Quantitive Photoemission Spectroscopy of Hydrogen Bonded Systems

Abstract

Water, the most crucial substance on our planet, is characterized by its hydrogen bond network. Although numerous investigations have been carried out to study the nature of the hydrogen bond, the electronic structure of hydrogen bonded water molecules on the level of molecular orbitals has not yet been revealed completely. In particular, no angle-dependent measurement of photoemission spectra has been performed in order to determine the anisotropy parameters for the valence orbitals of water molecules in liquid phase, so far. The spatial distribution of the photoelectrons ejected from a particular orbital is anisotropic and can be expressed in terms of a parameter β. Besides its dependence on ionizing photon energy, the anisotropy parameter β is determined by the topology of the corresponding orbital. As a consequence of formation of hydrogen bonds, deformation of involved orbitals takes place, which results in change of β value for each orbital.In the present work, an angle-dependent photoelectron spectroscopy experiment is performed in combination with a table-top EUV radiation source based on high harmonic generation (HHG). For the first time, the β values (hν=38.7 eV) for the 1b1, 3a1 and 1b2 orbitals in liquid water are determined to be 0.8, 0.7 and 0.6 respectively, in contrast to the values 1.4, 1.1 and 0.7 in the gas phase. The smaller β values related to the liquid phase, result from delocalization of the orbitals due to the significant interactions between hydrogen bonded molecules in liquid water. Moreover, the change of the photoemission anisotropy is different for the three orbitals: the 1b1 and 3a1 orbitals exhibit significant decrease of β value while the 1b2 shows only slight variation. This indicates that 1b1 and 3a1 orbitals have the most contribution to hydrogen bonding.Also the β parameter is determined for the 1b1 orbital in small-sized clusters (〈n〉 = 5), which amounts to 1.0. The small difference between the photoemission anisotropy for liquid water and small clusters is assumed to be an indication for the major influence of the first coordination shell on the local electronic structure in the hydrogen-bonded networks. Besides, it might be an evidence that the average coordination number is between 2 and 3 in liquid water.In order to reveal the nature of hydrogen bonds in systems comparable to water, an analog study is carried out on methanol. The comparative study on angular photoemission of methanol indicates the distinctly strong hydrogen bonding in water as compared to methanol.As an addition to the general topic of the present work, the development of a novel method for high harmonic generation is presented. Such modern EUV sources can advantageously be driven by Ti:Sapphire oscillators with low pulse intensity and are more compact than the existing HHG sources with rare gases as nonlinear media. Therefor, nano-structures with bow-tie shaped elements are applied to induce local field enhancement of the initial laser field.