Radu A. Miron

Accelerated molecular dynamics with the bond-boost method

We present a new method for accelerating molecular-dynamics simulations of infrequent events. The method targets simulation of systems that spend most of the time in local energy minima, with slow transitions in between, as is the case with low-temperature surface diffusion. The potential-energy surface is modified by adding a boost potential in regions close to the local minima, such that all transition rates are increased while relative rates are preserved. The boost potential is an empirical function determined by the deviation of the bond lengths of a specified set of atoms from equilibrium. The method requires no previous knowledge of the processes involved and it can be applied to a wide variety of interaction potentials. Application to the diffusion of Cu atoms on the Cu(100) surface using an embedded-atom potential yields correct rates for adatom hopping, exchange, as well as vacancy and dimer diffusion with speed-ups up to several orders of magnitude.

Triplet states of rubidium dimers on helium nanodroplets

Rubidium dimers in their 1 3Σu+ states are formed through collisions of Rb atoms that have been deposited on the surface of helium nanodroplets. Visible absorption spectra between 550 and 690 nm were probed by laser induced fluorescence and emission spectra measured for selected excitation wavelengths. A system absorbing around 595 nm with its emission to the ground state centered at 604 nm is identified as the Rb2 2 3Πg–1 3Σu+ transition. A broad unstructured band is measured near 667 nm. Following its excitation, two fluorescence channels are detected, one representing the 1 1Πu–1 1Σg+ (Rb2 B–X) transition and the other leading to atomic Rb D1 and D2 emission. Various explanations of this observation are discussed, each of which requires the presence of a third rubidium atom on the droplet. All spectra have been modeled using energy potentials from previous theoretical work and the results are compared. Relaxation after laser excitation experiences various bottlenecks, which show up in the vibrational e...

The Step and Slide method for finding saddle points on multidimensional potential surfaces

We present the Step and Slide method for finding saddle points between two potential-energy minima. The method is applicable when both initial and final states are known. The potential-energy surface is probed by two replicas of the system that converge to the saddle point by following isoenergetic surfaces. The value of the transition-state potential is bracketed in the process, such that a convergence criterion based on the potential can be used. We applied the method to study diffusion mechanisms of a small Ag cluster on a Ag(111) surface using an embedded-atom method potential. Our approach is comparable in efficiency to other commonly used methods.