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Low-energy electron-induced reactions in hydrated molecular complexes are important in various fields ranging from the Earth’s environment to radiobiological processes including radiation therapy. Nevertheless, our understanding of the reaction mechanisms in particular in the condensed phase and the role of water in aqueous environments is incomplete. Here we use small hydrogen-bonded pure and mixed dimers of the heterocyclic molecule tetrahydrofuran (THF) and water as models for biochemically relevant systems. For electron-impact-induced ionization of these dimers, a molecular ring-break mechanism is observed, which is absent for the THF monomer. Employing coincident fragment ion mass and electron momentum spectroscopy, and theoretical calculations, we find that ionization of the outermost THF orbital initiates significant rearrangement of the dimer structure increasing the internal energy and leading to THF ring-break. These results demonstrate that the local environment in form of hydrogen-bonded molecules can considerably affect the stability of molecular covalent bonds. Reactions induced by low-energy electrons in hydrated systems are central to radiation therapy, but a full understanding of their mechanism is lacking. Here the authors investigate the electron-impact induced ionization and subsequent dissociation of tetrahydrofuran, model for biochemically relevant systems, in a micro-solvated environment.

Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA–drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C 6 H 6 ) 2 —the simplest prototypes of noncovalent π – π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2 s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (<10 eV) and a pair of C 6 H 6 + cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process. Aromatic systems that interact non-covalently are important in many settings, such as base-pair stacking and DNA–drug interactions; however, their excited-state molecular dynamics are not fully understood. Now, intermolecular Coulombic decay in benzene dimers has been observed. The process is initiated by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules.

In weakly bound systems like liquids and clusters electronically excited states can relax in inter-particle reactions via the interplay of electronic and nuclear dynamics. Here we report on the identification of two prominent examples, interatomic Coulombic decay (ICD) and radiative charge transfer (RCT), which are induced in argon dimers by electron collisions. After initial ionization of one dimer constituent ICD and RCT lead to the ionization of its neighbour either by energy transfer to or by electron transfer from the neighbour, respectively. By full quintuple-coincidence measurements, we unambiguously identify ICD and RCT, and trace the relaxation dynamics as function of the collisional excited state energies. Such interatomic processes multiply the number of electrons and shift their energies down to the critical 1–10 eV range, which can efficiently cause chemical degradation of biomolecules. Therefore, the observed relaxation channels might contribute to cause efficient radiation damage in biological systems. Inter-particle reactions in weakly bound systems are often difficult to pinpoint by detecting exclusively the kinetic energy of the produced ions. Here the authors present a full-coincidence experiment, where inter-particle reaction channels are determined by the measurement of all outgoing particles.

Intermolecular interactions involving aromatic rings are ubiquitous in biochemistry and they govern the properties of many organic materials. Nevertheless, our understanding of the structures and dynamics of aromatic clusters remains incomplete, in particular for systems beyond the dimers, despite their high presence in many macromolecular systems such as DNA and proteins. Here, we study the fragmentation dynamics of benzene trimer that represents a prototype of higher-order aromatic clusters. The trimers are initially ionized by electron-collision with the creation of a deep-lying carbon 2s −1 state or one outer-valence and one inner-valence vacancies at two separate molecules. The system can thus relax via ultrafast intermolecular decay mechanisms, leading to the formation of C 6 H 6 + ⋅ C 6 H 6 + ⋅ C 6 H 6 + trications and followed by a concerted three-body Coulomb explosion. Triple-coincidence ion momentum spectroscopy, accompanied by ab-initio calculations and further supported by strong-field laser experiments, allows us to elucidate the details on the fragmentation dynamics of benzene trimers. Higher-order aromatic clusters are prevalent in biochemical systems, but a full understanding of their structural and dynamical properties is lacking. Here, the authors demonstrate that inner-valence ionization can induce ultrafast relaxation and further fragmentation mechanisms in benzene trimers.

Real-time imaging of transient structure of the electronic excited state is fundamentally critical to understand and control ultrafast molecular dynamics. The ejection of electrons from the inner-shell and valence level can lead to the population of different excited states, which trigger manifold ultrafast relaxation processes, however, the accurate imaging of such electronic state-dependent structural evolutions is still lacking. Here, by developing the laser-induced electron recollision-assisted Coulomb explosion imaging approach and molecular dynamics simulations, snapshots of the vibrational wave-packets of the excited (A) and ground states (X) of D 2 O + are captured simultaneously with sub-10 picometre and few-femtosecond precision. We visualise that θ DOD and R OD are significantly increased by around 50 ∘ and 10 pm, respectively, within approximately 8 fs after initial ionisation for the A state, and the R OD further extends 9 pm within 2 fs along the ground state of the dication in the present condition. Moreover, the R OD can stretch more than 50 pm within 5 fs along autoionisation state of dication. The accuracies of the results are limited by the simulations. These results provide comprehensive structural information for studying the fascinating molecular dynamics of water, and pave the way towards to make a movie of excited state-resolved ultrafast molecular dynamics and light-induced chemical reaction. Capturing the detailed structural evolution of electronic excited states is a challenging but critical step to understand and control ultrafast molecular dynamics. Here, combining a Coulomb explosion imaging approach and molecular dynamics simulations, the authors retrieve the transient geometry of the ground and excited states of D2O mono- and dication with few femtosecond, few picometre accuracy.

