The paper deals with the main stages of scientific and organizing activities in spectroscopy of member- correspondent of USSR Academy of Sciences, Professor SA Altashuler, as theorist, worked in the fields of magnetic and spectroscopic phenomena and nuclear magnetism. His name is known with the prediction of neutron magnetic moment, acoustic paramagnetic resonance phenomenon discovery, and a series of fundamental publications on paramagnetic resonance.

The Cm³⁺ ion, atomic number 96, has the electronic configuration and is the actinide analog of the lanthanide ion Gd³⁺. The ground term has only approximately 87 percent ⁸S_7/2 parentage due to the large spin-orbit interaction as compared to the almost 100 percent ⁸S_7/2 parentage for Gd³⁺. The electrostatic interaction between equivalent electrons in the f⁷ shell results in a large gap between the octet state of the ground term and the sextet states of the excited multiplets. For Gd³⁺ this splitting results in a gap greater than 30,000 cm^-1. The greater spatial extent of the 5f electron shell results in a smaller electrostatic interaction between equivalent electrons in the 5f shell than in the 4f shell. Thus for Cm³⁺, this splitting is on the order of 16,000 cm^-1. The ground term multiplet splitting is small, because the largest component of the ground multiplet has zero angular momentum. However for Gd³⁺ in most crystals, this ground term splitting is much less than approximately 1 cm^-1. Early detailed studies of the Cm³⁺ optical spectra were performed with the ²⁴⁴Cm isotope. During the past decade or so, multi-milligram quantities of ²⁴⁸Cm have become available. A number of single crystals have been doped with the ²⁴⁸Cm isotope and optical studies of these samples have been performed using laser selective excitation and fluorescence techniques. Excited state absorption studies allowed the collected of data to 40,000 cm^-1 using two visible lasers. Results from these experiments will be reviewed and comparisons will be made between the 5f and 4f series.

We studied the optical spectra and luminescence kinetics of double chloride Kpb₂Cl₅:TR³⁺ crystals as a new luminescent material promising for UV, VIS and mid-IR lasers, pumped with laser diodes. Intensity parameters were determined by the Judd-Ofelt method, radiative and non- radiative transition probabilities were calculated. It is shown that low multiphoton relaxation rate in the se crystal together with high values of radiative probabilities leads to the evidence of high intensive luminescence in spectral domain from 360 to 5000 nm. These features make these crystals promising for practical applications as active media for UV, VIS and mid-IR solid state lasers and amplifiers.

Spectroscopic investigations of Tb doped KPb₂Cl₅ and CsCdBr₃ crystals and AgBr_1-xI_x polycrystalline fibers were carried out. Luminescence in 4.3-5.5 micrometers region was found in the KPb₂Cl₅:Tb³⁺ and CsCdBr₃:Tb³⁺ crystals at room temperature. The AgBr_1-xI_x:Tb³⁺ polycrystalline fiber is expected to have luminescence in 3- 10 μm region. Gain properties of the crystals are evaluated.

Optical absorption of Eu³⁺ and Eu²⁺ ions in the polycrystalline europium oxides and fluorides was studied by the diffuse reflectance spectroscopy method. For all europium compounds, characteristic narrow peaks of 4f-4f electron transitions are observe. Their positions are rather independent upon the type of a compound. However, their intensity and number significantly vary, especially from the transition oxides to fluorides. In addition to those, shoulders of fundamental absorption in the oxide R_d- spectra of the oxide materials arise. Substitution of Eu²⁺ ions for Eu³⁺ ions in the EuF₃ - lattice causes a sharp change of the spectral characteristics. It is accompanied by an appearance of the wide band due to 4f-5d-electron transitions.

The positions of the Charge Transfer (CT) and the 4f- 4f^n-15d (fd) transitions of the trivalent rare-earth ions in CsCdBr₃ are estimated using the Jorgensen and Dorenbos methods, respectively. The CT spectra of Sm³⁺, Eu³⁺, Tm³⁺ and Yb³⁺ and the f-d transitions of Pr³⁺ in CsCdBr₃ were measured. Experimental and theoretical results are compared.

