The phosphorescent triplet state of p-xylene in an isotopically mixed crystal at 1·2K: an investigation by E.S.R. and MIDP

The phosphorescence spectrum of p-xylene-h ₁₀ in a p-xylene-d ₁₀ host is first presented. Secondly we report E.S.R. experiments performed on p-xylene-α,α′-d ₆ in p-xylene-d ₁₀; from an analysis of the orientations of the principal axes in combination with the crystal structure and of the hyperfine splittings it is concluded that the in-plane principal axes of the zfs tensor make an angle of 68° with the molecular axes. Thirdly we discuss Microwave-Induced Delayed Phosphorescence (MIDP) experiments on p-xylene-h ₁₀ in p-xylene-d ₁₀ in which the relative radiative rate constants for decay from the zero-field spin components of the triplet state to single vibronic bands of the ground state have been obtained.

The results of the MIDP and E.S.R. measurements are shown to be mutually consistent in terms of a model in which the effect of the crystal field and the methyl substituents on the pseudo-Jahn-Teller unstable electronic structure of the triplet state of benzene is described.     

Mössbauer spectrum of an ensemble of57Fe nuclei interacting with radiation fields coupling the Zeeman sublevels of the nuclear excited 3/2 state

We have introduced the concept dressed nucleus in order to describe the interaction of a nucleus (in a static magnetic field) with a radiation field havingn photons of a certain mode. The idea is to consider the global system as one quantum system in the Schrödinger representation. The number of eigenvalues of the Hamiltonian of the global system is (2I+1)², withI the spin of the free nuclear state. Taking into account the form of the corresponding eigenvectors and the relevant selection rules, the total number of -transitions (for the nuclear transition 3/2^- 1/2^-) is 24. If a Mössbauer effect experiment is set up with a single-line absorber, the source being an ensemble of dressed nuclei, we would expect a spectrum showing 24 lines, whose relative positions and intensities are calculated.     

Relaxation of the magnetic state of a ferromagnetic–superconducting layered structure

We have proposed a real-time method of neutron reflectometry. The magnetic state of the Ta/V/FM/Nb/Si ferromagnetic–superconducting system has been analyzed. Relaxation of the inhomogeneous magnetic state with a characteristic time of several hours, which depends on the magnetic field magnitude and temperature, has been observed. The relaxation of the domain structure has changed upon a transition of the V and Nb layers to the superconducting state. It has been concluded that real-time reflectometry data for polarized neutrons are important for determining the origin of magnetism in ferromagnetic–superconducting layered structures.     

Calculation of the elastic scattering properties for cold and ultracold 39K atoms in a triplet state

The elastic scattering properties for collisions between cold and ultracold 39K atoms in a triplet state are investigated. Based on the recent theoretical and experimental results, the improved hybrid potential is presented for a triplet α3∑u^＋ ground state of K2. Our calculated value of the s-wave scattering length a by using the Numerov method for the triplet state is 79.578α0 and found to be in good agreement with the previous ones. The numbers of bound states are supported by the molecular potential. Pronounced shape resonances appear for the l = 3 partial waves for the α3∑u^＋ state. Furthermore, the s-wave scattering cross section, the total cross section and energy positions of shape resonances for the α3∑u^＋ state are calculated.     

First exclusive measurements of the K − pp state populated in the pp → K + Λp reaction at 2.85 GeV

We have analyzed data of the DISTO experiment on the exclusive pp → K ^?+? Λp process at T _p ?=?2.85 GeV to search for a K ^??? pp  (?≡?X) nuclear bound state to be formed in the pp → K ^?+??+?X reaction. The deviation spectra of the K ^?+? missing-mass ΔM (K ^?+?) and Λp invariant-mass M(Λp) with selection of large-angle proton emission revealed a structure with M _X ?=?2265 ±2 MeV/c ² and Γ _X ?=?118 ±8 MeV.     

Optically detected EPR effect in the ã3 Au triplet state of the oxalylfluoride molecule excited to the 413, 423 and 431 rotational levels of the 00(Ã1 Au ) vibronic state

In the present study, resolved OD EPR spectra were measured for the 4₃₁, 4₂₃, and 4₁₃ levels of the ù A_u 0⁰ vibronic state. Values of the fine and hyperfine constants were estimated from an analysis of these spectra for the triplet rotational levels, coupled by the intramolecular interactions with the singlet levels studied. It was shown that the microwave power dependence differs significantly for different lines of the OD EPR spectrum. This difference can be explained by a model where the microwave field saturation effect is observed for different OD EPR lines at different power values of the microwave field. The decay profile can be fitted by a biexponential function in the presence of both magnetic and microwave fields. The observed data were analysed using the electron and nuclear spin decoupling mechanism in the limit of low level density.     

