Two-particle Excitations Research Articles

A study of Cd and Te isotopes in the interacting boson approximation

An investigation of the properties of the Cd and Te isotopes is performed in the framework of the interacting boson approximation. Particular attention is paid to the presence of “intruder” 0 + and 2 + states at the energy of the two-phonon triplet in the middle-shell Cd isotopes. It is suggested that these states originate from two-particle two-hole proton excitations across the shell gap at Z = 50. The behaviour of these states is studied as a function of the neutron number. No clear evidence of similar phenomena is observed in the Te isotopes. The collectives features of these nuclei are found to be in satisfactory agreement with the predictions of the neutron-proton interacting boson model.

Collective bands in doubly even Sn nuclei

Starting from a simplified picture treating proton two-particle two-hole excitations coupled with spherical quadrupole vibrations, in interaction with the low-lying quadrupole vibrational states in the doubly even Sn nuclei, we are able to account for a regular ΔJ = 2 band structure on top of excited J π = 0 + states. We compare in some detail results for 116Sn concering energy spectra and E2 transition rates.

Indirect optical cooperative excitation of two-particle electronic excitations in doped organic molecular crystals

A theory on the indirect optical cooperative excitation of a two-particle electronic excitation composed of one host exciton and one guest-molecule one-particle electronic excitation through one electric dipole-moment active singlet guest molecule one-particle electronic excitation is developed by considering the nonradiative coupling bstween one- and two-particle electronic excitations.

Dynamical Correlation Effects on the Quasiparticle Bloch States of a Covalent Crystal

A local-orbital formulation of the Dyson equation is presented for the one-particle Green's function with the self-energy expressed in terms of a dynamically screened interaction which includes local-field and excitonic effects. Also presented is a quantitative calculation of the quasiparticle states for diamond from first principles, which demonstrates the different roles played by two-particle excitations (electron-hole, plasmon). A discussion of the implications for local-density and empirical one-electron concepts is given.

Some features of two-particle exciton-phonon excitations in molecular crystals

In this paper we present the results of a theoretical study of the optical absorption line shapes in the two-particle exciton-intramolecular phonon (IP) excitation regime in molecular crystals. The exciton-IP coupling was handled considering the exciton and the IP as a pair of interacting pseudo-particles and exploring the conditions for branching-off a two-particle bound state (TPBS) from the two-particle-continuum (TP), without invoking the exciton-IP decoupling approximation. We were able to handle both symmetry-allowed electronic transitions as well as symmetry-forbidden, vibronically-induced, electronic excitations. For symmetry-forbidden transitions we have provided a systemic derivation of Rashba's, Koster—Slater type formula where the local perturbation results from quadratic IP coupling. For the case of symmetry-allowed electronic transitions we derived a new approximate expression for the line shape, incorporating both linear and quadratic coupling terms and elucidating the conditions for a peaceful coexistence of TP and TPBS. Next, we have considered the exciton-IP system coupled to intermolecular lattice phonons (LP), taking into account the effects of bath diagonal “strong” coupling and of off-diagonal “weak” coupling. The diagonal exciton-LP coupling results in the appearance of side bands which should be viewed as three-particle resonances corresponding to an exciton + IP + LP. Off-diagonal exciton-LP coupling results in the relaxation of the exciton-IP TPBS to the TP continuum at finite temperatures. Finally, we have explored the effects of static structural disorder on the exciton-IP system, considering the effects of Anderson-type diagonal disorder. Configurational averaging was performed utilizing the coherent potential approximation, while the disorder field was represented in terms of a lorentzian distribution of the site excitation energies. Structural disorder drastically affects the TPBS, while the TP continuum is only slightly affected by disorder scattering.

Wavefunctions of two-particle collective excitations

A formalism is developed by which the wavefunctions, as well as the energies, of two-particle collective excitations may be obtained using the corresponding two-particle Green function. Using a simple one-dimensional model of a polyene chain, the wavefunction of an excitonic state is calculated. This calculation shows that the electron and hole are closely bound. A similar calculation is performed for the spin-wave mode in antiferromagnetic gamma manganese.

Theory of Intrinsic Indirect Exchange Coupling in Semiconductors

Indirect exchange coupling through filled valence band and empty conduction band is investigated on the basis of an extended Anderson Hamiltonian by the perturbation theory. Coupling mechanisms are classified as (1) kinetic, (2) polarization, and (3) two-particle excitation. Relation to the band structure is shown for each mechanism. Sign, Fourier transform, and asymptotic behavior are investigated.