The narrow low intensity line with maximum at (lambda) ^-1 equals 15104 cm^-1 and width (delta) equals 0.7 cm^-1 at T equals 4.2 K have been detected in absorption spectrum of regular LiTmF₄ crystal. The observed line belongs to transition form ground state ³H₆((Gamma) ₂) to lowest level (Gamma) ₂ of ³F₂ term of Tm³⁺(4f¹²) ion. The perturbation, which induces (Gamma) ₂ - (Gamma) ₂ transition, does not break S₄ symmetry of Tm³⁺ sites. The perturbation of this kind is the interaction of Tm³⁺ 4f-electrons with electric field, created by quadrupolar moments of lattice ions, which are induced, int urn, by incident electromagnetic wave. To calculate the components of the quadrupolar moment tensors for ions in LiTmF₄ single crystal, we have formulated and solved the system of self-consistent linear equations. We take into account interactions between all induced dipolar and quadrupolar moments and their interactions with electric fields of incident wave. The calculated transition probailities per second are in qualitative accordance with experimental data.

Splitting of the first excited (Gamma) ¹₃₄(³H₆) doublet state of Tm³⁺ ions in the LiTmF₄ crystal in external magnetic fields up to 11 T has been studied using measurements of luminescence and far RI absorption spectra at low temperatures. In the magnetic field perpendicular to the S₄-symmetry axis of the crystal lattice, the splitting of the non-Kramers doublet is caused by the joint action of the nonlinear Zeeman effect and the piezo spectroscopic effect related to parastriction. Experimental dependences of the doublet splitting on the magnetic field strength and direction are in good correspondence with the theoretical estimations obtained in the framework of the semi phenomenological crystal field model. It is shown that spectroscopic data present direct information on the forced magnetostriction in the basal plane of uniaxial Van Vleck paramagnets that contain non- Kramers rare earth ions.

The effective method of decoding multicenter luminescence spectra permitting to separate lines of radiation of impurity centers with minimal difference of lifetimes no more than 5 percent is offered. It is achieved by using the computer simulation with utilizing a small amount of experimentally detected spectra. The efficiency of a procedure is shown on an example of the analysis of a multicenter luminescence spectrum of Yb³⁺ ions in the CsCsF₃ crystal with the use of only eight basis spectra.

The possibility of creation of self-frequency doubling laser crystal based on crystal KTP doped with chromium ions is investigated. The spectral properties of Cr³⁺ ions are researched. The magnitude of emission cross-section at the wavelength of 833 nm corresponding to the maximum of vibronic transition ⁴T₂-⁴A₂ of Cr³⁺ ion is estimated. The excited state lifetimes at room and liquid nitrogen temperatures are determined. The spectroscopic investigations show that impurity Cr³⁺ ions in KTP crystal are placed in positions with weak crystal field: Dq equals 1507 cm^-1, B equals 652 cm^-1, C equals 2800 cm^-1. The configuration diagram model with one excited state is discussed. The main features of fluorescence spectra and the expected laser emission in the near IR range form 700 to 1000 nm are considered.

In the absorption spectrum of the crystal there was stated the presence of Ti³⁺ ions at octahedral Ga positions in strong crystal field. Ti³⁺ ions arise due to some reducing conditions of the crystal growth. It was found that electron delocalization at higher temperature, e.g. In oxidation-reduction reaction: Ti⁴⁺ + e equals Ti³⁺ may appear in optical spectra as charge transfer bands in the NIR region giving rise to blue absorption. The observed fluorescence of ²T₂-²E transition of Ti³⁺ in the LaGaO₃/SrTiO₃ mixed crystals shows two bands centered at about 420 and 447 nm coming from excitation at 314, 340 and 384 nm.

Assignment of absorption lines of mixed valence Cr³⁺- Cr²⁺ pair center has been done by the analysis of the probabilities of exchange-induced electric-dipole transitions. Parameters of exchange interaction J₁ equals 44.5 +/- 5 cm^-1 and axial crystal field B⁰₂ equals -19.6 +/- 2 cm^-1 for the excited state have been estimated from the fine structure of absorption spectra of the pair. Using microscopic theory of superexchange interaction for the ground and excited states of the pair the value of the transfer integral of e_g-electron via the intermediate fluorine ion t_vv equals 3500 cm^-1 has been extracted.

