Multiscale approach combining nonadiabatic dynamics with long-time radiative and non-radiative decay: Dissociative ionization of heavy rare-gas tetramers revisited

Abstract

A multiscale approach is proposed to address short-time nonadiabatic dynamics and long-time decay. We show the role of both radiative and non-radiative processes in cluster decay mechanisms on examples of rare-gas cluster fragmentation after electron impact ionization. Nonadiabatic molecular dynamics is used as an efficient tool for theoretical study on femto- and picosecond scales and a multiscale approach based on kinetic rates of radiative as well as non-radiative transitions, both considered as parallel reaction channels, is used for the analysis of the long-time system relaxation spanning times over microseconds to infinity. While the radiative processes are typically slow, the system relaxation through non-radiative electronic transitions connected with electron-nuclear interchange of energy may, on the other hand, significantly vary in kinetic rates according to kinetic couplings between relevant adiabatic states. While the predictions of picosecond molecular dynamics themselves fail, the results of the multiscale model for the electron-impact post-ionization fragmentation of krypton and xenon tetramers are in agreement with experiment, namely, in leading to the conclusion that charged monomers prevail. More specifically, on microsecond and longer scales, mainly slow radiative processes are substantial for krypton cluster decay, while for xenon the radiative and slow non-radiative processes compete. In general, the role of slow decay processes through non-radiative transitions is comparable with the role of radiative decay mechanism. The novel multiscale model substantially improves theoretical predictions for the xenon tetramer decay and also further improves the good agreement between theory and experiment we reached previously for krypton.

On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

“…54,55 Very recently, Janecek et al have proposed a possible origin of the difference. 39 Their multiscale approach is complex; it involves spin–orbit coupling and different time scales to account for initial, nonadiabatic processes as well as radiative and nonradiative transitions between electronically excited states that occur on a microsecond time scale. Metastable dissociation of excited Ng dimer ions has been shown experimentally to occur on this time scale, 76−78 with profound differences between light (Ne, Ar) and heavy Ng’s.…”

“…Striking differences are found between the abundance distributions of He n Ng + for the heavy Ng’s (Kr, Xe) and He n Ar + ; a parallel to the different fragmentation dynamics of these systems is tentatively drawn. 38,39 We also critically evaluate previously published mass spectra and include recent results for He n Ne x + . 32 So far, few quantum mechanical treatments of He n Ng x + have been reported; the nature of the ionic core, charge delocalization, vibrational delocalization, and other quantum mechanical effects pose significant challenges.…”

Fragmentation dynamics of Ar4He1000 upon electron impact ionization: Competition between ion ejection and trapping

“…Other explanations could be the occurrence of double ionization 9,76 or possible secondary ionization of neutral Ar fragments. 19 Radiative relaxation on a longer (microsecond) time scale explored for heavier rare gases by Janeček et al 77 would not apply here since electronic relaxation is complete at the end of the nonadiabatic step of the simulation.…”

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.…”

“…Further on, clusters remaining in excited electronic states for extremely long lifetimes faced the theoretical treatment to another computational limitation. As an approximate solution, we proposed a multiscale treatment [20,21] including radiative and non-radiative transitions.…”