Investigation of 38Ar levels by 37Cl+p reactions : (II). Lifetimes, spins, parities and theory

The mean lives of 16 bound states of 38Ar have been determined from DSA measurements at 14 selected 37Cl(p, γ) 38 Ar resonances in the range E p = 0.9–1.5 MeV. Spins and parities of 20 (p, γ) resonances have been determined from a precision comparison with corresponding 37Cl(p, α 0) 34S resonances. Gamma-ray transition strengths combined with γ-ray angular distribution measurements at two resonances determine (or limit) the spins of 15 bound states of 38Ar. The experimental data on 38Ar are compared with the results from shell-model calculations. The negative-parity states, especially those with high spin, can be very well described in terms of a model with five active nucleons in the 1d 3 2 shell and one in either the 1f 7 2 or 2p 3 2 shell. For the positive-parity states, the new data underline the importance of two-particle four-hole excitations.

Doubly Excited Feshbach States ofCl−

An investigation of doubly excited states of the negative chlorine ion using the method of configuration interaction has located several Feshbach autodetaching resonances in the energy region 8.5-12.6 eV. Dominant two-particle excitations considered include $3{p}^{2}\ensuremath{\rightarrow}4{s}^{2}$, $4s4p$, $4{p}^{2}$, and $5{s}^{2}$. The results obtained are useful in interpreting the recent experimental structure observed in the electron spectrum resulting from collisions of ground-state ${\mathrm{Cl}}^{\ensuremath{-}}$ ions with helium- and hydrogen-gas targets.

Shell-model study of 56Ni, 58Ni, 54Fe and 56Fe

Shell-model calculations of 56Ni, 58Ni, 54Fe and 36Fe are presented. States are assumed to belong to configurations involving no excitation and two-particle excitations from the 1 f 7 2 shell. A limited number of core-excited states corresponding to four-particle excitations are also taken into account in the cases of 56Ni and 58Ni. The effective two-body interaction in the (1f, 2p) shell is deduced from a non-local potential which reproduces the nucleon-nucleon scattering data. Calculated energy spectra and transition rates are not in good agreement with experiment. This is certainly related to the truncation of the basis. Some improvements are obtained when single-particle energy spacings are allowed to vary.

Cooperative exciton spectra in molecular crystals

It is shown that the singlet exciton spectrum in the crystal is coupled with that of the two-particle excitations (singlet, triplet excitons, magnons, triplet exciton-magnon). Splitting on the excitation spectrum occurs when the composite energy of the two-particle excitations is close to that of the singlet exciton.

Stabilization of the axially symmetric shape and (pf) 2 mixing in sd shell nuclei

Two-particle excitations into the pf shell from the sd shell is shown to be important to make nuclei such as 22Ne and 24Mg more axially symmetric. The level structure of 22Ne can be reasonably understood, but a complete understanding of the level structure and electromagnetic properties of 24Mg is still lacking. Two-hole eight-particle states are also considered in 22Ne.

One- and two-particle excitation with good isospin

One-particle excitation operators with good isospin are introduced and classified as isoscalars or isovectors. They are applied to some gamma-ray transitions to determine selection rules. Similarly, two-particle excitation operators with good isospin are introduced and classified as isoscalars, isovectors, or isotensors. They are applied to pairing excitation modes of nuclei with N > Z.

A shell-model study of43Sc

The odd parity levels of 43Sc are calculated allowing the three extra-core particles to occupy the complete 1f2p shell. It is found that the energy levels are fairly insensitive to changes in the two-body interaction, and that, although agreement is obtained with some of the experimental data, several features of the empirical spectrum are not accounted for this way. These results, and stripping data from the 42Ca(3He, d)43Sc reaction, suggest that there are low-lying states arising mainly from two-particle excitations of the 40Ca core.

The half-life of the 2211 keV state in 136Ba

The decay of 12.9 d 136Cs was studied with a delayed coincidence circuit. It was found that the 2211 keV (6 +) state in 136Ba is delayed with respect to the 342 keV beta rays with a half-life of T 1 2 = 3.3±0.3 nsec , and the half-life of the 2144 keV (5 −) state is smaller than about 1 nsec. Gamma rays and conversion electrons were measured with Ge(Li) and Si(Li) detectors to obtain gamma-ray intensities and conversion coefficients of some of the transitions. Transition probabilities of the four transitions de-exciting the 2211 keV state are deduced as follows: 0.45 (340 keV, E2), 4.8 (153 keV, E2), 2.2 × 10 −6 (176 keV, E1) and 2.7 × 10 −5 (67 keV, E1) in Weisskopf units. The excited states in 136Ba are discussed in terms of two-particle excitations on the basis of data thus far obtained in the decay of 136Cs.

