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

Abstract

Post-ionization fragmentation of small ionic krypton clusters, Kr (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 P 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.

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

“…Note also that alternative names like hemiquantal method [14,16,22,23,24,25,26,27] and semiclassical method [19,28,29,20,30,21,31] were also used in our previous studies.…”

“…approach, it is typically about one attempt per ∆t Q ≈ 10 − 100 fs, while for the MFQ-TFS method, it is usually by an order of magnitude shorter (an attempt per ∆t Q ≈ 1 − 10 fs) because the rejection rate is much higher (see, e.g., references 19,31). Note also that if a quenching attempt is performed each time step, the MFQ-TFS approach becomes very close to the Tully's Fewest Switches one.…”

“…2.5.1), with K fixed to the selected atom and m = x, y, z and s z = ±1/2. Since, in addition, we found from the energy considerations that a (four-dimensional) subspace of this atomic state space corresponding to the P 3/2 term cannot contribute to the production of ionic monomer fragments due to lack of energy, only the (twodimensional) orthogonal complement of the P 3/2 space (corresponding to P 1/2 ) was considered in later calculations [21,31].…”

Relaxation of electronic excitations in Kr₂⁺ ions in cold krypton plasma.

“…OPEN-13-2) and multiprocessing work stations of the parallel computational center of Toulouse. [22] As discussed elsewhere [21], radiative processes do not play an important role during the collision compared to non-radiative processes and are thus neglected here. Moreover, since under ambient conditions the time during which the dimer ion undergoes collisions is much shorter than the time it spends as a free molecule, the effect of radiative processes taking place during the collisions may, in addition, be also considered negligible with respect to the free-flight phase.…”

“…Two kinds of transition processes are considered in the present work, (non-radiative) collision-induced transitions and spontaneous radiative transitions in free ionic dimers. As discussed elsewhere [21], radiative processes do not play an important role during the collision compared to non-radiative processes and are thus neglected here [22]. Since, in addition, the probability of non-radiative transitions is very small in free Kr + 2 ions (exactly zero if the spin-orbit coupling is neglected), the non-radiative free-flight transitions are neither considered here.…”