Infrared spectroscopy of negatively charged water clusters: Evidence for a linear network

Abstract

We report autodetachment spectra of the mass-selected, anionic water clusters, (H2O)n−, n=2, 3, 5–9, 11 in the OH stretching region (3000–4000 cm−1), and interpret the spectra with the aid of ab initio calculations. For n⩾5, the spectra are structured and are generally dominated by an intense doublet, split by about 100 cm−1, which gradually shifts toward lower energy with increasing cluster size. This behavior indicates that the n=5–11 clusters share a common structural motif. The strong bands appear in the frequency region usually associated with single-donor vibrations of water molecules embedded in extended networks, and theoretical calculations indicate that the observed spectra are consistent with linear “chainlike” (H2O)n− species. We test this assignment by recording the spectral pattern of the cooled (argon solvated) HDO⋅(D2O)5− isotopomer over the entire OH stretching frequency range.

A Drude-model approach to dispersion interactions in dipole-bound anions

“…For example, whereas Ayotte et al 59 have proposed that the observed IR spectra of (H 2 O) 6 -and it deuterium substituted isotopomers are due to a chain-like isomer, Kim and coworkers have suggested a more three-dimensional structure as the origin of the spectrum. 110,120,122 One might expected that comparison of the experimental IR spectrum with the calculated spectra for different isomers would permit definitive assignment for a cluster the size of (H 2 O) 6 -.…”

“…100 (H 2 O) n -clusters have also received considerable attention from the experimental and theoretical communities. 59,77,[108][109][110][111][112][113][114][115] There remain several fundamental questions about these species, including the size cluster for which the electron is more stable in an interior rather than a surface state, and how the anions observed experimentally are formed.…”

“…Recently, the Johnson group has succeeded in measuring vibrational spectra in the OH stretching region for the (H 2 O) n -Ar m, n = 5-9, clusters. 58,59,123 The presence of attached Ar atoms assures that the clusters are cold (T 50K). The comparison of measured spectra with ab initio calculated spectra for various isomers of the bare (i.e., non-Ar solvated) clusters greatly narrows down the possible structures.…”

“…Although there is a consensus that the observed forms of (H 2 O) n -, n = 2-4, have chain-like structures, the situation regarding the larger clusters is less clear. 59,110,[120][121][122] Neither the mass spectra nor the measured electron detachment energies allow one to definitely establish the geometrical structures. Recently, the Johnson group has succeeded in measuring vibrational spectra in the OH stretching region for the (H 2 O) n -Ar m, n = 5-9, clusters.…”

“…57 The recent resurgence of interest in dipole-bound anions and the closely related "solvated-electron" systems has been fueled by important advances on the experimental front. These advances include the use of Ar-atom vibrational predissociation spectroscopy, enabling one to obtain vibrational spectra of cold anionic clusters, 40,58,59 femtosecond photodetachment spectroscopy, 60 a new generation of experiments using electron transfer from highly excited Rydberg states, 61,62 and improved field ionization methods 63 for estimating the electron binding energies of weakly bound anions.…”