Computer experiments on molecular ejection from an amorphous solid: Comparison to an analytic continuum mechanical model

Abstract

Molecular-dynamics simulations are performed to investigate the ejection into the gas phase of large molecules in an amorphous van der Waals solid due to a rapid expansion of a cylindrical "track" of material. Such an excitation geometry may be caused by a fast ion penetrating a solid. The ejection yield, the angular distribution of ejected particles, and the crater size are investigated as a function of the expansion rate and energy, sample thickness, and angle of incidence. Comparisons are made with results from an analytic continuum mechanical model which estimates the ejection from the transiently pressurized region. Although the model is described for point particles and is independent of excitation mechanism, it is able to describe many aspects of the simulation.

Direct Evidence for Projectile Charge-State Dependent Crater Formation Due to Fast Ions

“…Depending on the electronic heat and electron density, the plasma dynamics [6] or a gas flow out of the ion track at high atomic temperatures (as discussed also for macroscopic volcano eruptions [26]) may also be important. One may speculate that the inner part of the crater is shaped by short-ranged effects such as melting and evaporation in the thermal spike, whereas the ridge might be related to interactions of longer range such as a pressure pulse [22]. However, an influence of long range stress fields after rapid cooling of molten tracks (the so-called ion-hammering effect) cannot be excluded, as shown qualitatively for hillock formation in vitreous silica [24].…”

“…On purpose, however, we avoid using our experimental charge-state scalings as a test of the reliability of specific models, because different versions of the same type of model predict different scalings and different types of models (e.g. thermal or hydrodynamical) may predict the same scaling [22,27,28]. A quantitative modeling of the complex process of crater formation by fast atomic projectiles is far from being accomplished.…”

“…Intense pressure, sound or shock waves [22] may lead to a collective and correlated motion and possible rearrangement of all atoms close to the track. In addition, thermal-spike effects invoked by nonequilibrium potentials or by hot electrons [23,24] or the late phase of the Coulomb-explosion spike [23,25] (a mutual repulsion of target ions) may lead to a disordered atomic motion.…”

Confinement Effects of Ion Tracks in Ultrathin Polymer Films

“…In pure thermal models (as in the analytical thermal spike model), it is the energy density at the surface that controls particle ejection and cratering or hillock formation [7,19]. MD simulations have also shown the importance of coherent movement of a volume of atoms (pressure waves), in addition to local heating, at high excitation densities [20,21]. Such correlated momentum transfer seen on simulations and experiments with organic materials [6] has been analytically described by expansion models like the pressure pulse model [22].…”

“…The procedure to estimate the time-dependent total deposited energy density ϵðρ; z; t; hÞ from the addition of all point sources along the track is described in detail in Refs. [20,22] and outlined in the Supplemental Material [16]. An example of 2D maps of the normal component of the transferred momentum p z ðρ; z; hÞ for a thick and a thin film is given in Fig.…”