Fragmentation of KrN+ clusters after electron impact ionization. Short-time dynamics simulations and approximate multi-scale treatment.

Post-ionization fragmentation of small ionic krypton clusters, KrN+ (N = 3-13), has been investigated using a semiclassical non-adiabatic dynamics approach consisting of classical treatment of atomic nuclei and full quantum treatment of electrons, and an extended diatomics-in-molecules model including the spin-orbit coupling as well as leading three-body interaction corrections. Electronic quantum decoherence has also been considered via a simplified scheme proposed previously. The positive charge has been initially localized on a randomly selected atom in the form of a localized 2P1/2 positive hole. It follows from the calculations that the data are not converged at timescales usually considered in dynamical calculations (t = 200 ps in this work) and that an extension to t ≈ 1 µs is needed. An approximate multi-scale treatment developed recently has been used to provide such an extension of the output of dynamical calculations. A qualitative agreement with available experimental data has been achieved, in particular, the experimental observation that the monomer fragment, Kr+, completely dominates has been reproduced. Interestingly, stabilized neutral dimer and trimer fragments have been observed in our calculations at non-negligible abundances despite extremely weak bonding in these species.

Fragmentation of KrN+ clusters after electron impact ionization II. Long-time dynamics simulations of Kr7+ evolution and the role of initial electronic excitation.

Long time simulations, up to 100 ns, have been performed for the fragmentation of Kr7+ clusters after electron impact ionization. They rely on DIM approaches and hybrid non-adiabatic dynamics combining mean field and decoherence driven either by Tully fewest switches (TFS) algorithm or through electronic amplitude (AMP) calculations. With both methods, for the first time, when the initial electronic excited state belongs to group II correlating to P1/2 atomic ions, the fragmentation ratio in mainly monomer and dimer ions agrees very well with known experimental results. A complex non-adiabatic dynamics is found where initial neutral monomer evaporations due to gradual deexcitation over electronic states of group II are followed by a non-adiabatic transition across a wide energy gap of the spin-orbit origin to electronic states of group I. The resulting excess of kinetic energy causes the final fragmentation of charged intermediate fragments to stable ionic monomers or dimers. Characteristic times of these processes have been estimated. The kinetic energy distribution of the neutral and ionic monomers (the dominating final fragments) has been analyzed in detail. Interestingly they exhibit some signature of the initial excited electronic state which could allow for an experimental identification.

Photodissociation of medium-sized argon cluster cations in the visible region.

Semiclassical methods for non-adiabatic dynamics simulations, based on a semiempirical diatomics-in-molecules model of intracluster interactions and the mean-field dynamical approach with the inclusion of quantum decoherence, have been used to study the photodissociation of argon cluster cations, Ar(N)(+)(N = 6-19), at E(phot) = 2.35 eV. Time periods upto t = 200 ps have been considered and abundance of ionic and neutral fragments, their time evolution and stability have been investigated and compared with available experimental data as well as earlier theoretical studies. A good agreement has been achieved between our predictions and the experimental data and deviations from earlier dynamical calculations are discussed.

First principles transport coefficients and reaction rates of Ar2(+) ions in argon for cold plasma jet modeling.

Momentum-transfer collision cross-sections and integral collision cross-sections for the collision-induced dissociation are calculated for collisions of ionized argon dimers with argon atoms using a nonadiabatic semiclassical method with the electronic Hamiltonian calculated on the fly via a diatomics-in-molecules semiempirical model as well as inverse-method modeling based on simple isotropic rigid-core potential. The collision cross-sections are then used in an optimized Monte Carlo code for evaluations of the Ar 2 (+) mobility in argon gas, longitudinal diffusion coefficient, and collision-induced dissociation rates. A thorough comparison of various theoretical calculations as well as with available experimental data on the Ar 2 (+) mobility and collision cross-sections is performed. Good agreement is found between both theoretical approaches and the experiment. Analysis of the role of inelastic processes in Ar 2 (+)/Ar collisions is also provided.

On the competition between linear and perpendicular isomers in photodynamics of cationic argon trimers.

Photoabsorption and subsequent photodissociation of two structural isomers of Ar(3) (+) are studied via semiclassical non-adiabatic dynamics simulations. Several experimental observables are simulated under various plausible experimental conditions with the main emphasis on the differences between the data produced for the two isomers. They include photoabsorption cross section, total kinetic energy released, fragments kinetic energy distributions, and distribution of the total kinetic energy among photofragments represented via Dalitz plots. The ability of the parameters to discriminate between the two isomers is analyzed through a thorough comparison with available experimental data. We show that the recently recorded experimental Dalitz plots [V. Lepère, Y. J. Picard, M. Barat, J. A. Fayeton, B. Lucas, and K. Béroff, J. Chem. Phys. 130, 194301 (2009)] correspond to a hot mixture of distorted linear-like and perpendicular-like structures where linear-like structures prevail.