Non-adiabatic dynamics combining Ehrenfest, decoherence, and multiscale approaches applied to ionic rare-gas clusters photodissociation, post-ionization fragmentation, and collisions

Ternary recombination of excited Ar+(P1/22) ions, established experimental results reinterpreted with a new extended model

For many years, the recombination of excited ions of argon, Ar+(P1/22), has been assumed negligible under ambient conditions as compared to the recombination of ground-state ions, Ar+(P3/22). This opinion was confronted with detailed experimental results that seem to clearly support it. Here, we propose a new interpretation in light of our recent calculations, which shows that the recombination efficiency is comparable for both fine-structure states. Noteworthily, in our model leading to a picture consistent with the experiment, residual dimer ions emerge from Ar+(P1/22) due to non-adiabatic dynamics effects and interplay in measured data.

Rotational Transitions in the N2 + Ion Induced by Collisions with Helium Atoms in Cold Helium Plasmas. A Quasiclassical Trajectory Study

Cross-sections of state-to-state rotational transitions in electronically ground-state 14N 2 + ${{\rm{N}}_2^ + }$ (X2Σ g + ${{\Sigma }_{g}^{+}}$ ) ions induced by collisions with 4 He atoms have been calculated using a quasiclassical trajectory method and a set of artificial neural networks representing theN 2 + ${{\rm{N}}_2^ + }$ /He potential energy surface. The training points for the neural networks have been calculated at a MCSCF (multi-configuration self-consistent field)/aug-cc-pVQZ level. A broad range of theN 2 + ${{\rm{N}}_2^ + }$ /He collision energy has been considered (E c o l l ≤ 100 ${{E}_{{\rm c}{\rm o}{\rm l}{\rm l}}\le 100}$  eV) and the efficiency of vibrational transitions in theN 2 + ${{\rm{N}}_2^ + }$ ion has also been analyzed. It has been found that vibrational transitions are negligible with respect to rotational transitions up toE c o l l ≈ 10 ${{E}_{{\rm c}{\rm o}{\rm l}{\rm l}}\approx 10}$  eV and that above this energy, both rotational and vibrational transitions inN 2 + ${{\rm{N}}_2^ + }$ are marginal in theN 2 + ${{\rm{N}}_2^ + }$ /He collisions.

Electronic effects in the dissociative ionisation of pyrene clusters

We present a combined experimental and theoretical study on the dissociative ionisation of clusters of pyrene. We measured the experimental appearance energies in the photon energy range 7.2-12.0 eV of the fragments formed from neutral monomer loss for clusters up to the hexamer. The results obtained show a deviation from statistical dissociation. From electronic structure calculations, we suggest that the role of excited states must be considered in the interpretation of experimental results, even in these relatively large systems. Non-statistical effects in the dissociative ionization process of polycyclic aromatic hydrocarbon (PAH) clusters may have an impact on the assessment of mechanisms determining the stability of these clusters in astrophysical environments.

Electron diffraction as a structure tool for charged and neutral nanoclusters formed in superfluid helium droplets

This perspective presents the current status and future directions in using electron diffraction to determine the structures of clusters formed in superfluid helium droplets. The details of the experimental setup and data treatment procedures are explained, and several examples are illustrated. The ease of forming atomic and molecular clusters has been recognized since the invention of superfluid helium droplet beams. To resolve atomic structures from clusters formed in droplets, substantial efforts have been devoted to minimizing the contribution of helium to diffraction signals. With active background subtraction, we have obtained structures from clusters containing a few to more than 10 monomers, with and without heavy atoms to assist with the diffraction intensity, for both neutral and ionic species. From fittings of the diffraction profiles using model structures, we have observed that some small clusters adopt the structures of the corresponding solid sample, even for dimers such as iodine and pyrene, while others require trimers or tetramers to reach the structural motif of bulk solids, and smaller clusters such as CS2 dimers adopt gas phase structures. Cationic clusters of argon clusters contain an Ar3+ core, while pyrene dimers demonstrate a change in the intermolecular distance, from 3.5 Å for neutral dimers to 3.0 Å for cations. Future improvements in reducing the background of helium, and in expanding the information content of electron diffraction such as detection of charge distributions, are also discussed.

Thermodynamics of small mercury clusters and the role of electronically excited states. A case study on Hg13

Classical Monte Carlo simulations in the isothermal-isobaric ensemble have been performed for the Hg13 cluster with the main emphasis paid to structural changes in this cluster induced by elevated temperature...

Electron Diffraction of Ionic Argon Nanoclusters Embedded in Superfluid Helium Droplets

We report electron diffraction of cationic argon nanoclusters embedded in superfluid helium droplets. Superfluid helium droplets are first doped with neutral argon atoms to form nanoclusters, and then the doped droplets are ionized by electrons. The much lower ionization energy of argon ensures that the positive charge resides on the Ar nanocluster. Using different stagnation temperatures and therefore droplets with different sizes, we have been able to preferentially form a small ionic cluster containing 2-4 Ar atoms and a larger cluster containing 7-11 atoms. The fitting results of the diffraction profiles agree with structures reported from theoretical calculations, containing a cationic trimer core with the remaining atoms largely neutral. This work testifies to the feasibility of performing electron diffraction from ionic species embedded in superfluid helium droplets, dispelling the concern over the particle density in the diffraction region. However, the large number of neutral helium atoms surrounding the cationic nanoclusters poses a challenge for the detection of the helium solvation layer, and the detection of which awaits further technological improvements.