Differential Scattering of Neutrons fromPb208at Resonant Energies

Measurements are reported on differential elastic scattering of neutrons of a few keV energy spread from 99.75% $^{208}\mathrm{Pb}$ in the energy region 0.715 to 1.761 MeV. The results include absolute differential cross sections at 20 resonant and 9 nonresonant energies. A least-squares phase-shift analysis provides the resonance parameters and reduced widths of 21 resonances. The observed ${s}_{\frac{1}{2}}$ potential-scattering phase shifts are in agreement with those expected from a type of phenomenological potential which reproduces the single-particle bound states of $^{209}\mathrm{Pb}$. Parity assignments and widths of the $J=\frac{1}{2}$ resonances are in disagreement with the prediction for 5 intermediate states arising from two-particle one-hole neutron excitations. There are, however, a number of resonances with widths corresponding to several percent of the single-particle limits which are likely candidates for some sort of intermediate states.

Nuclear structure calculations for the 2 − ( β)2 + ( γ)0 + decays of Rb 86 and Rb 84

Starting with the nuclear shell model, a calculation is performed for the non-relativistic nuclear matrix elements in the first-forbidden beta-decays of Rb 86 into Sr 86 and Rb 84 into Kr 84. In particular the ratios q≡ |∫iB ij| 2 |∫iB ij| 0 , w≡− ∫(− iσ· r) (∫iB ij) 2 x≡− C v ∫i r C A(∫iB ij) 2 u≡− ∫σ× r (∫iB ij) 2 are evaluated and compared with experimental data for the 2 − → 2 + → 0 + decay sequence. If a particle-surface interaction is assumed to cause the necessary admixtures, satisfactory agreement with the data is obtained (including the signs of w, x and u). It is concluded that the 2 + states of Sr 86 and Kr 84 cannot be understood in terms of two-particle excitations, or in terms of a purely collective picture, but show features of an intermediate description.

Correlation Effects in Many-Fermion Systems: Multiple-Particle Excitation Expansion

The ground-state wave function and energy of a finite system of interacting fermions are expanded in terms of multiple-particle excitations on an uncorrelated zero-order state. The resulting set of coupled equations constitutes a systematic variational generalization of Hartree-Fock theory. Comparison is made with many-body perturbation theory and it is shown that to any order the theory incorporates an infinite number of perturbation theory terms. Solutions of the equations for ground-state atomic systems are discussed and related to previous work using many-body perturbation theory. It is shown that the sums of perturbation terms necessary for convergence are automatically included in the equations for two-particle excitations. Application of the equations to open-shell atoms is described.

Low excited states of O16(I)

Abstract 1) Some general characteristics of the particle-hole energy are discussed by using a simple model. 2) Energy levels of the 0+ excited states due to the excitation of a single nucleon are estimated for O16 and Ca40 using the Rosenfeld force. Monopole transition probabilities of these excited states are also estimated and compared with the experimental results. 3) The energy level spacing between the 0+ and 2+ excited states of O16 is roughly estimated assuming the excitation of a single nucleon. The transition probability from this level to the ground state is calculated and is found to agree with experiment. Also the ratio of the two transition probabilities from the 2+ excited state to the 0+ excited and ground states is calculated and compared with experiments by Chevallier and Mennrath. The result obtained seems to suggest that it is necessary to take into account the two-particle excitation in order to explain the probability of the transition from the 2+ excited state to the 0+ excited state. 4) Some remarks are made on the ratio of the transition probabilities of the decays from the 1− state to the 0+ excited and ground state, and also on the two-γ emission probability of the 0+ excited state through the virtual excitation at the lowest 1− state.

Coupling of Angular Momenta in Two-Particle States in Deformed Even-Even Nuclei

The recent suggestion that low-lying intrinsic states in deformed nuclei are basically two-particle excitations is considered in more detail. The result of the strong-coupling model that the two-particle states are doubly degenerate with spins $\ensuremath{\Omega}=|{\ensuremath{\Omega}}_{1}\ifmmode\pm\else\textpm\fi{}{\ensuremath{\Omega}}_{2}|$ is re-emphasized. It is suggested on the basis of experimental evidence that the degeneracy of the two states of each configuration is removed largely by residual spin-dependent forces, analogous to the splitting observed in deformed odd-odd nuclei. Coupling rules are given for the lower-lying state of each configuration based on this assumption. As an example of the application of the coupling rules to actual nuclei the decay schemes of the two ${\mathrm{Ta}}^{178}$ isomers to levels in ${\mathrm{Hf}}^{178}$ are considered in detail.