On a relationship between the collective migration of surface atoms in microclusters and the saddle points on the potential energy surface

TL;DR

This paper investigates the potential energy surface of microclusters to understand atomic migration, revealing that surface atom movements involve low energy barriers, while deep interior rearrangements require high energy, with implications for surface diffusion.

Contribution

It provides a detailed analysis of saddle points on the PES of microclusters and links atomic motions to energy barriers, highlighting the role of surface atoms in cluster dynamics.

Findings

Surface atom gliding involves very low energy barriers.

Surface rearrangements with creation or annihilation of floaters have moderate barriers.

Deep interior atom rearrangements require high energy barriers.

Abstract

Plenty of saddles on a multidimensional potential energy surface(PES) of two-dimensional microclusters, where atoms are interacting via Morse potential, are numerically located. The reaction paths emanating from the two types of the local minima, which represent the compact and the non-compact shape of Morse clusters, to their neighboring saddles on PES are elucidated. By associating the reaction path crossing these saddles with the atomic rearrangements,we evaluate the barrier height corresponding to various characteristic atomic motion accompanied by the {\it floaters} (i.e. surface atoms popped out of the cluster surface). Our findings are summarized as: (i)The saddle points implying the {\it gliding motion} of a single {\it floater} over the cluster surface yields extremely small values of the energy barriers regardless of the shapes of clusters. In particular, the {\it gliding motion} of a train composed of a few surface atoms also appears as the low-lying saddles. As a result, the barrier height corresponding to the {\it simultaneous gliding motion}, which is a manifestation of the reaction path crossing the higher-index saddles on PES, is significantly low. (ii)A surface rearrangement, where {\it floaters} are created or annihilated, implies relatively high barrier energy which is still accessible below melting point. (iii)On the other hand, the atomic motion, where atoms located deep inside of the clusters are rearranged as well as surface atoms, yields extremely high barrier energies. Some relations between these results and the recent experimental study of the surface cluster diffusion are also pointed out.