Because the sequence of events that leads to laser ablation, which

may be defined as the collective ejection of matter (ions, atoms, clus-

ters, nanoparticles, etc.) following irradiation by ultrashort, intense

bursts of light, is formidably complex, the usual analytical tools of

theoretical physics are unable to account for the whole spectrum

of relevant processes taking place in the target, and thus cannot

provide a thorough understanding of the physical mechanisms

that underlie the phenomenon, as well as the physical nature of

the structural modifications inflicted to the system following the

absorption of energetic photons, notably in the so-called heat-

affected zone. To make the problem even more difficult, the process

takes place on an unusually wide range of length and timescales.

In view of these difficulties, computer simulations are, in spite of

their limitations, an excellent route to understanding the physics

of ablation [1-11], very nicely complementing experiment [12-14].

In particular, the numerical models developed by our group-

Perez et al. [5, 7-9] and Lorazo et al. [6, 10, 11]—have provided a comprehensive picture of the mechanisms that underlie ablation

in the thermal regime (as opposed to the non-thermal regime, where the physics is dominated by complex electronic effects

such as plasma formation and Coulomb explosion). It has been

demonstrated, in particular, that different routes are available for

ablation to occur, viz. spallation (ejection of fragments of material

following the passage of a tensile stress wave), phase explosion

(decomposition of a thermodynamically metastable homogeneous

liquid into a mixture of liquid droplets and gas), fragmentation

(disintegration of a homogeneous material into clusters under the

action of large strain rates) and vaporization (passage from the

solid or liquid to the gas phase), as a function of increasing fluence

[7]. Further, it has been demonstrated that, in the case of very

long (nanosecond) pulses in molecular solids, the way to ablation

is largely determined by the degree of local confinement, notably

depth into the target [9]. In contrast, the corresponding problem in

the presence of a liquid layer has remained largely unexplored, likely

because of the increased complexity of incorporating an additional

variable into an already extremely difficult problem. Nevertheless,

some progress has been realized using our simple two-dimensional

Lennard-Jones model [15].