Study of hydrated argon and carbon dioxide cluster using Velocity Map Imaging

Abstract

Carbon dioxide and argon can form hydrated clusters in water-rich environments, and their photoionization dynamics are of fundamental interest in atmospheric and astrochemistry. In this work, we combine photoelectron–photoion coincidence (PEPICO) detection with velocity map imaging (VMI) using 21.22 eV vacuum ultraviolet light from a helium discharge lamp to investigate the photoionization of ($CO_2)_m$, $Ar_m$, and $(Ar_m\cdot H_2O)$ clusters. A theoretical transfer-matrix model of the VMI spectrometer is carried out, and numerical simulations are performed to optimize the electrode configuration. Python-based software has been developed for coincidence-event filtering, false-coincidence suppression, and Abel inversion. Our Monte Carlo study of popular Abel inversion methods (pBasex and rBasex) indicates that the rBasex method is more robust and is therefore used in our data analysis.

Our experiments reveal several key findings. For both $Ar_m$ and $(CO_2)_m$ clusters, we observe a clear solvation effect: the measured binding energy decreases with increasing cluster size, showing the strong dependence of interactions on the local cluster environment. We quantified the delocalization effect of a homogeneous cluster ion, and to the best of our knowledge, this is the first experimental evidence of this effect in an argon cluster. For hydrated argon clusters, we observe a large ionization-core signal across all cluster ions, which might suggest evaporative dissociation from large cluster core or a strong polarization effect induced by the water molecule. Our data cannot distinguish between these two possibilities. For $(CO_2)_m$ clusters, our qualitative results suggest possible atomic rearrangement processes associated with the $B^{2}\Sigma_{u}^{+} \text{ and } A^{2}\Pi_{u}$ ionization states, which warrant further investigation.

These results show that combining photoelectron photoion coincidence spectroscopy with VMI is effective for probing cluster dynamics. The observed size-dependent binding energies and charge delocalization mechanism provide benchmark data for theoretical models of weakly bound clusters.

Popular Abstract

Molecular clusters—small aggregates of atoms or molecules—are the hidden building blocks behind many familiar phenomena, from clouds and snowflakes to interstellar ice and planetary formation. Yet because they are far too small to see with ordinary microscopes, studying how they form, evolve, and fall apart has long been a challenge. In this work, we use ultraviolet light combined with photoelectron spectroscopy and coincidence detection to capture both electrons and ions from the same ionization event, giving us a window into the cluster's behavior before and after excitation. To handle the complex data, we developed a custom Python software package that cleans up the signals and sharpens the images. We applied our method to argon, argon‑water, and carbon dioxide clusters. In pure argon, we found that the positive charge spreads over several atoms upon ionization, forming delocalized charge cores. For argon‑water clusters, we observed much weaker binding than in pure argon—explained by argon atoms forming a dense core with water on the surface, allowing charge to spread and loosen the structure, while water's strong interactions further reduce stability. To our knowledge, this is the first direct experimental evidence of charge delocalization and polarization effects in these systems. We also saw signs that ionization can trigger atomic rearrangement in $CO_2$ clusters, showing that molecules are not rigid but can reshape under excitation. These lab experiments simulate conditions in Earth's atmosphere, interstellar space, and planet‑forming regions, helping us connect molecular‑scale interactions to the macroscopic world—from aerosol particles to the chemical evolution of the universe.

• author: Liu, Tao
• supervisor: Mathieu Gisselbrecht; Shilpa Rani Sahu
• organization: Department of Physics; Synchrotron Radiation Research
• course: FYSM64 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Cluster, Hydrated clusters, Velocity Map Imaging, Photoionization dynamics
• language: English
• id: 9246159
• date added to LUP: 2026-07-13 09:36:39
• date last changed: 2026-07-13 09:36:39