There are several scientific activities performed during this project. Some of the activities are shortly described below:
1. Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons
In this study, the fragmentation of brominated cyclic hydrocarbons (bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane) upon ionization of inner-shell electrons (Br(3d) and C(1s)) was investigated using coincidence ion momentum imaging. Notably, a significant production of CH3+ fragments was observed, indicating intramolecular hydrogen migration, particularly evident with larger molecules. This contrasts with prior findings in linear hydrocarbon molecules. By analyzing the momentum correlations of fragment ions in three-body fragmentation pathways, it was deduced that CHx+ fragments with increasing hydrogen count are more likely formed through sequential fragmentation routes. The study's findings on molecular size-dependent kinetic energy releases and fragment kinetic energies were rationalized using classical Coulomb explosion simulations.
2. Dopant ionization and efficiency of ion and electron ejection from helium nanodroplets
The study focuses on effective ion and electron detection in photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets. Utilizing a commercial quadrupole mass spectrometer and a photoelectron–photoion coincidence spectrometer, the research systematically measures ion and electron yields in pure and doped He nanodroplets across varying sizes and ionization methods (direct and secondary after resonant photoexcitation). The optimal droplet size for maximum ejected ion yield is determined for two dopants: oxygen molecules (O2) and lithium atoms (Li). Optimal droplet sizes are identified, with peak detection efficiency of dopant ions relative to primary photoabsorption events reaching up to 20% for O2 and 2% for Li in charge-transfer ionization, and 1% for O2 and 4% for Li in Penning ionization. These findings offer crucial insights for refining conditions in mass spectrometric and photoionization spectroscopy investigations of molecules and complexes isolated within He nanodroplets.
3. Electron energy loss and angular asymmetry induced by elastic scattering in helium droplets
This study focuses on the interaction of ionizing radiation with helium nanodroplets, serving as an ideal model system for condensed matter. The research investigates how purely elastic electron scattering impacts the angular and energy distributions of photoelectrons emitted from He nanodroplets of varying sizes (10-109 atoms per droplet). In larger droplets, photoelectrons exhibit notable anisotropy in the direction of incident light due to shadowing within the droplets. However, the detected photoelectron spectra remain minimally affected. This discovery suggests potential for photoelectron spectroscopy of embedded dopants within droplets, as long as the dopants are smaller than the light's penetration depth and the emitted electrons' trapping range.
4. Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets
The study delves into radiation-induced damage in biological systems caused by ionizing radiation, predominantly through secondary processes like charge and energy transfer, resulting in DNA bond breakage. The investigation centers on the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters, and microhydrated versions following direct and indirect ionization from extreme-ultraviolet (XUV) irradiation. Comparative analysis of photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules with those of Cyt/Thy clusters and microhydrated forms formed in superfluid helium (He) nanodroplets highlights the impact of Penning ionization after resonant excitation, revealing reduced fragmentation compared to direct photoionization and charge-transfer ionization. The addition of water molecules to Cyt/Thy molecules suppresses fragmentation effectively, a trend similarly observed in homogeneous Cyt/Thy clusters formed within He nanodroplets. Penning ionization electron spectra (PIES) for Cyt/Thy exhibit broad features, while microhydrated derivatives indicate sequential ionization, leading to intact microsolvated Cyt/Thy cations.
5. Fragmentation of Water Clusters Formed in Helium Nanodroplets by Charge Transfer and Penning Ionization
Helium nanodroplets (HNDs) are commonly employed to create customized clusters and molecular complexes within a cold, transparent, and minimally interacting matrix. Evaluating species within HNDs using mass spectrometry is hindered by ion fragmentation and trapping. This study investigates fragment ion mass spectra of HND-aggregated water and oxygen clusters, focusing on charge transfer ionization (CTI) and Penning ionization (PEI). PEI exhibits approximately 10 times lower efficiency than CTI in ionizing embedded clusters, yet it generates larger water cluster sizes and higher relative yields of unprotonated cluster ions, constituting a 'soft ionization' method. However, the propensity of ions to remain bound to HNDs results in diminished detection efficiency for larger HNDs with over 10,000 helium atoms. These findings play a pivotal role in optimizing conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated within HNDs.
