Loren Ban

Last Name

Ban

First Name

Loren

ORCID

0000-0002-9312-2984

Organisational unit

06232 - ETH transfer IP und Lizenzen / ETH transfer IP and Licenses

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Publications1 - 10 of 22

Time-dependent photoemission from droplets: influence of size and charge on the photophysics near the surface
Ban, Loren; Tang, Hanchao; Yoder, Bruce L.; et al. (2022)
Faraday Discussions
Photoemission from submicrometer droplets containing a mixture of dioctyl phthalate and dioctyl sebacate was investigated by femtosecond and nanosecond photoionization. Photoelectron spectra recorded after ionization with single 4.7 eV femtosecond or nanosecond laser pulses showed marked differences between the two cases. These differences were attributed to ionization of long-lived states which only occurred within the duration of the nanosecond pulse. The tentative assignment of the long-lived states to dioctyl phthalate triplet states is discussed. A nanosecond-femtosecond pump-probe scheme using 4.7 eV (pump) and 3.1 eV (probe) pulses was used to investigate the decay dynamics of these long-lived states. The dynamics showed an accelerated decay rate at higher dioctyl phthalate concentrations. Furthermore, the dependence of the decay dynamics on droplet size and charge was investigated. The decay of the long-lived states was found to be faster in smaller droplets as well as in neutral droplets compared with both positively and negatively charged droplets. Possible mechanisms to explain these observations and the dominance of contributions from the droplets surface are discussed.
Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters
Gartmann, Thomas E.; Ban, Loren; Yoder, Bruce L.; et al. (2019)
The Journal of Physical Chemistry Letters
We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55−3.85 eV and a genuine anisotropy parameter in the range of 0.51−0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.
Low-energy electron scattering cross sections for nanoscale water clusters
Horn, Katinka; Tsizin, Svetlana; Ban, Loren; et al. (2025)
Physical Review Research
Electron scattering cross sections are essential for the quantitative description of electron-induced processes in nanoconfined aqueous systems. Here we report on energy- and size-dependent cross sections for water clusters with diameters between ∼1.0 and 2.5 nm and electron energies below ∼32 eV. The cross sections of the clusters are up to a factor of 2 larger than those of water, suggesting a reduced dielectric screening in nanoconfinement. The cross sections show no significant cluster size dependence. A slight increase of cross sections with increasing electron energy is attributed to the increasing contribution from electronic scattering channels. As the simplest form of nanoconfined water, nanoscale water clusters represent an important model system.
Below Band Gap Formation of Solvated Electrons in Neutral Water Clusters?
Ban, Loren; West, Christopher W.; Chasovskikh, Egor; et al. (2020)
The Journal of Physical Chemistry A
Below band gap formation of solvated electrons in neutral water clusters using pump-probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in the liquid and clusters. Similar relaxation times in the order of 200 fs and 1-2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The relaxation times independence from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the different solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half maximum suggests a dependence of the solvation time on the binding energy.
Electric Charge Effect on the Water Mass Transfer across Mixed Aqueous–Organic Droplet Interfaces
Azizbaig Mohajer, Mercede; Gleichweit, Michael; David, Grégory; et al. (2025)
The Journal of Physical Chemistry A
Understanding gas-particle mass transport is essential for predicting aerosol behavior in the atmosphere and in industrial processes. The mass accommodation coefficient, αM, is a key parameter describing this exchange. Defined as the probability of a gas-phase molecule adhering to a particle upon collision, αMpresents a highly surface-sensitive property. Aqueous atmospheric aerosol droplets carry electric charges, which accumulate near the surface; however, their influence on the gas-particle mass transport remains elusive. To access the influence of charge on αM, we combined an aerosol charging method with photothermal single-particle spectroscopy, enabling direct water mass exchange measurements on the surface of single aerosol droplets. We investigated charged and neutral aqueous glycerol and tetraethylene glycol droplets across a wide range of concentrations and temperatures. The micrometre-sized droplets carried approximately 10³elementary charges─exceeding typical atmospheric aerosol charge levels─yet our results show that αMis independent of the droplet charge and instead is dominated by composition and temperature. Theoretical estimates of the charge-dipole and dipole–dipole interaction energies corroborate this finding, highlighting that under atmospherically relevant conditions, electric charge plays a negligible role in the mass accommodation process.
Angle-resolved Photoelectron Spectroscopy of large Water Clusters ionized by an XUV Comb
Colaizzi, Lorenzo; Ban, Loren; Trabattoni, Andrea; et al. (2021)
