Preface

Summary

This book gathers all contributions to the International Conference on H₂in Space held in Paris, France, on September 28- October 1st, 1999. The attendance was 106 participants from 16 countries. The goal was to gather together representatives of three communities: experts in the physics of the molecule, including experimentalists, observers of the interstellar medium, in particular of the warm H₂ detected in infrared lines, and theoreticians studying H₂ formation and cooling in astrophysical objects, from the early universe to the present galaxies.

The electronic structure of the H₂ molecule has been well studied in the past, but it was shown that recent progress has been made on the theory of highly excited and long-range Rydberg states, both from calculations of line-strengths and comparison with experiments. New and more accurate data are now available for the rates of collisional excitation of rovibrational transitions by neutrals, as well as protons or electrons. It has been known for several decades that interstellar H₂ is formed on dust grains, but, until now, the efficiency of this process was poorly known. Recent experiments have reproduced this formation process on silicates and amorphous carbon and shown that the efficiency is strongly dependent on temperature. In particular, the mobility of H atoms on grains is much lower than previously thought. Laboratory experiments with trapped ions and nanoparticles have opened new avenues of investigation. The relationship of H₂ with PAHs is being addressed both in the laboratory and by means of astronomical observations.