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Droplet Photoelectron Imaging

Periodic Reporting for period 2 - eDrop (Droplet Photoelectron Imaging)

Reporting period: 2021-05-01 to 2022-10-31

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.
Light interacts differently with small particles compared with extended condensed matter because of the finite size of these particles. This also modifies light-induced processes in these particles, such as the formation and transport of electrons or related chemical reactions. To investigate such phenomena, a novel droplet photoelectron spectrometer with a femtosecond high harmonic laser light source has been built, tested and its performance characterized.

Exploiting such finite-size effects enabled us to retrieve accurate information about how slow electrons lose energy and change their direction when they travel through liquid water; i. e. information about electron scattering in liquid water. Detailed knowledge of electron scattering in water is, for example, crucial for a better understanding of energy dissipation processes that are relevant to radiation chemistry and biology.

The hydrated electron is a species that is supposed to play an important role in the chain of radiation damage processes in biological material. Hence, knowledge of its electronic properties and about its formation upon excitation of aqueous systems by light are important to assess its role in radiation damage. Additionally, the influence of spatial confinement on those properties needs to be assessed. We have performed a series of experimental studies, revealing that spatial confinement has no major influence on the electronic properties nor on the relaxation dynamics of the hydrated electron. However, a clear system size dependence was observed for its probability of formation.

Because of the finite size of aerosol particles, sunlight is amplified in their interior. Our investigations show that all light-induced reaction steps in atmospheric aerosol particles will take place 2 to 3 times faster in these particles as a result of the light amplification - likely with important implications regarding the role of such light induced processes in atmospheric processes.
The new photoelectron spectrometer has enabled us to obtain previously inaccessible information on light-induced processes in small particles. Progress beyond the state of the art was achieved mainly in the following two areas: (i) The retrieval of low-energy electron scattering data for liquid water now enables accurate electron scattering simulations for a better assessment of radiation damage in biological systems and for a more detailed analysis of electronic properties of solutes from liquid phase photoelectron spectroscopy. (ii) The quantification of the acceleration of light-induced reactions in aerosol particles by a factor of 2 to 3 will enable advances in the modeling of atmospheric aerosol chemistry, likely changing the perception of the importance of these processes in the atmosphere. We anticipate obtaining an even more detailed picture regarding electron transport and light-induced reactions from further planned investigations.
Photoelectron image (top) recorded after irradiation of a droplet with high harmonic light (bottom).