Coulomb explosion of simple metal clusters in intense laser fields

We present a molecular dynamics model giving a classical description of valence and core electrons in Na clusters irradiated by intense laser pulses. Both core electrons and valence electrons are treated at the same level as classical Coulomb interacting particles. A minimum of quantum information is embodied by giving to each electron a finite width adjusted to the energies and radii of the related atomic shells. This model provides a surprisingly stable cluster ground state and a fair description of optical linear response. We focus on electron emission under laser irradiation in the intermediate and high-intensity regime. With increasing laser intensity, we observe a clear threshold for core ionization at a critical intensity of ${10}^{16}\mathrm{W}∕{\mathrm{cm}}^2$. Excitation at the critical intensity produces a fragmentation of the cluster into single ions with an average charge state of $+2$ and a kinetic energy distribution, which clearly indicates a Coulomb explosion.

Figure 1

Number of emitted electrons (full line) vs laser intensity for excitation of ${{\mathrm{Na}}_{41}}^{+}$ with a laser of frequency $\omega =2.5\mathrm{eV}$ and a ${\cos }^4$ pulse with full width at half maximum (FWHM) of $20\mathrm{fs}$. The dotted line represents the number of valence electrons in the ground state.

Figure 2

Example of time evolution of ionization of the Na atoms after excitation of ${{\mathrm{Na}}_{41}}^{+}$ with a laser of frequency $\omega =2.5\mathrm{eV}$, an intensity $I={10}^{16}\mathrm{W}∕{\mathrm{cm}}^2$, and a ${\cos }^4$ pulse with FWHM of $20\mathrm{fs}$. The ionization stage is labeled by line type as indicated. The inset shows the final ionization distribution.

Figure 3

Kinetic energy distributions for the different Na ions after excitation of ${{\mathrm{Na}}_{41}}^{+}$ with a laser of frequency $\omega =2.5\mathrm{eV}$, intensity $I={10}^{16}\mathrm{W}∕{\mathrm{cm}}^2$, and ${\cos }^4$ pulse with FWHM of $20\mathrm{fs}$.