2021 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Detailed knowledge about photo-induced electron dynamics in water is key to the understanding of several biological and chemical mechanisms, in particular for those resulting from ionizing radiation [1] . While several studies reporting on detailed low-energy electron scattering cross sections in amorphous ice, liquid water and large water clusters [2] and a time-resolved approach to investigate electron scattering in water has been reported [3] , such investigations in gas-phase water clusters have shown to be a promising bridge in between the gas and liquid phase, allowing for many technological limitations to be overcome and setting a clear route to perform attosecond-resolved spectroscopy of hydrated molecules. Indeed, extreme ultraviolet (XUV) attosecond pulses may be used to photo-ionize a water sample and to investigate the electron dynamics and transport properties with extremely high temporal resolution [4] .
Low-energy electron escape from liquid interfaces: charge and quantum effects
Ban, Loren; Gartmann, Thomas E.; Yoder, Bruce L.; et al. (2019)
arXiv
The high surface sensitivity and controlled surface charge state of submicron sized droplets is exploited to study low-energy electron transport through liquid interfaces using photoelectron imaging. Already a few charges on a droplet are found to modify the photoelectron images significantly. For narrow escape barriers, the comparison with an electron scattering model reveals pronounced quantum effects in the form of above-barrier reflections at electron kinetic energies below about 1 eV. The observed susceptibility to the characteristics of the electron escape barrier might provide access to these properties for liquid interfaces, which are generally difficult to investigate.
Valence Electronic Structure of Interfacial Phenol in Water Droplets
Heitland, Jonas; Lee, Jong Chan; Ban, Loren; et al. (2024)
The Journal of Physical Chemistry A
Biochemistry and a large part of atmospheric chemistry occur in aqueous environments or at aqueous interfaces, where (photo)chemical reaction rates can be increased by up to several orders of magnitude. The key to understanding the chemistry and photoresponse of molecules in and "on" water lies in their valence electronic structure, with a sensitive probe being photoelectron spectroscopy. This work reports velocity-map photoelectron imaging of submicrometer-sized aqueous phenol droplets in the valence region after nonresonant (288 nm) and resonance-enhanced (274 nm) two-photon ionization with femtosecond ultraviolet light, complementing previous liquid microjet studies. For nonresonant photoionization, our concentration-dependent study reveals a systematic decrease in the vertical binding energy (VBE) of aqueous phenol from 8.0 +/- 0.1 eV at low concentration (0.01 M) to 7.6 +/- 0.1 eV at high concentration (0.8 M). We attribute this shift to a systematic lowering of the energy of the lowest cationic state with increasing concentration caused by the phenol dimer and aggregate formation at the droplet surface. Contrary to nonresonant photoionization, no significant concentration dependence of the VBE was observed for resonance-enhanced photoionization. We explain the concentration-independent VBE of similar to 8.1 eV observed upon resonant ionization by ultrafast intermediate state relaxation and changes in the accessible Franck-Condon region as a consequence of the lowering of the intermediate state potential energy due to the formation of phenol excimers and excited phenol aggregates. Correcting for the influence of electron transport scattering in the droplets reduced the measured VBEs by 0.1-0.2 eV.
Size-Resolved Electron Solvation in Neutral Water Clusters
Ban, Loren; Yoder, Bruce L.; Signorell, Ruth (2021)
The Journal of Physical Chemistry A
Cluster-size-resolved ultrafast dynamics of the solvated electron in neutral water clusters with n = 3 to ∼200 molecules are studied with pump–probe time-of-flight mass spectrometry after below band gap excitation. For the smallest clusters, no longer-lived (>100–200 fs) hydrated electrons were detected, indicating a minimum size of n ∼ 14 for being able to sustain hydrated electrons. Larger clusters show a systematic increase of the number of hydrated electrons per molecule on the femtosecond to picosecond time scale. We propose that with increasing cluster size the underlying dynamics is governed by more effective electron formation processes combined with less effective electron loss processes, such as ultrafast hydrogen ejection and recombination. It appears unlikely that any size dependence of the solvent relaxation dynamics would be reflected in the observed time-resolved ion yields.
Charge Effects on the Photodegradation of Single Optically Trapped Oleic Acid Aerosol Droplets
Parmentier, Evelyne A.; Arroyo, Pablo Corral; Gruseck, Richard; et al. (2022)
The Journal of Physical Chemistry A
It has recently been reported that reactions can occur faster in microdroplets than in extended condensed matter. The electric charge of droplets has also been suggested as a possible cause of this phenomenon. Here, we investigate the influence of electric charges on the photodegradation of single, optically trapped oleic acid aerosol droplets in the absence of other reactive species. The temporal evolution of the chemical composition and the size of droplets with charge states ranging from 0 to 104 elementary charges were retrieved from Raman spectra and elastic light scattering, respectively. No influence of the droplet charge was observed, either on the chemical composition or on the kinetics. Based on a kinetic multilayer model, we propose a reaction mechanism with the photoexcitation of oleic acid into an excited state, subsequent decay into intermediates and further photoexcitation of intermediates and their decay into nonvolatile and volatile products.

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Loren Ban

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