Research Projects

The ultrafast dynamics of multiply ionised molecules in weakly bound systems play a crucial role in various environments, including the interstellar medium (ISM), planetary atmospheres, and man-made plasmas, as well as in radiation therapy. Noncovalent interactions (NCIs) such as Van der Waals forces, dipole interactions, and hydrogen bonding govern molecular aggregation in nature. Dipole-driven π-π and CH-π stacking aid PAHs formation in the cold ISM, while hydrogen bonding between nucleobases stabilizes DNA strands. Furthermore, five- and six-membered heterocyclic rings are prevalent among PAHs in the ISM and also form essential structural units of DNA nucleobases and related biomolecules. In such weakly interacting systems, when a core-electron is removed by electron impact or ionising radiation, Intermolecular Coulombic Decay (ICD) can occur, wherein the “energy released by electronic relaxation of an excited molecule leads to ionization of a neighbouring one via Coulombic electron interactions”. This ICD process has also been demonstrated to be a sensitive probe for investigating NCIs in weakly bound systems. In ICD study on six-membered rings, Prof. Dorn and coworkers reported that increasing the number of nitrogen heteroatoms enhances the π-electron density and broadens the spectrum of NCIs such as π–π, CH–π, and hydrogen bonding. Amid rising interest, researchers have begun investigating five-membered heterocyclic systems such as thiophene, where Mootheril et al. reported that the presence of a sulfur heteroatom enhances the efficiency of the ICD channel. Despite recent advancements, a comprehensive and systematic study on five-membered heterocycles has not yet been reported. This proposal aims to investigate how different atom-centres, such as nitrogen and oxygen, in π-electron-rich five-membered hydrocarbon rings influence NCIs by studying electron impact ICD of homo- and hetero-dimeric combinations of cyclopentadiene, pyrrole, and furan. These ICD dynamics will be investigated utilizing the Cold Target Recoil Ion momentum spectrometer (COLTRIMS) constructed by PI in collaboration with Prof. Dorn, a pioneer in ICD to aromatic ring systems. Utilizing this apparatus, 3D momentum vectors of all the fragmented ions and electrons detected in coincidence will be reconstructed, giving us a complete kinematic picture of all fragmentation pathways. By comparing the electron energy loss spectra (EELS) of the scattered electrons and potential energy surfaces (PES) calculated by Co-PI, different core ionisation events localised in carbon, nitrogen, and oxygen atoms will be distinguished for objective-specific analysis. The measured kinetic energy release serves as a direct indicator of the distance between the positive charge localizations on the monomer moieties and reflects the strength of NCIs. In addition to ICD, both Auger decay, where an electronically excited molecule undergoes self-ionization, and Proton Transfer Mechanism (PTM) can originate from the same precursor state and compete on ultrafast timescales. In a recent intense-field photon-induced ICD study on pyrrole-water dimer, valuable insights were obtained on the ultrafast competition between ICD and PTM pathways, with the timescale of PTM measured to be ~52 fs. However, intense-field experiments often lead to bond softening and structural changes due to the ionizing field itself, complicating the interpretation of intrinsic relaxation dynamics. To avoid such distortions, we propose a novel single-photon core ionization approach in weakly bound systems using EUV pulses generated via high-harmonic generation, a technique recognized by the 2023 Nobel Prize. The combined EUV pump–near-IR probe and theoretical trajectories of dimer conformers by our Co-PI will provide insight into how molecular structure, hydrogen bonding, and heteroatom identity regulate the rich dynamics and dominance of ultrafast de-excitation channels in ISM and biological tissues.