The effect of the hydrogen fluoride chain on the aromaticity of C6H6 in the C6H6···(HF)1–4 complexes

The effect of the hydrogen fluoride chain ((HF)_n) on the aromaticity and π character of C–C bonds of C₆H₆ in the C₆H₆···(HF)_n (n = 1–4) complexes were investigated using density functional theory employing RM05 functional. It was found that the binding energy between C₆H₆ and different (HF)_n chains showed a maximum at n = 3 (C₆H₆···(HF)₃). Also, the π–hydrogen interaction (πHI) and the bifurcated fluorine interaction (BFI) increased and decreased the π character of the C–C bond of C₆H₆, respectively. In addition, the change of aromaticity of the C₆H₆ due to the interaction with the HF chains was also studied using three different aspects such as aromatic fluctuation index (FLU), average two centre index (ATI) and proton nuclear magnetic resonance (HNMR) spectrum. The most change in the aromaticity happens when the C₆H₆ interacts with (HF)₃ chain. The variation of aromaticity with the binding energy and the summation of two-body terms were investigated and very good linear correlations were observed.     

Cross sections of the interactions of4He nuclei with protons at 8·6 and 13·6 GeV/c

⁴He-p collisions at two values of⁴He momenta-8·6 GeV/c and 13·6 GeV/c — have been studied using the one-metre JINR hydrogen bubble chamber. Total, elastic and topological cross sections have been measured. The results are in agreement with other published data. The elastic differential cross section was compared with the predictions of the Glauber model formalism.Dedicated to Academician Václav Votruba on ths occasion of his eightieth birthday.We would like to thank Mrs. M. Stehlíková for drawing the figures.     

The description of the Kπ = 1+ isovector M1 state within a boson expansion formalism

A nuclear system of Z protons and N neutrons moving in an anisotropic potential well and interacting among themselves through an isoscalar plus an isovector quadrupole-quadrupole interaction is treated within a boson expansion formalism. The expansion is performed in terms of the particle-hole RPA bosons. Application refers to ¹⁵⁶Gd. Appreciable corrections to the RPA results concerning both the energies and the B(M1↑) values are obtained. The higher RPA corrections destroy the fragmentation of the RPA-M1 strength and most of the collectivity is concentrated in one state.     

The photodetachment spectrum of OHF−: the influence of vibration at a transition state

New ab initio potential energy functions for collinear OHF^? and OHF have been used with time-dependent wavepacket studies in a simulation of the photodetachment spectrum of the OHF^? anion. Franck—Condon excitation of OHF^? lies within the transition state region for the bimolecular reaction F + OH ? O + HF on the lowest triplet surface. The branching ratios for fragmentation of the excited OHF are such that the peaks in this spectrum may each be correlated with dissociation to a single vibrational state of the O + HF(v) or OH(v) + F products. The possible influence of vibrational excitation of the anion has been explored also. This is predicted to lead to marked changes in the spectra. Experimental observations of such spectra could be used in refinement of the potential as could an experiment in which the electron kinetic energy is detected in coincidence with the vibrational states of the molecular products or with the kinetic energy of the atomic products. The calculations revealed a number of metastable resonances on the OHF surface, although these are all at higher energies than are relevant to the photodetachment spectrum from the ground state anion.     

ENDOR amplitudes of triplet state molecules: II. Orientational dependence of the νp frequency line and S-T0 mixing

The specific ENDOR line at the free Larmor frequency ν_p in the low temperature spectra of triplet state molecules is caused by degenerate NMR transitions within theM _s=0 zero-level (ZL) electron spin manifold. This ZL line was found to be orientationally dependent for the diradical complex Zn(3,6-di-tert-butyl-o-semiquinone)₂Zn(DBSQ)₂: the ZL line dominates the ENDOR spectrum if it is detected at the perpendicular canonical components of the EPR spectrum, and vanishes if the complex is oriented with its ZFSz-axis parallel to the direction of the magnetic field, i.e., if detected at the parallel canonical EPR components. This effect is shown to result from the interaction between nuclear spin substates of the S and T₀ manifolds, their levels being close to each other for the Zn(DBSQ)₂ complex. Such an interaction mixes the states and shifts energy levels. Consequently, it cancels the degeneracy of the nuclear substates within the ZL manifold and reduces the rate of nuclear flip-flop relaxation. This specific relaxation mechanism has been shown to substantially affect the amplitude of the ZL line (Doubinskii A.A., Lebedev Ya.S., Möbius K.: Appl. Magn. Reson.13, 439 (1997)). The nuclear flip-flop relaxation effect is expected to be orientationally dependent since the S-T_o separation depends upon the orientation of the diradical with respect to the external magnetic field.     

Potential of the A 1Σ u + state of He2

The potential of the lowest excited singlet state of He₂ is calculated. The best function includes 209 configurations constructed from 10 σ basis orbitals. Excellent agreement with experimental quantities depending on the shape of the potential near the minimum (equilibrium interatomic separation, vibrational and rotational constants for the lower vibration levels) is obtained. The dissociation energy is 18 600 cm^-1, compared to the experimental 19 910±50 cm^-1. Agreement is not as good for the highest vibrational levels.     

