Accurate modeling of sequential decay in clusters over long time scales: Insights from phase space theory

Abstract

A general theoretical framework for describing the thermally induced sequential decay in atomic clusters is presented. The scheme relies on a full treatment of individual dissociation steps based on phase space theory (PST), built into a kinetic Monte Carlo (kMC) procedure. This combined PST/kMC approach allows one to follow the evolution of several statistical properties such as the size, the angular momentum, or the temperature of the cluster over arbitrarily long time scales. Quantitative accuracy is achieved by incorporating anharmonicities of the vibrational densities of states, the rigorous conservation of angular momentum via the effective dissociation potential, and a proper calibration of the rate constants. The approach is tested and validated on selected Lennard-Jones clusters in various situations. Several approximations, including a mean-field rate equation treatment, are critically discussed; possible extensions are presented.

On possible simplifications in the theoretical description of gas phase atomic cluster dissociation

“…20 As discussed in Sec. I, modeling a cluster dissociation cascade by means of a set of MEs necessitates use of either P tr ͑E , ⑀ tr ͒ ͑the distribution of E tr as a function of E and J͒ or P t ͑E,⑀ t ͒ ͑the distribution of E t as a function of E if the J-dependency is neglected͒.…”

“…15, and references therein͒ investigated the possibility of computing both the KER distribution ͑i.e., the probability of E tr assuming a specific value ⑀͒ and the probability of the cluster fragment to have a total angular momentum JЈ by means of PST, obtaining results that strongly suggested this theory to be capable of accurately predicting both quantities. Estimating the reaction rate, however, was indicated as a more demanding task due to the necessity of computing accurately the cluster density of states ⍀͑E͒, 20 an issue previously discussed in Refs. 13 and 18.…”

“…Needless to say, the importance of P tr ͑E , ⑀͒ and P t ͑E , ⑀͒ stems from the necessity of defining the internal energy EЈ of the fragment cluster after a dissociation event to predict the rate of, e.g., a second monomer ejection, and it is therefore of prime interest in the description of multiple cluster decays as thoroughly discussed in Ref. 20.…”

“…13 and 20. In the latter case, the simulation temperature was chosen matching MD and PST results 20 despite the fact that V eff ͑r͒ should, in principle, be E-dependent and determined using microcanonical simulations.…”

Effect of the cluster angular momentum J and the projectile orbital momentum L on capture probability and postcollision dynamics

“…Notice, however, that the absolute magnitude of such contribution is very low compared with the internal energy of the clusters, so that the effect of the higher kinetic energy injected in the emitted particle on the physical properties of the remaining aggregate is expected to be small. 25 From Fig. 6, it is also apparent that a larger fraction of ⑀ p is left in the cluster upon dissociation in the case of high speed collisions.…”