Evolution of the resonances of small water cluster anions (H₂O)_n19^- in helium droplets upon attachment of electrons

Using a high-resolution electron monochromator, we studied the formation of (H2O)(n <= 19)(-) cluster ions upon collisions of free low-energy electrons with water clusters embedded in helium droplets. The anion efficiency curves as a function of the initial electron energy were measured for the cluster sizes n = 2-8, 10, 13, 16, and 19. The present experimental results show that the shape of the resonance yields is dependent on the size of the water cluster anion. The results are discussed in terms of the different electron states available for the excess electron from a linear cluster structure to three-dimensional cluster structures as the number of water molecules within the cluster increases.