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The valence ionization of uracil and mixed water–uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure–property relationships and (ii) the coexistence of pure and mixed clusters in the water–uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core–shell structures. [ABSTRACT FROM AUTHOR]

We have investigated the effectiveness of molecular hydrogen (H2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H]+ peak intensities in the mass spectra may not be used for estimating H2-formation rates. [ABSTRACT FROM AUTHOR]

We present scaling laws for absolute cross sections for non-statistical fragmentation in collisions between Polycyclic Aromatic Hydrocarbons (PAH/PAH+) and hydrogen or helium atoms with kinetic energies ranging from 50 eV to 10 keV. Further, we calculate the total fragmentation cross sections (including statistical fragmentation) for 110 eV PAH/PAH+ + He collisions, and show that they compare well with experimental results. We demonstrate that non-statistical fragmentation becomes dominant for large PAHs and that it yields highly reactive fragments forming strong covalent bonds with atoms (H and N) and molecules (C6H5). Thus nonstatistical fragmentation may be an effective initial step in the formation of, e.g., Polycyclic Aromatic Nitrogen Heterocycles (PANHs). This relates to recent discussions on the evolution of PAHNs in space and the reactivities of defect graphene structures. [ABSTRACT FROM AUTHOR]

We report on measurements of the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) targets in Xe20 + + C16H10 and Xe20 + + [C16H10]k collisions and compare results for the two C16H10 isomers: pyrene and fluoranthene. For both types of targets, i.e., for single PAH molecules isolated in vacuum or for isomerically pure clusters of one of the molecules, the resulting fragment spectra are surprisingly similar. However, we do observe weak but significant isomer effects. Although these are manifested in very different ways for the monomer and cluster targets, they both have at their roots small differences (<2.5 eV) between the total binding energies of neutral, and singly and multiply charged pyrene and fluoranthene monomers. The results will be discussed in view of the density functional theory calculations of ionization and dissociation energies for fluoranthene and pyrene. A simple classical over-the-barrier model is used to estimate cross sections for single- and multiple-electron transfer between PAHs and ions. Calculated single and multiple ionization energies, and the corresponding model PAH ionization cross sections, are given. [ABSTRACT FROM AUTHOR]

Electron-scattering cross sections have been determined for trimethylene oxide, cyclic (CH2)3O molecule, both experimentally and theoretically. The absolute total cross section (TCS) has been measured over energies from 1 to 400 eV using a linear electron-transmission method. The obtained TCS generally decreases with rising energy, except for the 3–10 eV range, where some resonantlike structures are discernible. Integral elastic cross section (ECS) and ionization cross section (ICS) have been also calculated up to 3 keV in the additivity rule approximation and the binary-encounter-Bethe approach, respectively. Their sum, ECS+ICS, is in a good agreement with the measured TCS. Comparison of the TCS energy dependence for trimethylene oxide with that for its isomeric open-chain counterpart—acetone, (CH3)2CO, has also been made. Moreover, examination of experimental TCSs for the cyclic (CH2)nO, n=2–4, ether series reveals that the intermediate-energy molecular TCSs for members of that family can be nicely represented as a sum of the effective TCSs for particular constituents of the molecule, i.e., methylene groups and oxygen atom. Finally, based on these partial TCSs, the TCS for the c-(CH2)5O—the next member of the series—has been determined and compared with the respective ECS+ICS values computed here for this compound. [ABSTRACT FROM AUTHOR]

Absolute total cross sections (TCSs) for electron scattering from boron trifluoride (BF3) and phosphorus trifluoride (PF3) molecules have been measured using a linear transmission method. The electron energy ranges from 0.6 to 370 eV for BF3 and from 0.5 to 370 eV for PF3. The TCS energy dependence for BF3 exhibits two very pronounced enhancements: resonantlike narrow feature located near 3.6 eV with the maximum value of 19.2×10-20 m2, and intermediate energy very broad enhancement with two humps, one centered around 21 eV (18.8×10-20 m2 in the maximum) and the other near 45 eV (19.5×10-20 m2). For PF3 the TCS has quite different low-energy dependence: at 0.5 eV it has a high value of 70×10-20 m2 and decreases steeply towards higher energies. Beyond the minimum near 5.5 eV, the TCS reveals two distinct humps: the resonant one centered near 11 eV with the peak value of 32.9×10-20 m2 and the second one much broader around 35 eV (27.9×10-20 m2). The present TCSs for trifluorides are compared to each other as well as to previous TCS data for selected perfluorides and to results for their perhydrided counterparts. The differences and similarities in the shape and magnitude of TCSs are pointed out. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C60 molecules following collisions with Ar2 +, He2 +, and Xe20 + at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C60]n+ → C60++(n-1)C60 evaporation model. Excitation energies in the range of only ∼0.7 eV per C60 molecule in a [C60]13+ cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar2 + and He2 + collisions, we observe very efficient C119+ and C118+ formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C59+ or C58+ and C60 during cluster fragmentation. In the Ar2 + case, it is possible to form even smaller C120-2m+ molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe20 + collisions. [ABSTRACT FROM AUTHOR]