Exciton luminescence in orthorhombic-structure MnF<Subscript>2</Subscript> epitaxial films

The low-temperature photoluminescence (PL) of 100 to 300-nm thick MnF₂ epitaxial films of the α-PbO₂-type orthorhombic structure was studied. The PL spectrum consists mainly of a broad band peaking around 575 nm and a slowly decaying long-wavelength wing. The short-wavelength part of the main band revealed relatively weak spectral features, which are due to magnon replicas of the Mn²⁺ excitonic line perturbed by Mg and Ca impurities. These features were found to shift toward shorter wavelengths by 12 nm relative to their position in bulk MnF₂ crystals. The shift can be accounted for by a change in the crystal field acting on the Mn²⁺ ions in the orthorhombic phase.     

Molecular Constants of MnF6 −2 Anion

Abstract

The normal coordinate analysis of XY₆ type of anions and molecules employing different force fields is well established.^1,2 Recently, the vibrational spectra and complete frequency assignment of MnF₆ ^?2 has been reported by Flint.³ Using the available vibrational data the force constants have been evaluated employing three potential models viz. general valence force field (GVFF), Urey-Bradley force field (UBFF) and orbital valence force field (OVFF). The mean amplitudes of vibration and Bastiansen-Morino shrinkages have also been computed at two temperatures.     

Room-temperature photoluminescence in MnF2 under high pressure

A novel two-color photoluminescence (PL) is found in MnF₂ at room temperature (RT) under high pressure. In contrast to the low-temperature PL, the observation of room-temperature PL is unusual in transition-metal concentrated materials like MnF₂ since the de-excitation process at RT is fully governed by energy transfer to non-radiative centers. We show that room-temperature MnF₂ emissions originate in the pressure-induced cotunnite phase. Both the nine-fold Mn²⁺ coordination and the Mn–F–Mn exchange pathway inhibit exciton migration among Mn²⁺, favoring two-band PL at RT. The electronic structure and the excited-state dynamics are investigated by time-resolved emission and excitation spectroscopies under pressure. The two PL bands at 2.34 and 1.87 eV above 15 GPa are assigned to Mn²⁺ emissions arising from two distinct Mn²⁺ centers formed in the MnF₂ high-pressure phase. The microscopic origin of the two-color PL is analyzed in terms of exciton dynamics in the MnF₂ cotunnite structure.     

Theory of infra-red and optical spectra of antiferromagnets

The contributions of magnons to the optical properties of antiferromagnets having the rutile structure are discussed. The properties considered are electric-dipole active two-magnon absorption in the infra-red, and magnon sidebands on sharp-line exciton transitions in the visible. The discussion is based on a thorough treatment of the properties of excitons and magnons in the antiferromagnetically ordered state. The site-group and space-group symmetries of the magnetic excitations are derived and the selection rules for electric and magnetic dipole transitions are determined. The occurrence of magnetic Davydov splittings of the excitons is investigated, and their symmetry properties throughout the Brillouin zone are derived. The functional dependences of exciton energy on wave vector are calculated. Applications of the theory are made to experimental results on excitons and magnons in MnF₂, FeF₂ and CoF₂.

The possible mechanisms for two-magnon and magnon-sideband absorption are discussed, and the influence of crystal symmetry on these mechanisms is described. The two-magnon state responsible for electric-dipole absorption is identified and selection rules for electric-dipole activity are presented. A spin Hamiltonian for the two-magnon process is set up and used to derive expressions for absorption coefficients for electric vector parallel and perpendicular to the crystal c-axis. Comparison with experiment for MnF₂ yields numerical values for the parameters of the basic coupling mechanism. The exciton-magnon states which give rise to magnon-sideband absorption are explicitly constructed and electric-dipole selection rules are derived for all possible types of sideband. Spin Hamiltonians for the various magnonsideband absorption processes are presented and used to derive expressions for sideband shapes. The results are applied to the experimental spectra for MnF₂ and FeF₂ and the sideband shapes in MnF₂ are calculated numerically. The sideband shapes observable in emission spectra are also briefly discussed.     

