Single Crystal Host Research Articles

A study of ferric trisoxalate photochemistry in single crystals by electron paramagnetic resonance. An intermediate exchange-coupled ferrous bisoxalate-radical species

The EPR of exchange-coupled Fe(C 2O 4) 2 2− and C 2O 4 − is observed after the photolysis and heating of Fe(C 2O 4) 3 3− in NaMgAl(C 2O 4) 3·9H 2O or K 3Al(C 2O 4) 3·3H 2O single-crystal hosts. The small 0.052 cm −1 antiferromagnetic coupling is determined from the g anisotropy (the x, y, z principal values are 2.0891, 2.0346 and 1.9970 respectively) and the temperature dependence of the EPR of the lowest Framers doublet of the coupled spin system. The Fe(C 2O 4) 2 2− spin Hamiltonian parameters ( D = 17 cm −1, E = 3.3 cm −1, g xx = 2.35, g yy = 2.47 and g zz = 2.077) and the axial and rhombic distortion parameters D = −173 cm −1, E = −27 cm −1, respectively, were also determined from the data. The axial direction of the distorted square-planar arrangement of the inner oxygens of Fe(C 2O 4) 2 2− lies only 14.2° from the two-fold axis of the Fe(C 2O 4) 3 3− precursor and the axis of the rhombic distortion (the x axis) lies 54.8° from the three-fold axis of the precursor. It is suggested that the population distributions, the anisotropies and the differences in the spin-lattice relaxation rates between the Fe(C 2O 4) 2 2− spin levels lead to the observation of EPR in only one of the Kramers doublets. The well-oriented nature of the product weighs against the dissociation of C 2O 4 − into CO 2 − and CO 2 in the crystal. The absence of proton hyperfine structure in the spectrum shows that the protonation of C 2O 4 − (from the crystal waters of hydration) does not occur, as it evidently does in solution. Comparison of results in the two crystal hosts shows that the ratio of thermal recombination of the photolysis product (which has no EPR spectrum) to Fe(C 2O 4) 3 3− and dissociation to Fe(C 2O 4) 2 2− and C 2O 4 − varies from ligand to ligand and from host to host. The ratio appears to be very sensitive to the orientation of the reacting ligand and to the crystal packing around it.

The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting ΔvQ of the nuclear hyperfine interaction due to an electric field gradient Vzz. MAPON has successfully been applied to measure ΔvQ as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes56Co,57Co,58Co and60Co have been studied in iron single crystal hosts, yielding ΔvQ consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe system. The variation of ΔvQ with direction of magnetization, measured in58CoFe and60CoFe single crystal samples, is also described. Further MAPON measurements are described for a56CoFe polycrystalline sample, for which the most probable value and width of the distribution of Vzz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.

Excited state absorption of Sm2+ in SrF2 and SrCl2

The excited state absorption spectrum of Sm2+ diluted as an impurity in the single crystal hosts SrF2 and SrCl2 has been observed. The absorption peaks associated with the 5D0(4f6) excited state were found to be displaced by approximately 3000 cm−1 from the absorption bands arising from the 7F0(4f6) ground state when the 5D0–7F0 energy was taken into account. The observed peak separation is ascribed to the 4f–5d exchange interaction. From the measured peak positions and band intensities, the exchange energy for the Sm2+ impurity in the two host crystals was determined to be about the same order of magnitude as that expected for the free ion. The implications of the properties of the excited state absorption bands for the performance of CaF2:Sm2+ and SrF2:Sm2+ lasers are considered.

Pressure dependence of the electric field gradient at Po impurities in Bi single crystal

The electric quadrupole coupling constants for210Po(6+) isomers in a Bi single crystal host have been measured at ambient, 3 Kbar and 6 Kbar hydrostatic pressures. Assuming a linear pressure dependence of the EFG we find the logarithmic derivative dlnq/dP=−8(5)Mbar−1. This result is analyzed in a quantitative way in a point charge model, considering the elastic and lattice constants of Bi, and in a qualitative way in the covalent bond picture.

Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets

Magnetic behavior of nonet tetracarbene as a model for one-dimensional organic ferromagnets

Electric quadrupole moment of the 211Rn (63/2)- isomer: Absence of core deformation at very high spins.

