Approximation of the electron excitation cross sections for triplet states excited from the 23 S 1 metastable state in helium

The method of approximated four-parameter representation of the electron-impact cross sections for a helium atom excited from the 2³ S ₁ metastable state into higher triplet states is applied and discussed. The approximation consists in interpolation over the whole set of the cross section values for each helium atomic level measured in our experiments and reported by other researchers. The approximation parameters and the cross sections calculated using these parameters for the maxima of the corresponding excitation functions are presented for 19 triplet levels of the S, P, and D HeI series with n=2–10. The interpolated values are compared to the theoretical cross sections. The serial regularities were investigated for the S, P, and D levels studied and a decrease in the cross sections for excitation from the given metastable state within each series, described by the approximate law Q=Cn ^?5, was revealed. Validity of the similarity relationship and the Bethe approximation for cross sections in the 2³ S?n ³ P series was verified. It is shown that the cross sections for a triplet level excitation from the 2³ S ₁ metastable state exceed the corresponding values for excitation from the ground state of helium by a factor of approximately 10³ for n=2 and 3 and 10 2 for the higher levels. It is concluded that the proposed method of representation of the cross sections for the electron-impact excitation of triplet levels from the metastable state increases accuracy and more importantly, reliability of the final results.     

Triplet proximity effect in FSF trilayers

We study the critical temperature T _c of FSF trilayers (F is a ferromagnet, S is a singlet superconductor), where the triplet superconducting component is generated at noncollinear magnetizations of the F layers. An exact numerical method is employed to calculate T _c as a function of the trilayer parameters, in particular, mutual orientation of magnetizations. Analytically, we consider limiting cases. Our results determine the conditions necessary for the existence of recently investigated odd triplet superconductivity in SF multilayers.     

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.     

Spin-correlations and low lying excited states of the spin-1/2 Heisenberg antiferromagnet on a square lattice

The ground state and the lowest excited states of the spin 1/2-Heisenberg model are investigated by exact diagonalization and variational Monte Carlo techniques. Our trial state represents a generalization of a wave function introduced by Hulthen, Kasteleijn and Marshall. The long range character of the spin-correlation function is in excellent agreement with exact diagonalization and also with recent neutron scattering results for La₂CuO₄. The asymptotic behavior of the spin-correlation function is found to differ from spin-wave theory. From the exact (N<=20 spins) and variational (N<=400) ground state energies we determine as asymptotic values 1.3025 and 1.288, respectively. We calculate the dispersion for the spin-wave excitations and identify an excited triplet which becomes degenerate with the ground state in the thermodynamic limit. This triplet state allows spontaneous symmetry breaking to occur atT=0 K. Quantum fluctuations reduce the sublattice magnetization to an effective value of 0.195 (3) as compared to the Néel-state value of 1/2.     

Spinaustauschstreuung von Kalium an Caesium

The differential spin exchange cross sectionI _ex(?, E) for the system K-Cs has been measured as a function of angle for 2°???40° and collision energy for 1.2 · 10^?13 erg ?E?2.1 · 10^?13erg. Confirming previous experiments it exhibits a maximum neargj≈10°. The angular position of maximum changes with collision energy. Increasing the energy lowers the maximum angle. The analysis of the cross section with regard to information about the interaction of the collision partners turns out to be very complicated. A first few model calculations are presented and discussed. The most obvious, qualitative relations between the singlet and triplet interaction potentials,V ₀ andV ₁, and the differential spin exchange cross sectionI _ex(?) appear to be: 1. The average magnitude ofI _ex is markedly different below and above the rainbow angle \(\vartheta _{R_1 }\) of the shallow triplet potential V₁. From the observed intensity drop towards large angles we estimate a lower bound for the triplet potential well depth? ₁?1.4 · 10^?14 erg. 2. The expected relation between the integral spin exchange cross sectionσ _ex and the difference potentialΔV(R)=V ₁-V ₀ at large distances is also borne out by the differential spin exchange cross section I_ex(?) below \(\vartheta _{R_1 }\) . Relative to the intensity beyond \(\vartheta _{R_1 }\) the cross section increases if range and strength ofΔV is increased.     

