Search

In recent years it has been shown that two paramagnetic ions could be coupled together by the virtual emission of a phonon at one site and its subsequent re-absorption at another site (1-4). This (~virtual-phonon exchange ~ (VPE) is analogous to photon exchange, which leads to the Coulomb interaction between charged particles (5). Recently the origin of nondipolar interactions between rare-earth ions in ethyl sulfates has been at tr ibuted to this mechanism (r However, numerical estimates based on Orbach's procedure (7) for estimating the ion-phonon interaction parameters lead to results which are three order of magnitude smaller than the experimental data (s,~). A more thorough and detailed calculation of the individual contributions to VPE in terms of the normal co-ordinates of the complex surrounding the paramagnetie ion has been presented (~o). Preliminary results seem to indicate that, at least in the case of Tb 3+, Ce 3+ in the ethyl sulfates, the observed departures from dipolar interactions arc due to VPE. In order to check the general validity of this result a systematic investigation was undertaken for all rare-earth ions for which experimental data were available. We start from a system composed of two ions in a crystal lattice each having a doubly-degenerate ground state. The zeroth-order states of the two-ion system have the form IA> ~ la(ll, a(2)> = la(1)> | la(2)>, where ]a(~> denotes one of the two wave functions [• of the ground doublet of the paramagnetic ion number s. The eigenstates of the whole system, ions plus phonon field, have the form

43 strains of E. coli observed in B and/or C bile from patients in the University of Catania Medical Clinic between 1965 and 1970 were subjected to 16 antibiotics in vitro. Sensitivity to streptomycin fell from 55.86% in 1965 to 16% in 1970, whereas a fluctuation from 35 to 55% was observed for kanamycin over the same period; there was a general fall in effectiveness in the case of the other 14 antibiotics. The clinical advantages nd technical simplicity of the antibiogram are noted.