The electric quadrupole moments of $^{211}\mathrm{Rn}$ isomers were determined as $Q({\frac{17}{2}}^{\ensuremath{-}})=19(2)$ e.fm2 and $Q({\frac{63}{2}}^{\ensuremath{-}})=160(22)$ e.${\mathrm{fm}}^{2}$ via measurements of the quadrupole interaction in a Bi single-crystal host. The results indicate that the spherical shape is preserved in the lead region even at the core-excited very-high-spin regime, in contradiction to theoretical predictions.

Nuclear polarization and signs of quadrupole moments of high-spin Gd isomers

Time-differential perturbed-angular-distribution (TDPAD) experiments have been performed on polarized intermediate- and high-spin isomers 144 Gd(10 +) and 147 Gd( 13 2 +, 27 2 −, 49 2 +) . Excited Gd nuclei populated by ( 28 Si, x n) reactions were polarized by the tilted-multifoil technique and implanted in a Gd single-crystal host. Observation of the subsequent electric quadrupole interaction with the known electric field gradient yielded negative values of the sign of the deformation for all isomers studied. The magnitude of the induced nuclear polarization P I was also determined from the TDPAD data and compared to model calculations in order to deduce the average atomic angular momentum and polarization of Gd ions at v/ c ∼ 0.018.

Low temperature spectroscopy of internally hydrogen-bonded 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in a mixed crystal

Low temperature fluorescence and fluorescence excitation spectra of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole (MeHPB) in a single crystal biphenyl host are reported which exhibit sharp vibrational structure. The spectra are analyzed in terms of seven vibrational modes including three phenyl modes, three normal modes involving N–N and C–N stretching and bending motion, and a low frequency torsional vibration about the central C–N bond. The fluorescence exhibits a normal Stokes shift indicating that excited-state intramolecular proton transfer is precluded in this mixed crystal probably because the MeHPB molecule is held in a nonplanar conformation by molecular packing forces. A model of the MeHPB molecule based on semiempirical molecular orbital calculations is also presented which provides support for these conclusions.

Intercalate structure, melting, and the commensurate-incommensurate transition in bromine-intercalated graphite

In situ high-resolution x-ray scattering experiments have been carried out to study in-plane intercalate structure and phase transitions as a function of temperature in a single crystal graphite host. For the case of bromine-intercalated graphite the intercalate plane has three sublattices and each sublattice has a centered ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}7$) rectangular structure with four ${\mathrm{Br}}_{2}$ molecules per two-dimensional unit cell in the commensurate phase. The coherently ordered in-plane bromine regions exceed 10 000 \AA{} in size. Above the commensurate-incommensurate transition (342.20\ifmmode\pm\else\textpm\fi{}0.05 K), a stripe domain phase becomes established in a single domain of a sublattice along the sevenfold direction. The incommensurability as a function of reduced temperature exhibits a power law with an exponent of 0.50\ifmmode\pm\else\textpm\fi{}0.02, confirming the existing theories. The relative shifts observed for the various harmonics are accurately predicted by a sharp-domain-wall model with $\frac{4\ensuremath{\pi}}{7}$ phase shifts. A power-law line shape is observed for the incommensurate intercalate layer, yielding values for the exponent $\ensuremath{\eta}$ consistent with model calculations. Results on the temperature dependence of the intensities, linewidths, and line shapes of several Bragg peaks around the melting transition are presented. The intercalate layer exhibits a continuous melting transition from a two-dimensional solid phase to an anisotropic fluid phase, occurring at 373.41\ifmmode\pm\else\textpm\fi{}0.10 K for a stage-4 compound.

Nuclear quadrupole alignment in Zn, Lu, and Re and the quadrupole moments of69m Zn and177m Lu

We report low-temperature nuclear quadrupole alignment of69m Zn,177m, g Lu and186, 188Re in the corresponding metal single crystal hosts. A comparison with data on these systems from other laboratories is given, and sources of systematic error discussed. In the Lu system, measurements on in situ irradiated and implanted samples are found to be in agreement. From the measured quadrupole frequencies, we derive the following quadrupole moments: Q(69m Zn, 13.86 h, 9/2+)=−0.45(7)b,Q(177m Lu, 161 d, 23/2−) =4.23(67)b,Q(186Re, 90h, 1−)=0.57(9)b, andQ(188Re, 16.7h, 1−)=0.38(17)b.