On the calculation of <H 2> molecular integrals

The density matrix equations of motion arising in the triplet mechanism of chemically induced electron spin polarization are solved exactly without the imposition of the Redfield approximation. It is shown that the triplet spin relaxation time occurring in the final expression is not the true relaxation time because the spectral density involved depends both on the rotational correlation time and on the quenching rate. The effective spin relaxation time differs only slightly from the true time. The equations are extended to the case where the initial triplet passes on its polarization to the secondary triplet and exact solutions for the polarizations of the latter's doublets are obtained in the form Π_B = cΠ_A; an explicit expression for c is presented. The consequences of the secondary triplet being able to pass back its polarization to the initial triplet are explored and a ‘coherence effect’ on the polarization on the first triplet's doublets is analysed.     

Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy

High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective formation of triplets after excitation. We studied these processes with fs pump–probe spectroscopy. The time behaviour of transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution. For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function. Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation. Both processes could be described by a time constant of 62 ps±9 ps. For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield increased with higher excitation energy. The kinetics becomes bi-exponential with increasing amplitude of the short time constant of 2 ps at increasing excitation energy. The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are – besides the high spin-orbit coupling factor introduced by the sulphur atom – the almost isoenergetic positions of the S ₁ and T ₂ states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state S ₁, ultrafast intersystem crossing takes place. Received: 6 December 1999 / Published online: 2 August 2000     

Tryptophan phosphorescence of ribonuclease T1 as a probe of protein flexibility

The phosphorescence properties of Trp-59 of ribonuclease T₁ fromAspergillus oryzae were monitored as a function of temperature, pH, salt concentration, and complex formation with substrate analogues and, also, in the presence of glycerol as viscogenic cosolvent. The results establish a rough correlation between the internal flexibility of the macromolecule, as derived from the triplet lifetime, and its stability (G orT _m ) toward unfolding. Below 10°C or in 70% glycerol the triplet probe distinguishes at least two gross conformations for the protein, which are characterized by a large difference in phosphorescence lifetime. It is pointed out that such structural heterogeneity does not correspond with the heterogeneity inferred from fluorescence decays and acrylamide quenching rates. Further, implications of the phosphorescence data with regard to the interpretation of acrylamide quenching of fluorescence are discussed.     

Collisions of alkali K and Rb atoms in ground state: Modification of interaction potentials

The interaction of alkali K and Rb atoms that reside in the ground state is considered in the range of collision energies E = 10⁻⁴ to 10⁻² au. The singlet (X ¹Σ⁺) and triplet (a ³Σ⁺) interaction potentials available in the literature are analyzed and modified. For the KRb dimer in the range of interatomic distances 15–21a ₀, we chose analytical representations of the singlet and triplet potentials that more accurately describe the interaction of alkali Rb and K atoms in the ground state. Complex cross sections of the spin exchange are calculated for the first time that permit one to calculate the processes of polarization transfer and relaxation times, as well as shifts in the magnetic resonance frequencies caused by K-Rb spin exchange collisions.     

The influence of intermolecular interactions on the Kerr effect in gases

Expressions are derived for the second and third Kerr virial coefficients, B _K and C _K, of spherical top molecules in terms of irreducible cluster polarizabilities, and values are calculated using the dipole-induced dipole model for argon, krypton, xenon, methane, tetrafluoromethane, neopentane and sulphur hexafluoride. For mixtures of rare gases it is shown that the collision-induced dipole moment makes a negligible contribution to B _K. The effect of the choice of intermolecular potential function on the calculated second Kerr virial coefficients is also demonstrated. It is found that the predominant contributions to C _K arise from the pair polarizability, and that the triplet polarizability is only of minor importance.     

