Electron paramagnetic resonance spectra of nearest neighbour pairs have been measured for the light lanthanide ions Ce3+ and Nd3+ in lanthanide nicotinate dihydrates, for comparison with the properties of the heavy lanthanide (HLn) ions Tm3+, Dy3+, Tb3+ and Er3+. Their properties +are found to be quite different: (a) the g–matrices have one small and two large principal values in contrast to one large and two very small for HLn, (b) the g-matrix is much less changed by replacing a nearest neighbour in a dimer containing two Ln3+ ions by a La3+ ion than for HLn and (c) the interaction between a nearest neighbour pair has a large non-dipolar component in contrast to almost purely dipolar interaction for HLn.

Results of EPR measurements of the Van Vleck paramagnets (monoclinic holmium nicotinate dihydrate and activated cubic complex fluoride KY3F10:Ho3+) are presented. Spectra were taken with a wide-band spectrometer operating in the frequency range 65–535GHz at liquid helium temperature for magnetic fields up to 1T. Transformations of the hyperfine structure of the EPR spectra with the frequency of the radiation and the directions of the constant and alternating magnetic fields have been studied and compared with results of calculations based on a diagonalization of the complete electronic–nuclear Hamiltonian. Crystal field splittings between the ground and first excited singlets of 7.3cm−1 in holmium nicotinate and 5.8cm−1 in KY3F10:Ho have been estimated using extrapolations of the resonance frequency dependences on magnetic field to zero field strength.

The majority of the experimental results of previous measurements of optical absorption and fluorescence, including the Zeeman effect, of magnetic susceptibility and electron paramagnetic resonance, on lanthanide nicotinate dihydrates of the heavier half of the lanthanide group, have been accounted for by a model of the crystal field. This crystal field is constructed within the framework of the exchange charge model in an approximation with only four fitted parameters.

Electron paramagnetic resonance (EPR) measurements have been made on Dy3+ and Er3+ in lanthanum nicotinate dihydrate (LaND) single crystals, for comparison with the information described in the previous paper about single crystals of DyND and ErND. For both DyND and ErND, as for TmND, the measurements were found to be consistent with a ground state with a large amplitude of Mj ═ ±J, an almost Ising-like g-matrix, and a purely magnetic dipole-dipole interaction between nearest neighbour (NN) Ln3+ ions, where Ln is either Dy or Er. In contrast, for Ln3+ dilutely substituted into LaND, although the g-matrix has one principal value much larger than the other two, the principal direction is different from that for LnND. EPR in crystals of LaND containing between 0.01 and 0.1 mole fraction of Ln shows that isolated ions have a g-matrix very similar to that in more dilute crystals, and NN pairs have a g-matrix similar to that for LnND and a purely magnetic dipole-dipole interaction. Hence, the Kramers ions Dy3+ and Er3+ are seen to exhibit the same features as were found for the non-Kramers ions Tb3+ and Tm3+ in the lanthanide nicotinate dihydrates.

Measurements have been made in lanthanide nicotinate dihydrate (LnND) single crystals, where Ln = Dy and Er, of optical absorption, including the Zeeman effect, of magnetic susceptibility between 1.4 and 4.2 K, of electron paramagnetic resonance between about 9 and 35 GHz, and of magnetic resonance at 22 MHz. As in the case of TmND (Baker et al. 1986a), the measurements are consistent with a ground state with a large amplitude of M(J) = +/- J, an almost Ising-like g-matrix, and a purely magnetic dipole-dipole interaction between nearest-neighbour Ln3+ ions. In this respect, the Kramers ions Dy3+ and Er3+ exhibit features similar to the non-Kramers ions Tb3+ and Tm3+ in the nicotinate dihydrates.

Optical absorption and fluorescence, magnetic susceptibility and electron magnetic resonance experiments have been made of Tb3+, 4f8, ions in lanthanide nicotinate dihydrate single crystals. Optical absorption spectroscopy in undiluted terbium nicotinate dihydrate (TbND) indicates that the ground state of Tb3+, 7F6, is a singlet exhibiting a quadratic Zeeman effect, and this is confirmed by the observation of a temperature-independent Van Vleck paramagnetism below 4.2 K. This is what one would expect for a non-Kramers ion in a crystal field of no symmetry. In spite of this, electron paramagnetic resonance (epr) is observed from Tb3+ ions both in dilute LaND containing 0.01 mole fraction of Tb and in TbND. In the dilute material, in spite of the absence of site symmetry, the results show a nearly degenerate non-Kramers doublet with a g value of 17.2(8), close to the maximum possible value of 12 gj; the ground state must therefore be predominantly Mj = ±6. The epr in TbND arises from some type of defect site involving two interacting dissimilar Tb3+ ions. The spectrum of Tb3+ ions observed by optical fluorescence is also attributed to defect sites.

