Revealing the Role of N Heteroatoms in Noncovalent Aromatic Interactions by Ultrafast Intermolecular Coulombic Decay.

Despite the widely recognized importance of noncovalent interactions involving aromatic rings in many fields, our understanding of the underlying forces and structural patterns, especially the impact of heteroaromaticity, is still incomplete. Here, we investigate the relaxation processes that follow inner-valence ionization in a range of molecular dimers involving various combinations of benzene, pyridine, and pyrimidine, which initiate an ultrafast intermolecular Coulombic decay process. Multiparticle coincidence momentum spectroscopy, combined with ab initio calculations, enables us to explore the principal orientations of these fundamental dimers and, thus, to elucidate the influence of N heteroatoms on the relative preference of the aromatic π-stacking, H-bonding, and CH-π interactions and their dependence on the number of nitrogen atoms in the rings. Our studies reveal a sensitive tool for the structural imaging of molecular complexes and provide a more complete understanding of the effects of N heteroatoms on the noncovalent aromatic interactions at the molecular level.

Intermolecular Charge Transfer Induced Fragmentation of Formic Acid Dimers.

We investigate the intermolecular nonradiative charge transfer process in a double hydrogen-bonded formic acid (FA) dimer, initiated by electron-collision induced double ionization of one FA molecule. Through fragment ions and electron coincident momentum measurements and ab initio calculations, we obtain direct evidence that electron transfer from the neighboring FA molecule to fill one of the two vacancies occurs by a potential energy curve crossing of FA^{++}+FA with FA^{+}+FA^{+*} curves, forming an electronic excited state of dicationic dimers. This process causes the breaking of two hydrogen bonds and subsequently the cleavage of C─H and C─O covalent bonds in the dimers, which is expected to be a general phenomenon occurring in molecular complexes and can have important implications for radiation damage to biological matter.

Structural and dynamical studies of CH-πbonded CH4-C6H6dimer by ultrafast intermolecular Coulombic decay.

The inner-valence ionization and fragmentation dynamics of CH4-C6H6dimer induced by 200 eV electron impact is studied utilizing a multi-particle coincidence momentum spectroscopy. The three-dimensional momentum vectors and kinetic energy release (KER) of the CH4++C6H6+ion pairs are obtained by coincident momentum measurement. Our analysis on the absolute cross sections indicates that the intermediate dication CH4+-C6H6+is preferentially produced by the removal of an inner-valence electron from CH4or C6H6and subsequent relaxation of ultrafast intermolecular Coulombic decay followed by two-body Coulomb explosion. Combining withab initiomolecular dynamics (AIMD) simulations, the real-time fragmentation dynamics including translational, vibrational and rotational motions are presented as a function of propagation time. The revealed fragmentation dynamics are expected to have a potential implication for crystal structure imaging with various radiation sources.

Ultrafast Charge and Proton Transfer in Doubly Ionized Ammonia Dimers.

We investigate the ultrafast energy and charge transfer processes between ammonia molecules following ionization reactions initiated by electron impact. Exploring ionization-induced processes in molecular clusters provides us with a detailed insight into the dynamics using experiments in the energy domain. We ionize the ammonia dimer with 200 eV electrons and apply the fragment ions coincident momentum spectroscopy and nonadiabatic molecular dynamics simulations. We identify two mechanisms leading to the doubly charged ammonia dimer. In the first one, a single molecule is ionized. This initiates an ultrafast proton transfer process, leading to the formation of the NH2+ + NH4+ pair. Alternatively, a dimer with a delocalized charge is formed dominantly via the intermolecular Coulombic decay, forming the NH3+·NH3+ dication. This dication further dissociates into two NH3+ cations. The ab initio calculations have reproduced the measured kinetic energy release of the ion pairs and revealed the dynamical processes following the double ionization.

Triple ionization and fragmentation of benzene trimers following ultrafast intermolecular Coulombic decay.

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[Formula: see text]C[Formula: see text]C[Formula: see text] 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.

Concerted Double Hydrogen-Bond Breaking by Intermolecular Coulombic Decay in the Formic Acid Dimer.

Hydrogen bonds are ubiquitous in nature and of fundamental importance to the chemical and physical properties of molecular systems in the condensed phase. Nevertheless, our understanding of the structural and dynamical properties of hydrogen-bonded complexes in particular in electronic excited states remains very incomplete. Here, by using formic acid (FA) dimer as a prototype of DNA base pair, we investigate the ultrafast decay process initiated by removal of an electron from the inner-valence shell of the molecule upon electron-beam irradiation. Through fragment-ion and electron coincident momentum measurements and ab initio calculations, we find that de-excitation of an outer-valence electron at the same site can initiate ultrafast energy transfer to the neighboring molecule, which is in turn ionized through the emission of low-energy electrons. Our study reveals a concerted breaking of double hydrogen-bond in the dimer initiated by the ultrafast molecular rotations of two FA+ cations following this nonlocal decay mechanism.

Two-body dissociation of C3H4 isomers investigated by 50 keV/u Ne8+ impact.

The fragmentation of two isomers of C3H4, propyne (CH3CCH) and allene (CH2CCH2), is investigated by 50 keV/u Ne8+ impact. Obvious isomer effects are observed by comparing the time-of-flight spectra generated from the two isomers. Six two-body fragmentation channels of C3H4 2+ dications are identified for each isomer. CH2 + + C2H2 + is found to be the most favored CC bond breaking channel for both isomers, indicating that CH3CCH2+ intends to rearrange to the structure containing the CH2 group before fragmentation. For CH bond breaking channels, it is found that the CH3CCH which contains a CH3 group is more efficient for H2 + and H3 + ejection. In addition, two-body dissociation channels of C3H4 3+ trications are identified. While the H+ + C3H3 2+ channel is observed in the fragmentation of both isomers, the H2 + + C3H2 2+ channel only occurs in the fragmentation of CH3CCH3+. For CH2CCH2 3+, the peak and shoulder structures in the kinetic energy release spectrum of the H+ + C3H3 2+ channel are attributed to different geometries of the C3H3 2+ product.