IR spectrum and localization of excitation of reaction center of photosystem II in triplet state

Using the density functional theory, vibrations of different model forms of the chlorophyll a molecule in the ground and triplet states have been calculated. The assignment of the experimental difference IR spectrum corresponding to the formation of the triplet state of the photosynthetic reaction center of photosystem II has been proposed on this basis. It has been shown that molecules of accessory chlorophyll B _A and B _B located between the special pair and H _A and H _B pheophytin molecules can be involved in the intermolecular hydrogen bond with the water molecule. The energy of this interaction in the triplet state of molecules for B _A is larger on 6 kcal/mol. This allows us to relate this pigment to the location of the triplet excitation at the reaction center of photosystem II.     

Magnetic field and spin effects from sequential p-type and d-type triplet mechanisms

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br- and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical ‘p-type’ triplet mechanism) and modification of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer (‘d-type’ triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V _d for the pure d-type and V _pd for the combined p- and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic field and heavy atom effects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters.     

Partial-wave analysis for elastic <Emphasis Type="Italic">p</Emphasis><Superscript>13</Superscript>C scattering at astrophysical energies

A standard partial-wave analysis was performed on the basis of known measurements of differential cross sections for elastic p ¹³C scattering at energies in the range 250–750 keV. This analysis revealed that, in the energy range being considered, it is sufficient to take into account the ³ S ₁ wave alone. A potential for the triplet ³ S ₁-wave state of the p ¹³C system in the region of the J ^p T = 1⁻¹ resonance at 0.55 MeV was constructed on the basis of the phase shifts obtained from the aforementioned partial-wave analysis.     

Photophysical Processes in the Gas Phase at High Levels of Vibrational Excitation of Triplet Molecules of Benzophenone and Fluorenone

By the delayed fluorescence activated by direct multiphoton excitation of triplet molecules by CO₂–laser radiation we have studied the prevailing deactivation pathways of triplet molecules with a high store of vibrational energy E _vib. The dependences of the kinetic characteristics of delayed fluorescence on the presence of vapors and foreign gases have been used to estimate the rates and efficiencies of intermolecular vibrational relaxation in the vibrational quasi–continuum of the triplet state T ₁. By the changes in the intensities and decay rates over a wide range of vibrational energies we have established the E _vib dependences of reversible intercombination conversion between the states T ₁ and S ₁ and interconversion from T ₁ to the ground electronic state S ₀ for both the case of isolated excited molecules and at a steady vibrational temperature. It is shown that at high vibrational temperatures the radiationless transition from the T ₁ state to S ₀ has an activation character and is accomplished through the energy barrier. The conditions for going to an exponential dependence have been determined. It has been found that the obtained dependences are in good agreement with the known experimental results. The influence of molecular and environmental characteristics on the decay rate of triplet molecules is compared.     

On the Absorption of antibaryons in nuclei

Antiproton (ˉp) and antilambda (ˉΛ) production has been measured for minimum bias in p+A collisions and central A₁+ A₂ collisions at the CERN-SPS by the collaborations NA35/49 and NA44. The measurements are extrapolated from rapidity distributions to absolute minimum bias cross sections. It is shown that the ˉp cross sections divided by A₁· A₂ follow an exponential trend as a function of a characteristic length obtained from a Glauber type absorption model, while the ˉΛ cross sections divided by A₁· A₂ are constant. The exponential trend also holds for ˉp production at the lower energies of the Brookhaven AGS. A discussion of the physics interpretation of the established trends in terms of an effective absorption cross section is presented. Received: 15 July 1999 / Revised version: 10 November 1999     

Forward jet production in deep inelastic scattering at HERA

The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (x_Bj) and as triple differential cross sections d³σ/dx_BjdQ² , where Q² is the four momentum transfer squared and is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.     