The intra-center 4f-shell transitions ⁴I_13/2 yields ⁴I_15/2 of Er³⁺ ion introduced into Si crystal under original Sublimation Molecular Beam Epitaxy (SMBE) process are investigated by low temperature photoluminescence spectroscopy with resolution down to 0.5 cm^-1. Er-related optically active centers detected in SMBE Si:Er/Si structures are classified based on their spectroscopic features. The novel optically active Er- related center Er-1 specific for SMBE Si:Er/Si structures is identified. A novel type of Si:Er/Si structures namely selectively Er-doped multilayer structures with enhanced luminescence efficiency grown in SMBE process is demonstrated. The features of the energy transfer to Er³⁺ ions in SMBE Si:Er/Si structures as compared with ion implanted Si:Er/Si structures are discussed.

We have studied the f-f absorption spectra of Er³⁺ in GaN crystal grown by gas phase epitaxy in chlorine system. A detailed study of ⁴I_15/2 yields ²H_11/2 transitions in the region 19000-19260 cm^-1, has been carried out at 293, 77 and 2K. At T equals 2K the ⁴I_15/2 yields ²H_11/2 spectrum contains 6 lines in accordance with theoretical predictions for Er³⁺ in noncubic crystal field. The levels of ²H_11/2 multiplet are situated at: 19041, 19049, 19074, 19108, 19191, 19246 cm^-1. The number of lines observed at T equals 2K and its small width indicate that in our samples Er³⁺ ions occupy predominantly a unique position in GaN lattice. Most probably, Er³⁺ substitutes Ga atoms or enters in interstitial position.

The effect of rare earth and transition metal luminescent impurities on the optical properties of As₂S₃ glass is studied in a wide spectral region. The Raman, IR, band- to-band and edge absorption spectroscopies are employed to obtain information about the incorporation of impurity ions in the host glass structure and the corresponding changes in intrinsic optical characteristics. The effects of light- soaking and thermal treatment on the doped As₂S₃ glasses were examined as well. In the fundamental absorption region a reflectivity maximum at 2.98 eV shows blue or red shift depending on the electro negativity of the impurity, in accordance with the corresponding variations of the glass structure. Near the edge absorption the impurity affects strongly the slope and the magnitude of the weak absorption tail. The observed effects of metal dopants on the As₂S₃ glass are discussed in connection with the expected behavior oft he impurities in the glass.

Many fewer 3d ions are known to be incorporated at lattice sites in point defects in diamond than in silicon. In diamond, only nickel and cobalt have been positively identified. There appears to be a great variety of nickel- containing sites, and several different cobalt-containing sites, some of these also involving substitutional nitrogen. The evidence for incorporation of other 3d atoms is much less certain, but there is possibly some for Ti, Cr, Mn, Fe, Cu and Zn. Some general inferences may be drawn from these data.

All known solid-state UV/VUV active media based on the interconfigurational 5d-4f transitions of trivalent rare earth ions in wide-band gap fluoride crystals are solarizable under intense UV/VUV pumping. Active media- specific pumping-induced color centers, on one hand, would absorb the laser radiation and reduce the laser efficiency. On the other hand they also get bleached by the pump laser radiation so that the current value of total intracavity losses is governed by dynamic equilibrium and depends on some active medium microparameters as well as the pump flux and is cavity specific. Presented here are the model of dynamic processes in a UV solarizable active medium under pumping conditions and the new technique of laser experimental data analysis for variable intracavity losses based on this model. It is shown that properly set laser experiment enables us to obtain the necessary active medium microparameters, including the pump excited state absorption and color center absorption of laser radiation cross- sections, photoelectron trapping rate by host lattice defects and their recombination rate. The results of this technique application to the laser experiment analysis for the series of Ce³⁺:LiYb_xLu_1-xF₄ single crystals are presented.

The results of spectroscopy and laser action of disordered calcium niobium gallium and calcium lithium niobium gallium garnets doped with Nd³⁺, Er³⁺, Eu³⁺, Ho³⁺ and Tm³⁺ are displayed.

The dynamics of the intracavity absorption was studied in a cw Co:MgF₂ laser. The linear growth of the intracavity absorption was observed for the generation time values form 0 to at least 4 ms which corresponds to a 1200-km effective absorption path length.

The luminescence e properit4se of ytterbium ions in strongtium haloborates were studied under optical, x-ray and synchrotron excitation. The emission spectra of Sr_2(1-x)Yb₅O₉R consists of a blue and a UV band. The former is due to 4f¹³5d yields 4f¹⁴ transtions on the Yb²⁺ ions. The latter is tentatively assigned to charge transfer luminescence of Yb³⁺ ions. The influence of the structural features of Sr₂B₅O₉R and some preparative conditions on the luminescent behavior of Yb ions is discussed.

