Size-resolved infrared spectroscopic study of structural transitions in sodium-doped (H₂O)n clusters containing 10-100 water molecules.

In water clusters containing 10-100 water molecules the structural transition takes place between "all surface" structures without internally solvated water molecules to amorphous water clusters with a three dimensionally structured interior. This structural evolution is explored with rigorous size selection by IR excitation modulated photoionization spectroscopy of sodium-doped (H2O)n clusters. The emergence of fully coordinated interior water molecules is observed by an increased relative absorption from 3200 to 3400 cm(-1) in agreement with theoretical predictions and earlier experimental studies. The analysis has also shown that the intermediate-sized water clusters (n = 40-65) do not smoothly link the structures in the largest and smallest analyzed size regions (n = 15-35 and n = 100-150) in line with previous reports suggesting the appearance of exceptionally stable water cluster isomers at n = 51, 53, 55, and 57. In the size range from n = 49 to n = 55 a reduced ion yield, a plateau in the total IR signal gain and signatures in the distribution of free OH stretch oscillator absorption have been observed. Recently reported putative global minima structures for n = 51 and n = 54 point to the presence of periplanar interior rings in odd-numbered clusters in this size range, which may affect cluster (surface) stability and the shape of the free OH stretch absorption peak. Potential links between pure and sodium-doped water cluster structures and the signatures of solvated electrons in photoelectron spectra of anionic water clusters are discussed.