster component and 1/T*(1b) as the slower component, respectively. It was shown that 1/T*(1a) is strongly affected by the CH(3) group dynamics of the SAR1 center. The complete temperature dependence of 1/T*(1a) was described by invoking several relaxation mechanisms that involve hindered motion of the CH(3) group from classical rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (Delta E/k=1570 K). On the other hand the temperature dependence of 1/T*(1b) is related to the motional dynamics of the neighborly NH(3) and CH(3) groups. We found a larger average potential barrier for this motion (Delta E/k=2150 K) corresponding to smaller tunneling frequencies of the neighbor groups.

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine.

The stable L-alanine radical induced by gamma-irradiation was examined by electron paramagnetic resonance (EPR), transfer saturation EPR and electron nuclear double resonance (ENDOR) in the temperature region of fast motion of the methyl group (180-320 K). From the obtained spectral line broadening and spectral intensity the correlation time for the methyl rotation was estimated. The complex processes determining the relaxation rate were examined in the same temperature interval. It was shown that important contributions to the relaxation rate arise from non-secular and pseudo-secular types of contributions. The non-secular contribution involves intramolecular dynamics while the pseudo-secular contribution originates from intermolecular motions. The obtained values for the dynamical parameters have been compared with those obtained by pulse EPR methods and by proton nuclear magnetic resonance (NMR) on undamaged crystals.

ESR Study of Electron-Nuclear Dipolar Relaxation for AsO4 4- Spin Probe in the Paraelectric Phase of KH2 AsO4

Saturation behavior of allowed and forbidden ESR transition of AsO4 4- paramagnetic probe in KH2 AsO4 was studied in the wide temperature interval around the paraelectric-ferroelectric phase transition, T c . The ratios between forbidden and allowed line intensities were employed to deduce information on the electron-nuclear dipolar (END) relaxation mechanism. It was shown that a proton END relaxation mechanism exhibits an extremal temperature behavior in the paraelectric phase around 230 K. The extremal temperature behavior was described by employing a model of proton hopping along the O-H...O bonds around the paramagnetic centers, and the correlation time of this hopping was estimated in the wide temperature range in the paraelectric phase (150-330 K). The temperature dependence of effective proton distance from the neighbor oxygens was obtained, and it was discussed in terms of a localization of the spin density on these oxygens caused by charge inbalance in the As-O bonds in the ferroelectric phase.

A Double-Modulation ESR Study of Internal Dynamics in the Glassy Polymer Matrix Detected by a Nitroxide Spin Probe

The double-modulation ESR spin-probe technique was employed to study slow-motional dynamics in amorphous polymer matrices of polybutadiene, polyisobutylene, poly(methyl methacrylate), and paraffin. It was shown that the low-lying excitation levels of the polymer matrix, found in the range from 77 to 190 K below the internal spin-probe transition, affect the effective relaxation rate of a perdeuterated nitroxide probe. The interrelationship between the effective relaxation rates and the local molecular dynamics of side-chain groups attached to the polymer backbone was observed. Possible relaxation mechanisms are discussed within the electron-nuclear dipolar interaction.

Free radicals from single crystals of deoxyguanosine 5'-monophosphate (Na salt) irradiated at low temperatures.

In single crystals of the DNA nucleotide 2'deoxyguanosine-5'phosphate (5'dGMP) X- or gamma-irradiated at 4.2 K or 15 K, two primary radical species can be discriminated and assigned to the cation and anion of the guanine base, G(+) and G(-). Both species are unstable. G(-) partially transforms into a secondary radical at 4.2 K, the latter being the precursor to the dominant 300 K species formed by net H-addition to carbon C8. The secondary radical, together with another intermediate appearing at 77 K and perhaps connected with the anion decay could not be structurally identified. The guanine cation G(+) transforms upon annealing to temperatures above 77 K into a more stable species by deprotonation at position N1.

An e.s.r. study of stable radicals in gamma-irradiated single crystals of uridine 5'-phosphate (Na salt).

Electron spin resonance has been used for the study of the radical in gamma-irradiated single crystals of 5'-UMP (sodium salt). Six types of radicals have been identified at room temperature. They are formed by addition of H atoms to C5, C6, 02 and 04, and by an OH addition to C5 of the uracil ring. Five of these radicals have an appreciable spin density on C6. The radicals with the addition to C5 or C6 are less stable than than the essentially planar radicals with the H addition to 02 and 04.