For the first time the result of luminescent properties investigations of Ce doped Si_3-xAl_xO_yN_4-y solid solutions are presented. The interest of the sialon study is due to high radiation hardness and chemical stability of this system. The aims of Ce doping are: 1) the efficient phosphor creation, and 2) the investigation of the host matrix by using Ce as a control mark.

Among rare-earth ions Ce³⁺ has the simplest electron configuration - 4f¹. Its 4f-5d transitions have an energy of approximately 6.3 eV for a free ion and are quite allowed. Thus, the introduction of Ce³⁺ into the crystal with wide enough forbidden gap makes it possible to analyze the lattice environment influence on the excited state. During the present research the following results were obtained via spectral-kinetic investigations of the excited states of Ce³⁺ impurity in various third group metal oxides: (a) luminescence spectrum of Ce³⁺ has a doublet structure in all examined oxides. This happens due to the energy split of ²F_5/2 and ²F_7/2 terms of Ce³⁺ ground state by reason of spin-orbit interaction. (b) In Al₂O₃ a broad 5-component band corresponding to 4f-5d transitions is observed in the long- wave region of Ce³⁺ luminescence excitation spectrum. In the short-wave region of excitation spectrum, one can eliminate a non-structured band, which hypothetically consists of the exciton band and the charge transfer band, and as well as an outstanding narrow exciton-band. (c) In YAP bands of 4f-5d transitions are divided into two groups in excitation spectrum: 3-component in the region of 4-5 eV and 2-component in the region of 5-6 eV. (d) In YaG 4f-5d transitions appear as separate bands in excitation spectrum: 2.9, 3.9, 4.6, 5.6, 6.6 eV. Due to the closeness of the fundamental absorption edge in YAG, the band 6.6 eV is observable only at low temperatures. (e) At temperatures under 100 K the energy transfer form exciton to impurity is not detected for all the crystals.

Electronic relaxation of selectivity excited LiYF₄:Er³⁺ was investigated by means of time-integrated and time-resolved photoluminescence spectroscopy, using synchrotron radiation for excitation purposes. The range of excitation covered both Er³⁺ and host excitation up to 200 eV. Following Er³⁺ excitation, radiative cascades and cross relaxation were established. Under soft x-ray excitation, inelastic scattering of photoelectrons contributes to electronic relaxation.

There is a very great interest to obtain luminophors with a large quantum efficiency. One way to realize this is the employment of photon cascade emission. We present result of a search for new materials with cascade emission in Pr³⁺ intraconfiguration f-f transitions. The influence of host lattices, temperature, Pr concentration of f-f and d-f transitions has been studied.

Recently it was found that birefringence is induced in CaF₂ crystals when they are illuminated with laser light at 157 nm. Taking into consideration that CaF₂ is the only optical material used in 157 nm photolithography today, the possibility to use new wide band gap fluoride crystals as optical elements for 157 nm photolithography, even those of non-cubic symmetry, should be considered. Additionally fluoride dielectric crystals with wide band gaps doped with trivalent rare-earth (RE) ions can be used as passive or active optical elements int eh VUV. For doped crystals, applications depend on the structure of the energy level pattern of the 4f^n-15d electronic configuration and RE ion concentration. In this work we are exploiting the use of wide band gap fluoride dielectric crystals doped with RE ions. The laser induced fluorescence spectrum at 157 nm, and the absorption spectra of the LiCaAlF₆ crystals, pure and doped with RE ions, were investigated in the VUV region of the spectrum. A new m4tehod for monitoring RE concentration in wide band gap fluoride crystals, that is based on vibrating sample magnetometer measurement is presented as well.

As for other fluorescence light emitters, rare-earth (RE)- doped solids usually follow the well known principle of the Stokes law which simply states that excitation photons are at higher energy than emitted ones or in other words that out-put photon energy is weaker than input photon energy. Such principle is valid of course only when one ion system is considered.

