Size-Resolved Infrared Spectroscopic Study of Structural Transitions in Sodium-Doped (H₂O)_n Clusters Containing 10–100 Water Molecules

Abstract

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.

Can Electric Fields Drive Chemistry for an Aqueous Microdroplet?

“…As such, they are also direct reporters of surface electric elds as shown by Cooper et al using Infrared Photodissociation (IRPD) spectroscopy in which the measured Stark shifts are linearly proportional to the local electric eld at the surface, 41 depend on cluster size, and the absence or presence of ions and their identity. 42 Figure 1A displays the experimental IRPD measurement which shows that the neutral water and anionic water clusters exhibit a red-shift of the free O-H band with increasing cluster size 42,43,44 , whereas for positively charged cations there is a blue-shift of the free O-H band when progressing to larger clusters, accompanied by a transition in slope that correlates with the ion's charge (n = 100 and 30 for Ca 2+ and Na + , respectively). This trend is not reproduced using simple xed charge models that instead exhibits a linear function of the Stark shift with 1/r 2 (Figure S1), and is an important indicator that the surface features of microdroplets arise from many-body effects such as charge transfer 35,45 and intramolecular and intermolecular polarization.…”

Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

“…Experimental evidence for a small blue-shift of the free OH band in neutral water clusters with decreasing cluster size has been reported, although the origin of this effect has not previously been explained. 95 , 96 In the absence of an ion, a Stark shift could arise from electric fields generated by the intrinsic structure of interfacial water. At the bulk air–water interface, the electric field can have a non-zero component directed in the surface normal direction owing to the net dipolar orientation of water molecules.…”

The temperature and size distribution of large water clusters from a non-equilibrium model

“…Note that in a recent spectroscopic investigation of the OH-stretch vibration of size selected sodium doped water clusters in the size range from ten to hundred, we observed a transition to special structures with the sodium atom at the surface of the intact water clusters. 33 This arrangement leads to signal enhancement in the experiment that was not taken into account in the numerical simulation. There is a general shift of the distributions to smaller sizes.…”