All natural-parity states of 18 0 have been studied with high accuracy with the 14 C( 7 Li, ty) ls O coincidence and 14 Cfo,y) 18 0 radiative-capture reactions. The four-particle, twohole 0 2 + , 1", 2 3 + , and 3 3~ states deexcite with consecutive enhanced El and E2 crossover transitions having B (El) ^ 10~2 Weisskopf units (W.u.) and B(E2) ^ 20 W.u. These data suggest the existence of an a + 14 C dipole band in 18 0 similar to those discussed recently by Iachello and Jackson.PACS numbers: 21.60. Gx, 21.10.Pc, 23.20.Ck, 27.20.+n The A = 18 system is an attractive one for study of nuclear structure 1 in that it gives access to both charge symmetry and charge independence of the nucleon-nucleon interaction, as well as interplay of the single-particle and deformed collective-quadrupole degrees of freedom. 2 " 4 Indeed, the coexistence of core-excitation deformed states [e.g., the four-particle, two-hole (4p-2h) 0 2 + state at 3.63 in 18 0] and simple twoparticle shell-model states is now well established in 18 0.Recently, it has been suggested 5 that certain nuclei may display an altogether new collective degree of freedom. When the nucleus can be described as a dinuclear molecular system, as this suggestion implies, the relevant degree of freedom is the separation vector of the nuclear centers and the pertinent variables are the length of this vector and two of the three Euler angles which define its spatial orientation. The dinuclear molecular system can be described by a classical geometrical description 6 as well as a group-theoretical algebraic picture. 5 In this latter case the molecular spectra are considered to be generated by one S boson and three P u (-1 ^ /! ^1) bosons, the generators of U(4). Two quite distinct physical situations are possible. In the first, the participant nuclei do not themselves deform and only the length of the separation vector undergoes oscillation while the entire system can rotate about its center of mass. Such motion would be expected to lead to a conventional vibration-rotation spectra involving rotational bands having spin sequences 0 + , 1", 2 + , 3", 4 + , etc.; additionally it would be expected that enhanced collective El intraband transitions would be observed in non-self-conjugate systems. 7 In the second physical situation the participant nuclei interpenetrate as the separation vector oscillates. Such motion can give rise to a spectrum having equidistant multiplets of dipole vibrational character and again enhanced collective El transitions (between states of different multiplets) are predicted in non-self-conjugate systems. 7 Recently this enhancement of the radiative widths of transitions linking molecular states has been examined in a model-independent fashion 6 and sum rules have been derived for El, E2, and E3 transitions. These sum rules together with the usual Wigner limits of reduced widths for particle decay of the presumed molecular states provide an effective signature for such structure as well as a measure of the degree of collective enhancement. They a...