We report on studies of the up-conversion process in YLiF₄ single crystals co-doped with Er³⁺ and with Yb³⁺. Er³⁺ has a well known complicated energy level system within the 4f shell which gives rise to the up- conversion process. Yb³⁺ with a broad absorption band int eh regime 940 nm <EQ (lambda) <EQ 1000 nm essentially represents a two level system which absorbs IR light. It can transfer the absorbed energy resonantly to the Er³⁺ system thereby increasing the quantum efficiency of the up- conversion process considerably. We have performed steady state and time-resolved measurements of emission and absorption spectra as well as luminescence excitation spectra following either excitation with a light emitting diode, a laser diode or a pulsed dye laser. We have tried to trace some of the channels for energy transfer, relaxation and up-conversion and to optimize the system for an optimum up-conversion efficiency. At optimum conditions we can estimate quantum yields for up-conversion of RI to green light in the order of about 2.5 to 3 percent. Three-step up- conversion to UV emission is also observed. These observation suggest that YliF₄ single crystal co-doped with Er³⁺ and with Yb³⁺ may be considered as suitable candidates for diode-laser-pumped displays and for solid state laser system int eh visible and near UV region.

The detailed site-selective and time-resolved fluorescence laser spectroscopy and kinetic measurements with high spectral and nanosecond temporal resolution were applied to analyze the M and N absorption bands at the ⁴I_9/2(1) yields ⁴G_5/2(1) crystal field (CF) transition in the CaF₂:Nd³⁺ crystal at 4.2 K. It was found that at helium temperatures the dynamically split spectral line at the ⁴I_9/2(1) yields ⁴G_5/2(1) CF transition of coherently coupled Nd³⁺ ions in the pair M- and quartet N-centers is inhomogeneously broadened. Small variation of fluorescence lifetimes of the ⁴F_3/2 and ⁴D_3/2 levels due to small variation of distances between Nd³⁺ ions in the pairs was found. Approximately 1 percent variation of the Nd-Nd distance in the pair -center was determined form lifetime variation of the ⁴F_3/2 manifold.

In CsCdBr₃ the trivalent Rare-Earth (RE) ions substitute for the divalent Cd ions. For reasons of charge compensation and the specific crystal structure, RE centers of the form RE³⁺ - (Cd vacancy) - RE³⁺ are preferentially formed. These symmetric pair centers are very suitable for studying cooperative effects of RE ions. These include processes between the ions of the pair as well as processes between the coupled pair and the electronic excitations of the host lattice. Experiments will be shown and discussed on: (i) cross relaxation and up-conversion by direct ion-ion interaction; (ii) cooperative cross relaxation of the Pr³⁺ pairs with the host lattice; and (iii) cooperative vibronic emission of the Pr³⁺ pairs with high phonon progressions.

Photon-echo measurements have been performed in gamma- irradiated ruby and in ruby with ⁵³Cr³⁺.

The paper describes experimental investigations of two-color optical super-radiance (SR) in LaF₃:Pr³⁺ crystal. Optical excitation was made on the transition ³P₀- ³H₄(0) at liquid helium temperature. When the power of the pumping 10ns-pulse was higher than a certain threshold, the resonant medium emitted an optical SR pulse on ³P₀-³H₄(0) transition with a duration of 8-10 ns in the direction of the pumping pulse delayed in time by 12-15 ns with respect to the pumping pulse. When the peak power of the pumping pulse increases up to 40 kW, an SR pulse was also observed for the ³P₀-³H₆(1) transition. The delay time of this pulse was about 30 ns. Thus, the two-color SR in the employed crystal was observed.

Optical second harmonic generation (SHG) in NiO in spectroscopy and topography experiments reveals a pronounced signal below the Neel temperature which couples quadratically to the antiferromagnetic order parameter. The SH process originates in combined magnetic-dipole and electric-dipole transitions between the (3d)⁸ levels of the Ni²⁺ ion in the crystal field. It is greatly enhanced by being resonant both in the incoming and the outgoing light waves. The polarization dependence reveals a sensitivity of the SH process to both T and S domains which are investigated in imaging experiments.

In this article we report the adiabatic and dynamic solutions of the pseudo Jahn-Teller (JT) problem in the orbital triplet ⁴T₂(t²₂e) of Cr³⁺ ion resulting from the combined effect of tetragonal (E) vibrations and trigonal crysatl field in sulphide spinels lattice. We discuss the general properties of the adiabatic surfaces that give a clear qualitative insight on the main features of the optical bands. The shapes of the absorption bands are calculated on basis of the quantum-mechanical solution of the dynamic pseudo JT problem taking into account the effective electric-dipole moment involving odd- parity crystal field potential. Pseudo JT (⁴A₁ + ⁴E) (direct product) e interaction is shown to produce a complicated structure of the multiphoton optical bands and leads to a specific polarization effect. Calculated shape function for the U-band absorption proves to be in a good agreement with the excitation spectrum of (alpha) - ZnAl₂S₄ tiospinel doped with chromium ions, the experimentally detected peculiarities of the structure of this band are shown to be an evidence of strong pseudo JT interaction in the excited state.