The series of compounds with stoichiometric formula [Ln(C 5 H 4 NCO 2 ) 3 (H 2 O) 2 ] 2 , in which Ln is a trivalent lanthanide ion, have a dimeric molecular crystal structure with space group P2 1 /c.Optically clear single crystals of the thulium compound, with typical dimensions 3 mm x 2 mm x 1 mm, have been grown from aqueous solution. The structure is such that Tm 3+ ions occur in dimeric pairs with separation 0.4346 nm. The two ions in the pair are related by inversion symmetry, but there is no point symmetry at each thulium ion site. The crystals have been studied by optical absorption (partly with the use of a tuneable dye laser) at 490 nm wavelength; magnetic resonance spectroscopy at 35, 17, 10 and 0.02-0.06 GHz ; and magnetic susceptibility measurements. In spite of the absence of site symmetry, the results show that the ground state of each Tm 3+ ion, 4f 12 , 3 H 6 is a non-Kramers doublet with a single g value of 13.59(5), Close to the maximum possible value 12 g J = 13.97. The ground state must therefore be predominantly M J = | ± 6 >. The axis of the unique g value lies about 10° from the inter-ion vector. The magnetic properties are dominated by interaction between the two Tm ions within each dimer ; in zero applied field this splits the fourfold ground state of the pair by 0.927(15) cm -1 , i.e. 27.79(45) GHz, almost identical with the value of 0.926(72) cm -1 corresponding to pure dipolar interaction. Although the lower level is that with parallel spins, there is no evidence of longer range cooperative ordering down to 0.55 K.

E. p. r. at microwave frequencies in single crystals of lanthanum nicotinate dihydrate (LaND) doped with ca. 0.01 mole fraction of thulium shows no spectrum of isolated Tm3+ ions, but does exhibit spectra from coupled pairs of nearest-neighbour Tm3+ ions. The spin hamilton parameters are approximately the same as those measured for such pairs in pure TmND; in the dilute material the 169Tm hyperfine structure is resolved and a small zero-field splitting of the ground doublet of each Tm3+ ion is measured, described by a term δSx in the spin hamiltonian. A careful measurement of the conditions for excitation of e. p. r., and of the relative intensities of the lines, indicates that the transitions are driven by matrix elements of the Zeeman operator, involving components of the microwave magnetic field parallel to the principal axis of the g matrix, which are non-zero only if there is a term in δ. It is suggested that there is a similar term in δ in pure TmND, whose influence upon line positions is unresolved, but which is necessary to produce the observed transition intensity. However, such a term would render ∆MS = ±1, ∆mI = 0 transitions unobservable at r. f. frequencies (15-30 MHz) as it would produce too large a separation between the energy levels ; so it would leave us unable to account for the magnetic resonance at r. f. frequencies in pure TmND. Further experiments are required to produce an understanding of the transition mechanisms in pure TmND.

Magnetic susceptibility and optical absorption measurements, and EPR spectra at 20 MHz, and 8, 17 and 35 GHz, confirm the presence in thulium nicotinate dihydrate [Tm(C5H4NCO2)3(H2O)2]2 of interacting dimers of Tm3+ ions, each in a non-Kramers doublet ground state. The level corresponding to antiparallel spins lies 0.86(2) cm-1 above that of parallel spins.

A provisional sequence was determined by the plus and minus method (Sanger et al. 1977).1 It has now been completed using the termination method of Sanger et al. (1977).2 About 30 alterations have been made and therefore the numbering is somewhat different from that used in the provisional sequence. The DNA sequence given is that of the circular viral (plus) strand. Numbering is from the unique Pst site, but the sequence is written starting at the first nucleotide after the termination of the gene- H protein. The letters in the left-hand margin indicate the proteins whose sequence is given on the corresponding line. Restriction enzyme recognition sites are indicated by underlining. The single-letter code for the restriction enzymes is as follows: A, Alu I; F, Hin fI; H, Hha I; M, Mbo II; P, Pst I; Q, Hph I; R, Hin dII; T, Taq I; Y, Hap II; Z, Hae III. The position of the origin of viral-strand replication is based on the work of Langeveld et al. (1977).3 Other references are given in our earlier paper (Sanger et al. 1977)1 and elsewhere in this volume. The cs 70 mutation is cold sensitive and is believed to be a point mutation in the gene- F protein (C. A. Hutchison III, pers. comm.).

The method of electron nuclear double resonance has recently been employed by Feher for investigation of some solid-state problems. In this letter the application of this method for an investigation of Nd/sup 3+/ in LaCl/sub 3/ crystals is discussed. A tabulation is made of the frequencies so far observed at which the second resonance for Nd/sup 145/ occurs when the c axis is parallel to H. The remainder of the measurements are in progress. (A.C.)

The paramagnetic resonance spectrum in crystals of GdCl3 diluted with LaCl3 and CeCl3 has been examined. The splitting of the ground level 8S7,2 of Gd3+ in zero magnetic field is small, and has a remarkable temperature dependence. A theory to account for the experimental results is developed, and it is shown that perturbation mechanisms both linear and quadratic in the crystalline field potential are important.

Paramagnetic resonance absorption in single crystals of ${\mathrm{U}}^{233}$${\mathrm{Cl}}_{3}$ diluted with La${\mathrm{Cl}}_{3}$ has been investigated. The values of the parameters in the appropriate Hamiltonian are given. The values of the nuclear spin, magnetic dipole moment, and electric quadrupole moment obtained from these studies are discussed. The ratios of the mo ments of ${\mathrm{U}}^{235}$ and ${\mathrm{U}}^{233}$, which can be obtained with much higher accuracy than the individual moments, are given.