The electronic structure of free water clusters probed by Auger electron spectroscopy

Abstract

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.

The photoelectron angular distribution of water clusters

“…Figure 2 shows two typical spectra from our measurements, containing overlapping contributions from the water monomers and from clusters. Similar spectra, but for larger water clusters, were recorded by Öhrwall et al 31 and by The partition for fitting of the spectrum that contains both the cluster and monomer signals. The area I corresponds to the 1b 1 cluster, IIa is for the pure molecule 1b 1 , the bottom IIb area is attributed half to 1b 1 cluster and the remainder to 3a 1 cluster, the area III is for 3a 1 both cluster and monomer, and the area IV is for 1b 2 cluster and monomer.…”

On the nature and origin of dicationic, charge-separated species formed in liquid water on X-ray irradiation

“…In Figure 2a the photon energy is 600 eV, i.e., sufficient to cause direct core-ionization, being more than 60 eV above the O1s ionization threshold of liquid water. 40 The spectrum in (see below), this peak has no gas-phase analogue, 27 and is the unambiguous electronicstructure signature of 1h•1h final states, whose energies are lower than for the 2h states, thus leading to higher KE of the outgoing electron in the autoionization event. Our interpretation of the shoulder at 510 eV KE is further supported by our constrained DFT calculations on small water clusters presented below and in the Supplementary Information (SI), Figures S1 and S2.…”

“…21 More recently ICD-like processes have also been observed for core-ionization and core-excitation of aqueous solutions. 20,[22][23][24][25][26] The coreionization induced processes of ICD type have been also observed and thoroughly discussed for water [27][28] and ammonia clusters. 29 An important difference between relaxation of coreholes and inner-valence holes is that single core-ionization energies are greater than the 3 lowest valence double-ionization energies; thus the Auger channel is always open upon coreionization whereas it might be energetically forbidden upon inner-valence ionization.…”

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy

“…The signal shape and position is consistent with the 1b 1 signal detected for water clusters. [70][71][72] Furthermore, the binding energy position of the new cluster feature systematically shifts from higher to lower binding energies as a function of pumpprobe delay, indicating an increasing cluster size.…”