Steady-state population of 7 lowest excited erbium levels in LiY_1-xEr_xF₄ (YLF:Er³⁺) crystals was studied under up conversion CW InGaAs laser-diode pumping with varied power density. Theoretical and experimental concentration and power dependencies of population of higher-energy radiative levels were obtained. Relative changes in populations of studied levels in YLF:Er³⁺ crystals were experimentally controlled by visible spectra of steady-state luminescence in the wavelength ranges corresponding to transitions ⁴S_3/2 yields ⁴I_15/2 micrometers and ⁴F_9/2 yields ⁴I_15/2 micrometers . IR-pumped luminescence kinetic curves of higher-energy transitions ⁴S_3/2 yields ⁴I_15/2 and ²H_9/2 yields ⁴I_15/2 were recorded. The energy-transfer mechanisms were determined, and the predominant mechanisms responsible for up conversion excitation were elucidated. Microparameters of energy transfer and concentration dependencies of the self-quenching rates and non-linear coupling were obtained on the basis of theoretical and experimental estimates of the rates of intra- and intercenter relaxation processes allowing for statistics of coupling between the impurity centers in the system. The steady-state dependencies of population on the erbium concentration and pumping power density were calculated within the framework of rate balance equations. Good agreement between the theory and experimetnal data was obtained.

A theoretical model for two characteristic photoluminescence (PL) bands in SBN, 'green luminescence' and 'red luminescence' is proposed on the basis of the extended photoluminescence experiments in SBN:Cr, and also in SBN:Ce and in nominally pure SBN systems under different conditions. While the RL-band is suggested to be connected with charge transfer vibronic exciton (CTVE) clusters induced by Cr³⁺ impurities in the Nb-sites, the GL- band is connected with Nb⁴⁺ electronic polarons in a new, charge transfer excited states. Here Nb⁴⁺ centers are the cores of the CTVE clusters induced by these charged scores. The PL mechanism is the in-cluster CTVE recombination for both bands under discussion. But the CTVE states are quasi-resonantly mixed here with ⁴T₂ states of the Cr³⁺ core in the RL-band case, and with 5s-states of the Nb⁴⁺ core in the GL-band case. The role of excitonic polarons of CTVE nature is also discussed in connection with 'green' luminescence origin in KTaO₃ and KNbO₃ crystals.

Experimental and theoretical studies of the optical and EPR spectra, and the spin-lattice relaxation in CsCdBr₃ crystals containing 0.1, 0.3 and 1.0 mol. Percent of impurity Yb³⁺ ions have been fulfilled. In the optical excitation spectrum, a broad charge transfer band has been detected in the wave-number range of 25000-35000 cm^-1. The hybridization of the excited electron configuration corresponding to the charge transfer from the ligand (Br) 4p closed shells on the ground 4f¹³ configuration of the ytterbium ion is considered to interpret the anomalously large crystal field splitting of het ²F_5/2 multiplet. The EPR spectra with comparable intensities of the axial single-ion and symmetric dimer centers in the sample containing 0.3 mol percent of impurity Yb³⁺ ions were observed. Spin-lattice relaxation rates and linewidth sin the EPR spectra of the single-ion and dimer centers were measured in the temperature range of 1.5-35 K at the frequency of 9.5 GHz at different directions of the applied magnetic field relative to the crystal symmetry axis. Experimental results are analyzed in the framework of the microscopic theory of the electron-phonon interaction with taking into account peculiarities of the phonon spectrum of the impurity CsCdBr₃ lattice. High- frequency EPR spectra of dimer centers in CsCdBr₃ crystals containing 0.2 and 0.5 mol. Percent of impurity Nd³⁺ ions were taken in the range of 205-250 GHz at 4.2 K in the magnetic field sup to 0.8 T parallel to the crystal symmetry axis. The crystal field splitting between the first excited and the ground Kramers doublets, and magnetic splitting factors of these doublets were determined. AN estimate of the isotopic ferromagnetic exchange constant A equals (2.3 +/- 0.3) 10^-3 cm^-1 in symmetric dimer centers formed by impurity Nd³⁺ ions was obtained from the zero-field splitting of the EPR signals.