Electron spin-echo detection of nuclear quadrupole resonance in photoexcited triplets

Electron spin-echo experiments have been performed on the photoexcited triplet state of phenazine in a fluorene single crystal host. The relatively strong nuclear modulation observed contains the nuclear quadrupole resonance spectrum of the nitrogen nuclei in the phenazine triplet. Detailed analysis gives a coupling of 4.10+/−0.07 MHz for e2qQ/h and 0.22+/−0.05 for η. The principal axes of the nuclear quadrupole interaction coincide with the principle axes of the zero field splitting. Electron spin-echo detection of nuclear quadrupole resonance is not limited to molecules where the quadrupole and the zero field splitting axes coincide and should therefore be a useful probe of the electronic and physical structure of excited triplets of complex organic and biological molecules.

The circularly polarised luminescence spectrum of [Cr(en)3]3+ in the uniaxial single crystal host 2[Rh(en)3Cl3].NaCl.6H2O

The circularly polarised luminescence spectrum of a transition metal ion complex in a uniaxial single crystal host has been measured for the first time; individual vibronic lines of the [Cr(en)3]3+(en = ethylenediamine) emission spectrum were found to be circularly polarised and two distinct chiral emitting species were detected.

Conformational effects in the absorption spectra and photochemistry of [2,n](9,10)anthracenophanes (n = 2,3)

The absorption spectra of [2.2](9,10)anthracenophane (1) and [2.3](9,10)anthracenophane (II), in condensed media, contain overlapping contributions from two conformational isomers with quite different spectra. The spectra of the two conformations of I and of one conformation of II have been separately determined by a solid state method. This involved the incorporation of the photoisomers ofI (1,2,7,8,-tetrahydro-2a,6b[1′,2′]:8a, 12b[1″,2″] -dibenzenodibenzo [a,c] dicyclo- buta[e.g]cyclooctene) and II (1,2,8,9-tetrahydro-2a,6b[1′,2′]:9a,13b[1″,2″]-dibenzeno-7H-dibenzo[a,e]cyclobuta[c]-cyclopenta[g]cyclooctene) in host single crystals of the photoisomer of 1,3-di(9-anthryl)propane, I and II were then obtained by thermal dissociation and their spectra measured at 8 K. The two conformers of 1 were separated by photoselection, the rotated conformer being between 4 and 5 times more photoreactive than the translated conformer. A modified molecular force field program was used to simulate the environment around the guest molecule (I) and the resultant geometries and orientations in the (disordered) site of the host crystal are in excellent agreement with the polarization analysis of the absorption spectra. Two crystal orientations of the rotated conformer were found. For II, the rotated conformation, with two orientations, dominates the absorption spectrum and the translated conformer could not be obtained by photoselection. The force field program was used to calculate the molecular geometries (gas phase) of both conformations of I and II and their photoisomers, I was also obtained by ultraviolet photodissociation of its photoisomer at 8 K. Both conformers were obtained with the same ratio as for the thermally treated crystals. This result considered with the photoselection experiments, demonstrates that the reversible photoisomerization does not proceed via biradical intermediate.

High-resolution ODMR spectroscopy of phenazine in single crystal and polycrystalline host matrices

Experiments have been performed on a model guest molecule, phenazine, in biphenyl single crystal and n-heptane polycrystalline host matrices to illustrate the two main contributions to inhomogeneous line broadening in ODMR spectroscopy. These arise from (a) inhomogeneous crystal fields experienced by the guest molecule in a host lattice and (b) unresolved hyperfine splitting. It is shown that the former may be reduced by making optical site selection during both excitation and detection. In the latter category, the most offensive group of nuclei is shown to be the guest protons and to a lesser extent the near-lying host protons. This contribution may be drastically reduced by fully deuterating both guest and host systems. Performing ODMR spectroscopy under these conditions allows the typical linewidths of 1–10 MHz to be reduced to ca. 200 kHz. The advantages of such high-resolution spectroscopy are manifold, not the least of which being that it forms a prerequisite for highly specific multiresonance experiments such as the recently developed ODNQR method.