Proton transfer underpins number of chemical and biochemical processes, yet its sub-100 fs dynamics have rarely been captured in real time. Here, we report direct and time-resolved observation of ionizing radiation-induced proton transfer in a heteroaromatic hydrate: the pyrrole-water complex. Both the electron-impact and strong-field laser experiments create a locally and doubly charged pyrrole unit (C 4 H 5 N 2+ ), which immediately (within 60 fs) donates a proton to the adjacent H 2 O, generating deprotonated C 4 H 4 N + and hydronium H 3 O + cations that subsequently undergo Coulomb explosion. The electron-impact experiments directly revealed initial states and provided dynamical insights through fragment ions and electron coincidence momentum imaging. The strong-field femtosecond laser experiments tracked the ultrafast dynamics of proton transfer; complementary ab initio calculations unraveled the dynamical details. The 50-60 fs proton transfer qualifies as one of the fastest acid-base reactions observed to date. This study offers a novel perspective on radiation-induced proton transfer in hydrated biomolecules. Proton transfer plays a key role in nature, yet its ultrafast dynamics remain elusive. Here the authors use coincidence spectroscopy and theoretical simulations to show that radiolytic doubly-ionized pyrrole triggers proton transfer to water within 60 fs.

Intermolecular Coulombic decay (ICD) is considered a general phenomenon that plays a key role in many fundamental and applied fields related to biological environments. In many cases, however, the mechanisms and efficiency of ICD have yet to be uncovered. A prominent example is heavy-ion cancer therapy. Here, we report the first detection of a damaging intermolecular relaxation cascade initiated by heavy-ion bombardment of hydrated pyrimidine clusters. The process can significantly contribute to the high biological effectiveness of heavy-ion irradiation and thus might play an essential role in many radiotherapy techniques. Inner-valence ionization of the cluster initiates ICD and triggers proton transfer between water molecules, producing destructive low-energy electrons, HO • radicals, and hydrated protons. Notably, the efficiency of ICD was found to increase dramatically with the number of water molecules, making ICD the dominant decay mechanism after inner-valence ionization. These findings indicate that the biological damage, caused by ICD in aqueous environments, is much more severe than was previously recognized.

Intermolecular Coulombic decay (ICD) is an important relaxation process of excited atoms and molecules in an environment, producing low-energy electrons that may contribute to radiation damage. Despite its significance, the mechanisms influencing ICD in molecular complexes remain unclear. Here, we investigate and unambiguously prove the ICD process in thiophene dimer, an aromatic ring with a third-row atom. Using multi-particle momentum coincidence spectroscopy, accompanied by high-level electronic structure calculations, we elucidate that the ICD process is initiated from the sulfur-containing inner-valence orbitals which are energetically below the Auger threshold. This leads to an enhancement in the emission of low-energy ICD electrons compared to other aromatic ring dimers. By utilizing this ‘ICD-only decay’ contribution we quantify ICD probabilities above the Auger threshold. This study reveals the pivotal role of sulfur in shaping the ICD electron spectrum, which can be implied to control the low-energy electron emission in biological systems. Low-energy electrons from intermolecular Coulombic decay (ICD) relaxation can contribute to radiation damage. Here, using multi-particle momentum coincidence spectroscopy and high-level electronic structure calculations, the authors show that the presence of sulfur in inner-valence orbitals leads to an enhancement in the emission of ICD low-energy electrons in thiophene.

Cell and gene damage caused by ionizing radiation has been studied for many years. It is accepted that DNA lesions (single- and double-strand breaks, for example) are induced by secondary species such as radicals, ions and the abundant low-energy secondary electrons generated by the primary radiation. Particularly harmful are dense ionization clusters of several ionization processes within a volume typical for the biomolecular system. Here we report the observation of a damage mechanism in the form of a non-local autoionizing process called intermolecular Coulombic decay (ICD). It directly involves DNA constituents or other organic molecules in an aqueous environment. The products are two energetic ions and three reactive secondary electrons that can cause further damage in their vicinity. Hydrogen-bonded complexes that consist of one tetrahydrofuran (THF) molecule—a surrogate of deoxyribose in the DNA backbone—and one water molecule are used as a model system. After electron impact ionization of the water molecule in the inner-valence shell the vacancy is filled by an outer-valence electron. The released energy is transferred across the hydrogen bridge and leads to ionization of the neighbouring THF molecule. This energy transfer from water to THF is faster than the otherwise occurring intermolecular proton transfer. The signature of the ICD reaction is identified in triple-coincidence measurements of both ions and one of the final state electrons. These results could improve the understanding of radiation damage in biological tissue.

Vibronic coupling and coherence are crucial in the charge and energy transfer of photoexcited molecules. Here we investigate the coupled electron-nuclear dynamics of the photoionized benzene molecule using the time-resolved Coulomb-explosion imaging method. A long-period oscillation is experimentally observed in the ion yields of the channel, as well as the C  + C , and the C  + C  + C Coulomb explosion channels. Quantum dynamics simulations reveal that this  ~600 fs oscillation, which notably exceeds the period of any vibrational modes, originates from pseudorotation of the benzene cation. This motion arises from quantum beating between two coherent vibronic states of the benzene molecule coupled via the Jahn-Teller effect around the conical intersection. The structural evolution of the benzene cation via pseudorotation is visualized by the time-resolved momentum imaging in the C  + C  + C three-body Coulomb explosion channel. Our work offers a comprehensive characterization of coherent vibronic dynamics of the benzene cation and demonstrates the power of the time-resolved Coulomb-explosion imaging for unraveling coupled electronic and nuclear motions in aromatic molecules.