TABLE OF CONTENTS A second effect, which has been observed in clusters of monovalent s-electron metals (alkaline and noble metals), is an odd-even effect, also apparent in the mass spectrum. Some examples are now given:
The measured ionization potentials of N-even clusters of the alkali metals Na N and K N with N < 20 have values systematically larger than their N-odd neighbors [27, 28].
The inverse effect is found for the electron affinities of the noble metal clusters, with N-odd clusters having larger electron affinities. Or, in terms of the quantities experimentally measured, anionic M N - clusters (M indicates a noble metal element) with TV odd have higher photodetachment thresholds [29]. Notice that a noble metal cluster M N - has N + 1 valence electrons (originating from the atomic s-electrons).
The mass spectra of positively and negatively charged noble metal clusters obtained by ion bombardment show an odd-even alternation in the abundances, with N-even clusters being less abundant than their N-odd neighbors [30, 31]. This effect is observed up to N ? 40. In the ion bombardment technique the nascent clusters are already charged.
The odd-even alternation is observed in the dissociation energies of small alkali clusters [32, 33]. This is the smallest energy to fragment the cluster in two pieces, which is usually the energy to evaporate one atom.
The dissociation energies of singly and doubly charged Cu clusters ( 
and
