Picosecond melting of ice by an infrared laser pulse: a simulation study.

Hexagonal ice (Ih) is the most common form of ice, consisting of water molecules with their oxygen atoms arranged in a tetrahedral lattice. Both in the crystal and in the liquid form, oxygen atoms are spaced by 0.275 nm, but the density of ice is 8% lower than that of water. This well-known property gives ice the ability to crack rocks or break a bottle of beer in the freezer. Each water molecule in ice forms four hydrogen bonds (two per molecule), and upon melting roughly 1.75 hydrogen bonds per water molecule are maintained. Ultrafast superheating and melting of bulk ice induced by infrared radiation was demonstrated recently using spectroscopy. Herein, we describe the process of ice-melting using computer simulations of molecular dynamics (MD). MD simulations are ideally suited to investigate processes like melting and freezing, as they allow us to simultaneously probe the structure and dynamics of the system under investigation at atomic resolution and on a femtosecond time scale. To describe the thermal melting of ice, ionization processes have to be avoided. In practice, this means that the amplitude of the pump laser should be low enough that multiphoton or field ionization processes are avoided, and the photon energy should be kept under the single-photon ionization threshold, that is, l> 140 nm. As a rough guideline for the laser intensity, we used Il< 10 W, where l is the wavelength of light and I is the power by area given by I= cE0 2 0/2, where c is the speed of light,E0 the maximum amplitude of the electric field, and 2 0 the permittivity of vacuum. Under these circumstances, the ionization probability is negligible. At room temperature the frequency of the molecular vibrations of water are comparable to kT, and in thermal equilibrium the molecules are primarily in the ground state. To consider non-equilibrium processes, such as vibration–relaxation, a quantum-mechanical description is desirable. However, accurate timedependent quantum calculations for large systems, such as the system studied herein, are not tractable. Therefore, in practice, a classical description augmented by quantum corrections is adopted for such calculations. The flexible water model used throughout this work (flexible TIP4P) includes a Morse potential for the OH bonds, an anharmonic coupling term between bond stretches, and a coupling between bond stretching and angle bending, which makes it suitable for simulating absorption of infrared radiation. The model has been shown to reproduce density, energy, and vibration spectra of liquid water. It also reproduces the red shift in the vibration spectra in liquid D2O by changing the weight of the hydrogen atoms. The model has also been employed to calculate the vibration lifetime of HDO in D2O. [8]