Infrared Predissociation Spectroscopy of Large Water Clusters:  A Unique Probe of Cluster Surfaces

Abstract

The vibrational OH-stretch spectra of large water clusters were measured by photofragment spectroscopy after the absorption of pulsed tunable infrared radiation in the frequency range from 3000 to 3800 cm -1 . The mean size of the clusters from 〈n〉 ) 20 to 1960 was measured by threshold photoionization of the water clusters doped with sodium atoms. The largest abundance of the fragments was that of water hexamers. The fragment intensities are measured for different excitation energies and different cluster temperatures as function of the cluster size. For the selected sizes 〈n〉 ) 48, 111, 631, and 1960 complete OH-stretch spectra have been measured. The comparison with calculations revealed that the method is mainly sensitive to the outer cluster surface which has for all sizes an amorphous structure dominated by 3-coordinated and to a lesser extent also by 4-coordinated molecules. The intensity of the hexamer fragments goes through a maximum at n ) 70 and drops to n ) 300 where it levels off with a different slope. This behavior is attributed to the number of available connected 3-coordinated water molecules and the influence of the emerging 4-coordinated molecules in these clusters.

Infrared Spectra and Hydrogen‐Bonded Network Structures of Large Protonated Water Clusters H+(H2O)n (n=20–200)

“…In accordance with these discussions, we suggest that formation of crystalline structures occurs in H + (H 2 O) n clusters with hundreds of water molecules. Such formation of a crystalline core in large clusters has been predicted by theoretical searches for low-energy structures and IR spectroscopy on average size-controlled clusters, [2][3][4]22] and the present results are consistent with these previous studies. Figure 3 plots the free OH stretching frequencies as a function of the cluster size.…”

“…The frequency seems to converge to 3699-3700 cm À1 . In the previous study on H + (H 2 O) n (n 100), from the fact that the OH frequency of 3699-3700 cm À1 is much lower than the previously reported value of about 3718 cm À1 for (H 2 O) hni=40,111 (hni: average size, Figure 3), [4] we concluded that an excess proton affects the structures of H-bonded network even in large clusters (n = 100). [10] To obtain more accurate data on the free OH frequency of large neutral water clusters, we recently measured free OH frequencies of phenol-(H 2 O) n clusters, in which the perturbation from phenol is negligible.…”

“…For example, the lowest-energy structures of (H 2 O) n (n 9 1000) on the empirical potentialenergy surface have been reported, and these studies suggested that crystal cores are formed in the size region of a few hundred water molecules or more. [2][3][4]22] We show clear spectroscopic signatures for the abundance of the interior (4-coord) water. Furthermore, the fact that IR spectral patterns approach those of supercooled water and ice with increasing cluster size suggests formation of more ordered H-bonded network structures.…”

“…This means that 4-coord water becomes dominant in larger clusters. [2][3][4] For a detailed investigation of the band position of 4-coord water molecules, we carried out density functional (DFT) calculations at the B3LYP/6-31 + G(d) level. [24] We calculated structures and their IR spectra for H + (H 2 O) 60 as a typical example of a large cluster ( Figure 2).…”

The photoelectron angular distribution of water clusters

“…6,26,27 We obtained mean cluster sizes of N = 58 for the 40 eV data set, and N = 84 for both 60 eV data set (more details are available in the supplementary material 28 ). For the 40 eV data set, the nozzle temperature had to be estimated from the heating power, instead of a thermocouple reading.…”