Structural and vibrational properties of C36 and its oligomers (C36) M = 2, 3, 4 by tight-binding molecular dynamics

Non-orthogonal tight-binding molecular-dynamics is employed to calculate structural and vibrational properties of C₃₆ and its oligomers (C₃₆) _{M = 2, 3, 4} . The lowest energy configuration of the C ₃₆ cage is confirmed to have D _6h symmetry. For the dimer, too, the D _2h structure reported in the literature is found. The vibrational spectrum is identified with the power spectrum of the displacement autocorrelation function. Additional vibrational properties are extracted from the dynamical matrix. For the monomer, fair agreement with available ab initio calculations is achieved, with comparatively smaller deviations in the Raman-frequencies than for published semi-empirical calculations. The features of the vibrational modes are correlated with the structural properties of the oligomers. Received 24 November 2000 and Received in final form 24 August 2001     

Critical-point anomaly in the EPR linewidth of two-dimensional antiferromagnets

Calculations are presented which satisfactorily explain the observed temperature and angular dependence of the EPR linewidth in the two-dimensional antiferromagnets Rb₂MnF₄ and K₂MnF₄ near T_N. These calculations are based on an earlier theory by Huber for the three-dimensional systems.     

High resolution angle-resolved photoelectron spectrum of CO2, excited with polarized resonance radiation

The He(I) excited and angle-resolved photoelectron spectrum of CO₂ is investigated using a focussing VUV-polarizer. High resolution combined with the additional information given by comparing spectra taken at different angles permits detailed analysis of the vibrational structure. β-values are given for all vibrational components hitherto observed in the photoelectron spectrum of the ≈X, ≈A,≈B and ≈C electronic bands. Single excitations of the v₃ mode with vibrational energies 181 meV in the estate and 279 meV in the ≈B-state are reported. The peak at 360 meV vibrational energy in the (≈C-state is reinterpreted as a single v₃ excitation. The β-values of the most intense peaks are also measured using Ne(I) (16.67 and 16.85 eV), Ne(II) (26.86 and 26.95 eV) and He(II) (40.81) resonance radiations.     

Absorption spectra of NH4MnCl3 and NH4MnF3

The absorption spectra of NH₄MnCl₃ and NH₄MnF₃ crystals have been measured down to 10 K in the 250 to 600 nm region. The observed bands are assigned to electronic transitions from the ⁶A_1g(S) ground state to various excited levels of Mn²⁺ ions in an octahedral crystalline field. The position of the bands have been fitted within the strong crystal field scheme. Resulting parameters at room temperature are B=741, C=2990 and Dq=520 cm⁻¹ for NH₄MnCl₃ and B=800, C=3139 and Dq=694 cm⁻¹ for NH₄MnF₃. At low temperature some bands show a rich fine structure in which some phonon progressions have been identified.     

High temperature spin wave dynamics of the uniaxial antiferromagnets

In the framework of the spin operators diagram technique the dynamical properties of the easy-axis type antiferromagnets at high temperatures are considered. The diagonalization procedure generalizing the Bogoliubov u-v transformation on the spin operators is presented. The spin wave (SW) spectrum allowing for the temperature and the magnetic field dependence of the sublattice magnetizations at T～T_～ is obtained. The SW relaxation frequencies due to the processes of SW scattering on the thermal spin fluctuations are calculated. The latter are in agreement with the data obtained on MnF₂ near the spin-flop transition¹⁰.     

One-magnon luminescence sidebands of the excitonic transition in MnF2

We report the first observation of the π-polarized one-magnon excitonic luminescence sideband in MnF₂. The theory of Loudon is used to fit the experimental spectrum quantitatively. An effective temperature of the crystal is deduced from the simultaneously observed antiStokes sideband emission.     

Optical observation of ion displacement in pyroelectric BaMnF4

Two anomalous behaviors are observed in the Raman spectra of BaMnF₄ at room temperature. One is that the number of Raman lines is smaller than that expected for the orthorhombic crystal structure with C _2ν ¹² symmetry, the other is that, of the b(aa)c, a(bb)c, and b(cc)a scattering geometries giving the same A₁-symmetry spectrum, the number of Raman lines in the b(aa)c spectrum is considerably smaller than those in the other two spectra. It is suggested that rotation of MnF₆ octahedra about c-axis occurring in the orthorhombic phase is responsible for the former behavior, and Ba²⁺-ion displacement along a-axis for the latter one.     

Determination of the parameters of the potential well of a diatomic molecule with the use of the experimental spectrum of an electronic transition to a repulsive branch of the state under study

It is shown that the lack of information on the rotational structure of the vibrational terms of a diatomic molecule in the problem of determination of the potential well of the molecule can be compensated by information on the intensity distribution in the spectrum of a bound-free transition to a repulsive branch of the state under study. The accuracy characteristics of the method proposed are studied on the example of a potential whose parameters are close to those of the state a ³Σ _u ⁺ of the Na₂ molecule.     

Two-magnon Raman scattering in two-dimensional antiferromagnets at finite temperatures

The temperature dependence of two-magnon Raman scattering in the quadratic-layer antiferromagnets K₂NiF₄ and K₂MnF₄ is found to agree with the higher-order Green function theory of Balucani and Tognetti. New experimental data on K₂MnF₄ are also presented.     

