anti-resonant dip at the maximum of study of the properties of high fre­ mode. The scattered intensity of a polariton absorption : the decrease of quency acoustc phonons and the in­ replica can be even higher than that efficiency indicates that re-absorption teraction between phonons and polari­ of the first order spectrum depending effects at exact resonance prevail tons. on the frequency of the incident beam. over the probability enhancement. Below the longitudinal exciton ener­ Such replicas are absent from the

(iv) Resonant increase and cooling gy EL "allowed" longitudinal acoustic spectra of pure crystals. of polariton fluorescence have been phonon and "forbidden" transverse Resonance occurs at a photon ener­ observed and connected with Reso­ acoustic phonon scattering processes gy close to the band gap. Replicas nant Brillouin Scattering. In a back- are observed, whose line shifts fit well have been observed also for higher- scattering configuration, one observes with the two-branch model of the pola­ order LO phonon combinations. These polariton escaping from the crystal riton dispersion. results are interpreted by a model in with a wave vector opposite to that Above EL a set of narrow strong which the relevant resonant inter­ of the exciting light. Polaritons scat­ lines is observed which cannot be mediate state is a localized exciton tered by one longitudinal acoustic explained by the predicted one pho­ strongly interacting with the LO- phonon give rise to the well defined non process based on the two-branch phonon. The different replica orders Resonant Brillouin Scattering peaks. model. correspond to resonance states for­ All other polaritons, having suffered The observation of a new resonance med by differently "dressed" trapped several successive processes are phenomenum was also reported by M. excitons. smoothly distributed and give rise to Balkanski, C. Hirlimann and J.F. It is obvious that the choice presen­ Resonant Polariton Fluorescence. Morhange : replicas of the first-order ted here is personal and purposely Thus the Resonant Brillouin Scattering phonon spectrum 6). The first-order limited to only very few of the many involves a memory of the initial state, Raman spectrum of mixed III-V semi- high-level, timely and important contri­ whereas the Resonant Polariton Scat­ conductor compounds contains the butions presented at this very stimu­ tering originates from a randomised characteristic normal modes of the lating scientific meeting. distribution of more or less therma- two compounds. At resonance con­ lized phonons. ditions, phonon replicas of this spec­ References The experiments carried out on CdS trum are observed, only in disordered References are to the Proceedings by Winterling and Koteles have de­ system ; the first-order spectrum is of the International Conference on termined the dispersion of the exci- entirely reproduced, shifted by the Lattice Dynamics (Flammarion Scien­ ton-polariton and have led also to the frequency of the longitudinal optical ces, Paris), 1978.

New President of EPS, Antonino Zichichi

Antonino Zichichi, a founding mem­ tre which has put a considerable nuclear physics. More recently, at the ber of EPS, is a high energy physicist, number of places at the disposal of plenary meeting of the European presently engaged on a systematic EPS and has offered advantageous Committee for Future Accelerators, he study of the proton structure at the terms to Divisions wishing to hold was elected chairman of the working CERN machines ISR and SPS. Pro­ conferences there. In June 1977 Zichi­ committee studying the project for fessor of Advanced Physics at the chi was elected President of the Ita­ a large electron-positron colliding () he was lian Istituto Nazionale di Fisica Nu­ beam device (LEP) that ECFA consi­ the Director of the Postdoctoral cleare (INFN) which promotes fun­ ders is the European high energy School of Physics at that University. damental research in sub-nuclear and machine for the 1980s. Zichichi is well known in the interna- tional physics community ; the study of lepton pairs produced in hadronic LASER DEPARTMENT interactions, the search of heavy lep- INSTITUTE OF APPLIED PHYSICS, UNIVERSITY OF BERN + tons produced in e e- annilations, Applications are invited for the position of : exemplify his pioneering contributions RESEARCH PROJECT LEADER in two branches of sub-nuclear phys­ ics where a decade was to pass Qualifications : Essential — Experience in Gas Laser Physics with preferably a knowledge of electronic transition lasers and before the phenomena were under­ related discharge techniques. stood. His research activity covers Desirable — Some knowledge of German. many fields : the high precision tests The position carries responsibility for the Gas-Laser group (staff complement : of Quantum Electrodynamics, the sys­ 2 PhD, 1 dipl. and 1 PhD candidate) which is part of the Laser Department (staff tematic study of the electromagnetic complement : 25). form factors of the proton and the The Gas-Laser group is presently working on CW high power lasers in the visible pseudscalar mesons, the first measu­ and near UV and on pulsed TEA discharges. rements of the (ω-to-$$) mixing angle, the Equipment available for studies is the following : discovery of the antideuteron and of — TEA discharge system for metal vapours the 2 decay of the X° meson, the — Powerful UV sources (coherent and incoherent), pulsed and CW dye lasers — 300 kW DC power supply (600 A or 1 kV) high precision measurement of the weak coupling constant... Additional Information may be obtained from, and applications accompanied by curricula vitae sent to : Prof. Dr. Heinz P. Weber He is the founder and the Director Institute of Applied Physics, University of Bern of the "" Centre for Sidlerstrasse 5 CH-3012 Bern Scientific Culture, established in 1962 and now consisting of 70 national and The position will be open from 1 July, 1978. international Schools - the same Cen-