On the magnetic susceptibility of the liquid Pd1−x Si x alloy system at 1825 K

The isothermal magnetic susceptibility (x) of the completely miscible liquid Pd_1–x Si _x alloy system shows a rapid monotonous decrease withx from strong paramagnetism to weaklyx-dependent diamagnetism. The measured susceptibility isotherm at 1825 K is analysed within 0x1 by using a semiphenomenological method of decomposing the magnetic susceptibility into its constituent parts. Because of the empirical similarity between liquid and glassy metals this interpretation also provides assertions about the magnetism of glassy Pd_1–x Si _x aroundx=0.2.     

ENDOR on the triplet state of the primary electron donor in the photosynthetic bacteriumRhodobacter sphaeroides — One step forward in a still unfinished story

The electronic structure of the primary electron donor (D) of the photosynthetic bacteriumRhodobacter sphaeroides is investigated by ENDOR in the photoinduced triplet state of D. Hyperfine (hf) splittings of the triplet state measured on frozen solutions are given and compared to the results obtained earlier (Lendzian F., van Willigen H., Sastry S., Möbius K., Scheer H., Feick R.: Chem. Phys. Lett.118, 145 (1985). The hf splittings found are consistent with a model of an asymmetric spin density distribution over the two bacteriochlorophyll molecules which constitute D, suggesting a mirror image symmetry of HOMO and LUMO coefficients. The could be relevant for electron transfer, in particular unidirectionality in the reaction center (RC). The first triplet state ENDOR experiments on single crystals of RC’s are also reported.     

Analysis of the K S K S system from the reaction π − p → K S K S n at 40 GeV

On the basis of experimental data from the 6-m spectrometer of the Institute of Theoretical and Experimental Physics (ITEP, Moscow), an amplitude analysis of 40 553 events of the reaction π ^? p → K _S K _S n induced by a negatively charged pion of energy 40 GeV is performed over a broad momentumtransfer range by using a new procedure. The results for |t| > 0.1 GeV² are obtained for the first time. In particular, resonances of mass 1700 and 1900 MeV and width 120 MeV are discovered in the D ₊ wave (there were no such resonances for |t| < 0.1 GeV²). In the region of low momentum transfers, the S wave exhibits a structure that lies in the mass region around 1370 MeV and which requires three resonances for its explanation. Two of these (that of mass 1234 ± 6 MeV and width 47 ± 33 MeV and that of mass 1478 ± 6 MeV and width 119 ± 10 MeV) were found in the studies of A. Etkin et al. [Phys. Rev. D 25, 2446 (1982)] and O.N. Baloshin et al. {Yad. Fiz. 43, 1487 (1986) [Phys. At. Nucl. 43, 959 (1986)]}. The third has a mass of 1389 ± 9 MeV and a width of 30 ± 24 MeV. At high momentum transfers, the S wave is found to feature resonances that have the following parameters: M = 1328 ± 8 MeV and Γ = 237 ± 20 MeV, M = 1440 ± 6 MeV and Γ = 121 ± 15 MeV, and M = 1776 ± 15 MeV and Γ = 250 ± 30 MeV. For the D ₀ wave, it is found that, in addition to the well-known resonances f ₂, a ₂, and f′ ₂, there appear the following resonances in this wave: a resonance of mass 2005 ± 12 MeV and width 209 ± 32 MeV and a resonance of mass 2270 ± 12 MeV and width 90 ± 29 MeV at low |t| and a resonance of mass 1659 ± 6 and width 152 ± 18 and a resonance of mass 2200 ± 13 MeV and width 91 ± 62 MeV at high |t|.     

Calculation of the lowest 1 S resonance state of the H− anion by the stabilization method

By the example of the lowest resonance state of the H^? system, two versions of the stabilization method are considered: with introduction of an external potential into the Hamiltonian and with enlargement of the single-particle function basis. A comparison of the results suggests the possibility of applying these methods to calculating the resonance parameters in many-electron systems.     

Zeeman effect on the nuclear quadrupole resonance of 81Br in single crystals of methyl 4-bromobenzoate, 4,4′-dibromodiphenyl-ether and 4,4′-dibromodiphenylsulphide at 77 K

The Zeeman effect on the N.Q.R. of methyl 4-bromobenzoate, 4,4′-dibromodiphenylether, 4,4′-dibromodiphenylsulphide has been studied at 77 K on single crystals, using the ‘geometric method’. The apparatus used in the measurements, which allows one to maintain any temperature between room temperature and 77 K, is described, and the possible significance of the slight variations in the molecular arrangement found in the present measurements and in those performed at room temperature is discussed.