The recent observation of low-lying collective bands built on 0(+) intruder states in the neutron-deficient Pb region is discussed within the context of two-particle-two-hole and four-particle-four-hole proton excitations across the Z = 82 proton shell closure.

We study the phenomenon of shape coexistence at or near single-closed shells. For that purpose, we describe multi-particle-hole intruder excitations on equal footing with regular excitations in the interacting boson model, by introducing the particle-hole degree of freedom, through a new quantum number, called I(ntruder)-spin. The resulting horizontal classification scheme of multi-particle-hole intruder excitations in terms of I-spin multiplets is useful to interpret data and to describe global properties of intruder excitations throughout a mass region. On the other hand, the creation and annihilation of particle-hole pairs, using a non-compact algebraic structure U(6,6), allows to account for local properties associated with mixing between regular and intruder excitations. We will illustrate the model by discussing a particular series of isotopes where shape coexistence may be present, Po192-210.

In the present discussion we concentrate on shape coexistence as obtained within a deformed single particle field as well as starting from the spherical shell model, incorporating deformation effects via the residual proton–neutron quadrupole interaction. We discuss in particular the appearance of shape coexisting phenomena in the Pb region. In a second part then, we present a number of experimental fingerprints that are able to recognize the appearance of shape coexisting phenomena or of shape mixing through the use of selective experiments (e.g., band structure, spectroscopic factors, static moments, E0 properties and α-decay).

We discuss the experimental data on the neutron-deficient Po isotopes in the light of the shape coexistence phenomenon. The existing level systematics can be viewed as resulting from the interplay of two structures with different deformation and their mixing. Combined results of several nuclear models support this interpretation.

We discuss both the strength of the Interacting Boson Model (IBM) as a unifying structure, emphasizing the robust elements as well as the limitations in an early U(6) group structure of interacting s and d bosons. We briefly address the rich structure of the IBM-2 approach, when treating proton and neutron degrees of freedom explicitly, which gives rise to non-symmetric spatial excitation modes of which a number have been detected experimentally. Finally, the very clear observation of particle-hole excitations near to closed-shell regions, leads to an extension of the IBM to include both particle and hole bosons. The basic concepts are presented as well as the compelling experimental evidence pointing out the existence of larger group-structures than the U(6) group structure.

An extension of the interacting boson model (IBM) is suggested to treat both particle-hole and regular excitations on an equal footing. Particle and hole bosons are introduced, each corresponding to a unitary U(6) algebra. Reduction of the corresponding product algebra leads to the definition of intruder-spin (I-spin). Embedding of the latter into a larger dynamical algebra U(12) gives rise to multiplets that connect states in different nuclei with the same I-spin. Similarly, embedding into a non-compact algebra leads to the classification of multi-particle-multi-hole excitations within one nucleus.

: The excitation energies of the single-particle normal and intruder levels in both 183 Tl and 187 Bi were measured for the first time via the α decay of 187 Bi produced in the 97 Mo( 92 Mo,pn) 187 Bi reaction. The previously unobserved 187 Bi ground state (h 9/2 ) to 183 Tl ground state (s 1/2 ) α transition was identified, establishing the 187 Bi intruder state excitation energy to be 112(21) keV, 70 keV less than that of the same level in 189 Bi.

An extension of the interacting boson model (IBM) is suggested by introducting both the particle-hole character and the charge character of the bosons as supplementary degrees of freedom. This allows us to consider an SU(4) symmetry. Within this new classification scheme, three applications are worked out.

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We present a comparison in strength for the magnetic dipole transitions to the O+ bandhead from the two possible extended proton-neutron interacting boson model 1(+) states with either a mixed particle-hole character or a mixed proton-neutron character.

Synthesis of pyrrolidinothieno (or [1]benzothieno)[3]azepines from the corresponding azepinediones or by direct cyclization of N-[thienyl(or[1]benzothienyl)]acetylprolinals.

Led by the observation that particle-hole excitations play an important role in nuclei throughout the whole nuclear mass table, we have formulated a general framework such that particle-hole excitations can be incorporated in a systematic way in the interacting boson model. The particle-hole degree of freedom is introduced in the interacting boson models IBM-2 and IBM-3. Corresponding algebraic structures and reduction schemes are studied. The coexistence of intruder spin, F spin, and isospin as good quantum numbers is investigated. The mixing of particle-hole excitations with ground state configurations is determined and compared with a commonly used IBM-1 treatment. [S0556-2813(98)06405-X].

The 9-benzoyl-7,8-dihydro-7,7,8,8-tetramethylthieno[3,2-f]indolizin-6(4H)-one 4a was successfully prepared via the intramolecular Heck cyclization of the appropriate activated enamidone 3a.

Thieno[b]quinolizidines were synthesized by reduction of thieno[b]quinolizidinones and the stereochemistry of their methiodide derivatives was studied. Hofmann degradation of the latter led to thienoazecines.

A high syn stereoselectivity, of the addition of an aromatic to the cyclic N-acyliminium ions generated from hydroxylactams 3a,b,e,f leading to the thieno[2,4]diazepines 4a,b,e,f is described.