On Cross Relaxation In Photoexited Triplet States

Variable frequency ODMR experiments on a photoexcited triplet state molecule oriented in a single crystal host have been used to show that an anticrossing exists in the energy level diagram of the guest electron spins plus host nuclear spins at cross relaxation fields. As a consequence, the rate of energy transfer between guest and host is governed by nuclear spin-spin rather than the much slower spin- lattice relaxation. This has been confirmed by a direct measurement of the cross relaxation time (110 μs) in perdeuterobenzophenone in 4,4'-dibromodiphenylether, using microwave pulse techniques.

ESR of triplet naphthalene pairs, evidence for non-resonance

ESR measurements on triplet naphthalene guest pairs in a host single crystal of perdeuterated naphthalene have been performed between 0.7 K and 4.2 K. Hyperfine splitting is partially resolved even at lowest temperatures, if the field H O is oriented along the principal pair axes. At arbitrary orientations however the width of the observed ESR line (M-line) decreases monotonically with increasing temperature, whereas its absolute value depends on crystal quality. The saturation of the ESR transitions at low temperatures produces a dip in the M-line. In order to explain the inhomogeneously broadened lines at lowest temperatures a model is developed in terms of the eigenstates of the hamiltonian of the pair including non-resonance between pair molecules of different excitation energies. We are able to correlate quantitatively the low temperature ESR line width with the width of the inhomogeneously broadened optical line which yields the distribution of guest energies within the crystal. The ESR line narrowing effect at higher temperatures is attributed to random phonon processes disturbing the coherence in the energy transfer.

Frequency-dependent endor spectroscopy. Absolute signs of the hyperfine elements of triplet states

A new method for determining the absolute signs of the hyperfine elements of triplet states using frequency-dependent electron—nuclear double resonance spectroscopy is described and applied to 3(n, π*) 13C-benzophenone-d10 in the single crystal host 4,4′-dibromodiphenylether.

Low-symmetry effects in the EPR of Gd 3+ in two double sulfates

The EPR of Gd 3+ in two host single crystals K 2Co(SO 4) 2·6H 2O and K 2Ni(SO 4) 2·6H 20 is studied at room temperature using a Q-band spectrometer. Two magnetically inequivalent sites which are equivalent in the (010) plane of the crystals are observed. The spectra are characterized by the noncoincidence of the magnetic axes of the transitions for a particular site. This is attributed to a local symmetry less than orthorhombic. Approximate values of the parameters are obtained using perturbation corrections up to fifth order and an iterative least-squares method is used to evaluate the parameters precisely. It is concluded from the crystal-field parameters expressed in the principal coordinate system that the local symmetry is approximately monoclinic. By assuming that Gd 3+ occupies a divalent site, it is found from the crystal structure data off (NH 4) 2Ni(SO 4) 2·6H 2O that the principal z and x axes are approximately along the oxygen atom and potassium vacancy directions, respectively. The local symmetry is found to be C s with the z axis perpendicular to the mirror plane.

Analysis of the ESR spectra of the cholestane spin label in oriented multibilayers of lecithin and cholesterol in the gel state

The ESR spectra of the cholestane spin label in macroscopically oriented multibilayers of dipalmitoyllecithin and cholesterol have been studied in the temperature region from −17 to 40°C, below the liquid crystalline transition temperature of these systems. A qualitative analysis of the angular dependence of the powderlike ESR spectra is given, showing that in the pure lecithin system the molecules are tilted, whereas in multibilayers with 10 mole % cholesterol or more, no tilt of the molecules is found. The ESR spectra were computer simulated, using a simple exponential distribution function for the orientation of the spin label molecules in the bilayer, in terms of a static tilt angle and spread angle. In addition the components of the g-factor and hyperfine tensors of the cholestane spin label have been determined. A comparison has been made of the components of the g-factor and hyperfine tensors obtained from the computer simulation and components reported in the literature, which have been obtained with spin labels in single-crystal hosts.

The effect of fluorine substitution on the zero-field splittings of naphthalene

Zero-field splitting parameters of 1-fluoro- and 2-fluoronaphthalene in single crystal hosts of durene and biphenyl have been measured in experiments near zero magnetic field. Disagreements with previous reports concerning the effect of fluorine on the ZFS parameters are discussed and are shown to arise due to crystal field effects.