On the absorption saturation in the vibrational-rotational bands of molecules

Analytical expressions describing saturation of the stationary absorption coefficient in a vibrational-rotational band in the spectrum of a molecular gas are obtained taking into account that many other bands are involved in the absorption process as well. The formulas are derived for an arbitrary spectral composition of the pumping radiation with an allowance for a difference between the rotational and vibrational relaxation rates and between the relaxation rates of the lower and upper vibrational levels. The influence of saturation on the appearance of the partial inversion and the negative absorption effects is considered. The general formulas are simplified and the vibrational and rotational contributions to the saturation are separated within the frame-work of the local Elsasser model for the rotational band structure. Explicit relationships between the dimensionless parameters describing the vibrational and rotational saturation mechanisms are obtained and analyzed, and conditions under which one of these mechanisms is dominating are determined. Particular calculations were performed for the (000)–(010) and (000)–(001) absorption bands of the O₃ molecule and features of the saturation effect in these bands were established.     

强红外场作用下BCl3振动态传能动力学研究

基于文献[1],本工作在较高泵浦激光能通量范围,测量并研究了BCl₃分子振动激发v_3吸收谱及时间演变,观察了v₃激发弛豫的几种能量转移过程,以及对泵浦激光能量BCl₃气压等参数的依赖关系。表明泵浦光脉冲产生一个振动态非热分布的系综,转动能量转移对引起这种非热分布有重要作用。用简化碰撞动力学模型讨论了BCl₃振动激发吸收谱的演变过程,得到振动态再分布的简单关系;Pτ_v-v≌c/K′(T_v,T₀,q)和等效振动温度、平均吸收光子数的分析表达式,与实验结果定性地符合。 关键词：     

An analysis of a superweak absorption spectrum of the SO<Subscript>2</Subscript> molecule using the variational procedure

The high resolution structure of a superweak 2ν₁+ν₂+ν₃−ν₂ band of sulfur dioxide, SO₂, is for the first time recorded with a Bruker IFS-120 HR Fourier spectrometer and analyzed using a specially derived procedure and a relevant computer code. As a result of the analysis, 115 lines have been found in the recorded spectrum, which allowed us to obtain high-accuracy values of the vibrational energy and rotational parameters of the highly excited vibrational state, (211).     

Excited states and photochemistry of saturated molecules. The vibrational structure in the electronic spectrum of ethane

Ab initio wavefunctions including single excitation CI are used to analyze the origin of the vibrational structure in the low-energy band of the electronic spectrum of ethane. The calculations suggest that the vibrational structure appears to be due to the overlapping progressions from two vertical states (1E_u and 1A_2u), both of which are unstable to distortion to C_2h symmetry, wherein they are characterized by significantly lengthened CC bonds.     

Photoelectron spectroscopic studies of some nitrosyl and nitryl halides and nitric acid

The He(I) photoelectron spectra of the compounds FNO, ClNO, BrNO, FNO₂ and ClNO₂ have been recorded on an instrument specifically designed to study reactive and short-lived species. The spectrum of HNO₃ (isoelectronic with the nitryl halides) is included for comparison. The spectra of the nitrosyl halides are interpreted by reference to SCF-MO calculations, by correlation with the known levels of NO, and by consideration of the electronic effect of the halide (X) substituent. The resulting assignments are in accord with weak bonding between the X and NO moieties. The assignments for the nitryl halides and nitric acid are assisted by correlation with the known levels of NO₂, by consideration of the electronic effects of the X substituent, by an analysis of vibrational structure, and by CNDO type MO calculations. On the basis of our assignments for these latter molecules we propose a reassignment of the photoelectron spectrum of CH₃NO₂.     

Vibrational spectroscopy of perfluoropropionic acid in the region between 1000 and 11 000 cm

Fully fluorinated compounds, known as perfluorinated compounds, are widely used in industrial applications. Recently, some perfluorinated acids have been detected in the atmosphere and the tissues of animals. Some perfluorocarboxylic acids are emitted to the atmosphere from the thermolysis of fluoropolymers and the degradation of fluorotelomer alcohols. The gas phase vibrational spectrum of a representative perfluorocarboxylic acid in the region between 1000 and 11 000 cm⁻¹ has been investigated, with emphasis on the vibrational overtone spectrum in the near-IR region. The most intense transition in the fundamental spectrum is the CF₃ stretch while in the overtone region, the O-H stretch carries most of the intensity. A comparison of the perfluorocarboxylic acid vibrational spectrum with the hydrocarbon analog acid is discussed.