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Droplet Photoelectron Imaging

Periodic Reporting for period 1 - eDrop (Droplet Photoelectron Imaging)

Reporting period: 2018-11-01 to 2021-04-30

Angle-resolved photoelectron spectroscopy of aerosol droplets (“droplet photoelectron imaging”) is a novel approach to study fundamental aspects of the electron dynamics in liquids and across interfaces. Our recent proof-of-principle studies demonstrate that droplet photoelectron imaging not only complements, but also significantly extends the range of accessible information over established methods. Two aspects are unique to droplets: Firstly, the droplet size can be varied over a wide range from submicrons to microns. While large droplets provide overlap with liquid microjet and bulk studies, small droplets offer additional control by acting as efficient optical resonators. These optical cavity effects can be exploited to control where in the droplet the photoelectrons are generated; e.g. surface versus volume. Secondly, comprehensive information about photoelectron kinetic energy and angular distributions can be obtained fast and in a straightforward way by velocity map imaging.

Building on our proof-of-principle studies, we propose to exploit the versatility of the droplet approach to address fundamental questions regarding electron dynamics in liquids and across interfaces: Can this new tool provide the missing data for low-energy electron scattering in water and other liquids and resolve the issue of the “universal curve”? How do slow electrons scatter across liquid-gas and buried liquid-liquid/solid interfaces and how does this depend on the composition and curvature of the interface? How is the ultrafast relaxation dynamics of electrons following above-band-gap excitation influenced by electron scattering and confinement effects? Low-energy electron scattering is a determining factor in radiation chemistry and biology and a central aspect of the solvated electron dynamics, while interfacial processes play a key role in atmospheric aerosols. Droplet photoelectron imaging opens up new ways to study such phenomena.
The new droplet photoelectron spectrometer with the femtosecond high harmonic laser light source was successfully built, tested and its performance was characterized.

Several experimental studies about the formation and relaxation dynamics of the solvated electron in large, neutral water clusters were performed, the experimental data was analyzed and the results were published. These studies reveal the importance of accounting for the effect of electron scattering as a prerequisite for accessing genuine properties of the hydrated electron. Similar binding energies, photoelectron anisotropies and solvent relaxation dynamics were observed in large water clusters as in liquid bulk water. The formation and survival probability, by contrast, was found to depend strongly on the cluster size - both probabilities increase with increasing cluster size. Sustaining solvated electrons was found to require a minimum cluster size of 14 water molecules. This size-dependence is of relevance to radiation chemistry in water.

Furthermore, pronounced charge and quantum effects were observed for the escape of low-energy electrons from the surface of submicron-sized droplets. This proof-of-principle study is of fundamental importance for the understanding of low-energy electron escape from liquid surfaces – essential information, for example, for understanding chemical reactions at surfaces.
Finally, by modelling of previous data from photoelectron experiments of liquid water and water droplets, we could provide strong evidence that electron scattering cross sections in liquid water and amorphous ice are very similar within uncertainties of about a factor of two. Such electron scattering data are essential for the modeling of radiation chemistry and biology.
Our investigations regarding the size dependence of the properties of the hydrated electron in uncharged aqueous systems finally clarify which properties depend on the system size and which do not. Thus, our experimental studies have answered an important open question in this research field, which has received broad attention over more than hundred years.

The new photoelectron spectrometer for droplets has enabled us to obtain previously inaccessible information on low-energy electron escape from liquid surfaces. The importance of this proof-of-principle study is also reflected by its being highlighted as Editor’s suggestion in Physical Review Letters and as Focus Article “Catching electrons as they escape a liquid” in Physics. Further investigations regarding electron transfer through interfaces are planned and we anticipate that we can clarify open questions in this area with our new measurement technique for droplets.

The comparison of low-energy electron scattering cross sections in liquid water and amorphous ice has resolved one of the most debated issues regarding electron scattering in water, by demonstrating the similarity of the cross sections in the two phases. This result removes a so far unknown, but controversially discussed source of uncertainty in the modeling of radiation chemistry in aqueous environments. Our resulting publication was highlighted as Editor’s suggestion in Physical Review Letters. Here, we plan to perform photoelectron studies on droplets and particles to generate further evidence on which to base a more extended comparison of liquid and solid scattering cross sections.
Photoelectron image (top) recorded after irradiation of a droplet with high harmonic light (bottom).