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

Abstract

Long time simulations, up to 100 ns, have been performed for the fragmentation of Kr 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 P 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.

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

“…To further verify the assumption that long times are needed to get converged dynamical simulations of the post-ionization fragmentation of cluster cations of heavy rare gases, a series of long-time calculations were performed [74] for the Kr + 7 ion (the largest parent size for which experimental data are available [68]) using an improved simulation protocol with periodical checks of the fragmentation state of the decaying ionic complex and with detected intermediate fragments treated separately (see Ref. [33] for more details).…”

“…In the theory, the DIM+SO+ID-ID(dmp) model has been used for small parents (N ≤ 5) and the DIM+SO+ID-ID(dmp)+N3(DDD) model have been applied to larger parents (N ≥ 6). resulting from the post-ionization fragmentation of the Kr + 7 ion initially excited to the highest electronic level (L21) as obtained [74] from the MFQ-TFS/S(sf) dynamical method and the DIM+SO+ID-ID(dmp)+N3(DDD) interaction model. Horizontal lines correspond to respective experimental values of monomer and dimer abundances as reported in reference [68].…”

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

“…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 6 . 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.…”

“…Alike, most of thedata needed to calculate therate constants of thecIST processes, equation (3), were reported in thetwo preceding studies and, here, they have been reanalyzed in amore detailed way. More specifically, final electronic states of thedimer (denoted by index β in equation (3)) have been considered explicitly for each 6 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.…”