Application of genetic programming for proton-proton interactions

The aim of the present work is to use one of the machine learning techniques named the genetic programming (GP) to model the p-p interactions through discovering functions. In our study, GP is used to simulate and predict the multiplicity distribution of charged pions (P(n _ch )), the average multiplicity (〈n _ch 〉) and the total cross section (σ _tot ) at different values of high energies. We have obtained the multiplicity distribution as a function of the center of mass energy ($ \sqrt s $ \sqrt s ) and charged particles (n _ch ). Also, both the average multiplicity and the total cross section are obtained as a function of $ \sqrt s $ \sqrt s . Our discovered functions produced by GP technique show a good match to the experimental data. The performance of the GP models was also tested at non-trained data and was found to be in good agreement with the experimental data.     

The fast and slow rotation approximations for the description of the kinetics of triplet radical quenching and electron spin polarization

The specific features of the mechanisms and kinetics of generation of net chemically induced dynamic electron polarization (CIDEP) in triplet radical quenching (TRQ) in liquids is analysed in detail. The problem reduces to the analysis of fairly strong non-adiabatic transitions between states of the triplet radical spin Hamiltonian which are known to determine CIDEP generation in TRQ. The analysis is performed in two limits of fast and slow rotation of the triplet molecule using the previously developed method of treatment for non-adiabatic transitions. The method made it possible to derive analytical formulas for the CIDEP generation probability P _e and rate K _e, and for the TRQ probability P _q and rate K _q in the case of relatively strong quenching. It is shown that the dependence of K _e on the relative diffusion coefficient D _r is of bell shape and cannot be described correctly by the usually applied relation K _e = K _q P _e.     

A precise measurement of the spin-dependent neutron scattering length of 3He

The poor knowledge of the spin-dependent neutron scattering length of ³He has until now handicapped nuclear four body theory and the interpretation of excitations in the quantum liquid. We have measured, for the first time directly, the real part of the bound incoherent neutron scattering length, b_i′ of ³He. A neutron spin echo spectrometer was used to detect pseudomagnetic precession of polarised neutrons passing through polarised ³He gas. Any absolute calibrations of sample and beam parameters were avoided using simple transmission measurements with non-polarised neutrons. The only a priory information required was the spin-dependent neutron absorption cross section of ³He. The result is b_i′ = -2.365(20) fm, which reduces the prior uncertainty by a factor 30. The corresponding new value of the bound incoherent scattering cross section is σ_i = 1.532(12) barn. Including the known value of the coherent neutron scattering length, we obtain new values for the real parts of the free triplet and singlet neutron scattering lengths, a_-′ = 7.370(58) fm and a₊′ = 3.278(53) fm.     

The effect of bromine substitution on the charge-transfer emission of the complex of phenanthrene and tetracyanobenzene

The absorption spectra of the charge-transfer complexes of sym-tetracyanobenzene (TCNB) with phenanthrene, 9-bromophenanthrene, and 9,10-dibromophenanthrene are measured in chloroform solutions at room temperature. The total emission and phosphorescence spectra of the donors and the complexes are measured at 77 K in rigid glasses. The phosphorescence decay lifetimes are determined for phenanthrene, TCNB, and for the phenanthrene-TCNB complex, and a decrease in the phenanthrene-TCNB complex lifetime relative to the lifetimes of the two components is observed. The luminescence spectra of the complexes exhibit both a red shift and a lack of structure as compared with the donor spectra. The results are interpreted, in agreement with the results of Iwata et al. for the phenanthrene-TCNB complex (1), as an indication that there is a considerable degree of charge-transfer character in the lowest triplet state (T₁). Bromine substitution leads to a decrease in the energy of the phenanthrene triplet state. As a result, the energy gap between the donor molecule triplet state and the complex charge-transfer triplet state decreases from phenanthrene, to 9-bromophenanthrene, to 9,10-dibromophenanthrene. The results suggest that the proximity of these two triplet states in 9,10-dibromophenanthrene and its charge-transfer complex leads to some local donor triplet state character in the emitting complex triplet state.