Size-Resolved Photoelectron Anisotropy of Gas Phase Water Clusters and Predictions for Liquid Water

Abstract

We report the first measurement of size-resolved photoelectron angular distributions for the valence orbitals of water clusters with up to 20 molecules. A systematic decrease of the photoelectron anisotropy is found for clusters with up to 5-6 molecules, and most remarkably, convergence of the anisotropy for larger clusters. We suggest the latter to be the result of a local short-range scattering potential that is fully described by a unit of 5-6 molecules. The cluster data and a detailed electron scattering model are used to predict liquid water anisotropies. Reasonable agreement with experimental liquid jet data is found.A detailed understanding of elastic and inelastic scattering of electrons in liquid water is of fundamental importance for the modelling of radiation damage in biological systems, the description of the behaviour of the solvated electron in chemistry, and for the quantitative interpretation of photoelectron spectra of liquid water and aqueous solutions [1][2][3][4][5][6][7][8]. For slow electrons (electron kinetic energy eKE ≲ 50 eV), detailed experimental scattering parameters (differential scattering cross sections and energy losses) were so far only reported for amorphous ice [9]; with the exception of very slow electrons (eKE ≲ 6 eV), for which liquid water data were recently obtained from photoelectron velocity map imaging (VMI) of liquid water droplets [10]. Since there is little reason to expect substantial differences between amorphous ice and liquid water for electronic scattering processes (eKEs ≳ 6 eV) the amorphous ice and liquid droplet data [9,10] should now provide a reasonable data set for scattering simulations of liquid water. In addition, electron attenuation lengths (EALs) for eKEs ≳ 3 eV are available for liquid water from various microjet studies [11-13], which, however, do not allow quantitative predictions of the scattering contributions.The photoelectron angular distribution (PAD) is particularly sensitive to electron scattering and has thus recently received increasing attention in this context [7,10,[13][14][15][16][17][18]. Often, the information in the PAD is described by a single anisotropy parameter β (see Eq. ( 1)). For the liquid microjet, this is an approximation which we also follow in the present work. For ionization from the O1s orbital of liquid water, Thürmer et al. observed a more isotropic PAD; i. e. a smaller β-value; for liquid water compared with gas phase water over the eKE range from ~12 -450 eV [13]. For core-level ionization, this reduction is assumed to mainly arise from electron scattering within the liquid. For the ionization from the valence orbitals 1b 1 , 3a 1 , and 1b 2 , additional changes in the initial state due to orbital mixing also mediated by hydrogen-bonding are expected to contribute to the difference in β-values between gas and liquid phase. While monomer gas phase β-parameters have been reported for the three valence orbitals at photon energies 18 eV ≤ hν < 139 eV [15,16,[19][20][21], corresponding values for liquid water have t...

Angle-resolved photoemission spectroscopy of liquid water at 29.5 eV

“…Comparison of anisotropy parameters determined in this work and those obtained by Hartweg et al 43 for (a) 1 b 1 , (b) 3 a 1 , and (c) 1 b 2 bands of water. Experimental values for H 2 O monomer and (H 2 O) n clusters and predicted values for liquid water were taken from Ref.…”

“…Hergenhahn and coworkers have measured photoemission anisotropy for neutral water clusters with the aggregation numbers ( n ) of 50–80 and attributed the diminished photoemission anisotropy to elastic scattering in the cluster 42 . Nahon, Signorell, and their coworkers have employed a coincidence imaging spectrometer at SOLEIL to measure photoemission anisotropy as a function of electron kinetic energy for neutral water clusters up to n  = 20 43 . Their results are reproduced in Fig.…”

“…The result indicated that the formation of hydrogen bonding network changes the character of ionized orbitals, especially 1 b 1 and 3 a 1 involved in hydrogen bonding, in small clusters. They have also predicted photoemission anisotropy of liquid water using the values of water clusters ( n  = 1 or 6) and Monte-Carlo simulations of the elastic and inelastic scattering of an electron in liquid water 43 . The green diamonds shown in Fig.…”

“…Experimental values for H 2 O monomer and (H 2 O) n clusters and predicted values for liquid water were taken from Ref. 43. Open black circles indicate the prediction based on the β 2 values of monomer, and open orange triangles indicate the prediction based on cluster values (n = 6).…”

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…These species should be observable spectroscopically, since recently Xie et al, using VUV photoionization in combination with IR spectroscopy, observed hemi-bonded structures in H2S clusters 106 and for (CH3SH)2 + bonded with H2O, (CH3)2CO, CH3SH and NH3. 107 Coincidence spectroscopy where mass selected photoelectron spectra can be obtained could also be applied to these PAH water clusters, as recently demonstrated for pure PAH 108 and water clusters 109 at the French synchrotron SOLEIL.…”