Z. Phys. D Atoms, Molecules and Clusters 18, 1-2 (1991) Atoms, Molecules zo,oo.r, and Clusters f~r Phy,~k D

© Springer-Verlag 1991

Wilhelm Hanle

on his 90 th birthday

In January 1991 Prof. Dr. Drs. b.c. Wilhelm Hanle celebrates his 90 th birthday. On this occasion "Zeitschrift fi~r Physik D - Atoms, Molecules and Clusters" honours Wilhelm Hante and his work by dedicating this issue to him. Wilhelm Hanle still follows science today, particularly in the field he initiated; even in 1990 he was active at his former institute. In 1989 he finished writing his memoirs 1. In his most recent publication (1988)2, he argues for a reasonable solution of the energy problem. The above photograph shows an active Wilhelm Hanle as he was on his 85 th birthday.

More than 65 years ago, in 1924, the results of his doctoral thesis were published in "'Zeitsehrift fi~r Physik 30, 93 (1924,)" under the title "Uber die magnetische Beeinflussung der Polarisation der Resonanzfluoreszenz", i.e. "Magnetic Influence on the Polarization of Resonance Fluorescence". In this fundamental paper Wilhelm Hanle explained the observation of a decrease in the polarization of fluorescence light with increasing magnetic field strength. This Hanle effect is caused by the coherent superposition of two Zeeman sublevels at zero field. Hanle's experiment was the first hint of this kind of "coherence of photons". His explanation was not believed until modern

1 W. Hanle (printed by: I. Physikalisches Institut, Justus-Liebig-Universit/it Giegen, 1989) 2 W. Hanle et al.: Kerntechnik 53, 120 (1988)

wave quantum mechanics could describe the coherence in the states involved. Due to the significance of this paper, his original publication is reprinted on the opening pages of this issue. Additionally, an English translation of the abstract and summary of the paper are inserted along with a translation of the appendix to his memoirs where he describes "The Hanle-Effect - Level Crossing". There, he also summarizes the multitude of applications of his effect. The articles published in this issue underline this large variety of applications and the significance the Hanle-Effect has today.

All the papers of this issue are dedicated to Wilhelm Hanle. Some of the papers are directly concerned with his effect; others deal with related topics such as polarization, alignment and orientation, correlation and coherence, level crossing techniques and so on. Zeitschrift ffir Physik D, the editors, and all the authors of the articles wish Wilhelm Hanle all the best, in particular good health for the future, and last but not least we hope he remains an active member of the community of physicists, as he always has been.

Paul Mokler GSI-Darmstadt, November 1990

It is a pleasure to thank all the authors who contributed their work to this issue. The advice and assistance from Arthur Scharmann (L Physikalisehes Institut, Justus-Liebig-Universit~it GieBen) are cordially appreciated. He kindly provided also the photograph of W. Hanle.