The present Volume of Lecture Notes in Chemistry fulfils one of the stated aims of the Series, that of disseminating results discussed and evaluated at recent scientific international conferences; in our case a Satellite Meeting of the well-known Conference Series on the Physics of Electronic and Atomic a:ollisions, the XIIIth ICPEAC, which took place in Castelgandolfo, near Rome, from 23 to 25 July 1983. Since the Satellite Meeting attracted a widely international and in­ terdisciplina~y audience whose general consensus was one of warm appro­ val for the scie'ntific level achieved during it, we hope that the pre­ sent collection of essays will be met by similar success, thus warran­ ting our having asked the participants to work still further for us. Before turning to their efforts, however, it is only just to thank the Italian National Research Council (Chemistry Committee and Physics Committee), the University of Rome, the C.N.R. Tnstitute H.A.I. of the Rome Research Area (Montelibretti) and the E.N.E.A. Organisation for their financial aid, which made the Castelgandolfo Meeting possible. We warmly acknowledge the professional expertise of the staff at Villa Montecucco and for their collaboration we are grateful to: Rita Abbasciano, Catherine Cajone, Lucilla Crescentini, .Roberta Fantoni, An­ tonio Montani, Amedeo Palma, Rosario Platania, Maurizio Venanzi.