Controlling the Coulomb Explosion of Silver Clusters by Femtosecond Dual-Pulse Laser Excitation

Silver clusters grown in helium nanodroplets are excited by intense femtosecond laser pulses resulting in the formation of a hot electron plasma far from equilibrium. The ultrafast dynamics is studied by applying optically delayed dual pulses, which allows us to pursue and control the coupling of the laser field to the clusters on a femtosecond time scale. A distinct influence of the optical delay on the ionization efficiency gives strong evidence that a significant contribution of collective dipolar electron motion is present, which is verified by corresponding Vlasov dynamics simulations on a model system. The microscopic approach demonstrates the outstanding role of giant resonances in clusters also in intense laser fields.

Figure 1

Fragment spectrum of silver clusters with a mean number of atoms $N=40$ in helium droplets exposed to ${\omega }_L=800\mathrm{n}\mathrm{m}$ (1.54 eV) fs laser radiation at $4\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The resulting ${\mathrm{A}\mathrm{g}}^{z+}$ signals are highlighted. In this example the charge distribution reaches up to $z=11$.

Figure 2

Normalized dual-pulse signals for ${\mathrm{A}\mathrm{g}}^{5+}$ from the Coulomb explosion of silver clusters in helium droplets, resulting from the excitation with 130 fs pulses. The autocorrelation function is given for comparison. Negative delays correspond to a reversal of the two pulses. The signal at higher intensity was shifted by an offset (see right axis). Data points are fitted using a function that is symmetric with respect to zero delay in order to account for identical laser pulses (solid lines). The peak positions shift to a shorter delay when a higher laser intensity is applied (see dashed vertical lines for comparison).

Figure 3

Response of ${\mathrm{N}\mathrm{a}}_{55}$ for dual-pulse excitation with $I=4\times {10}^{12}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$, $\hslash {\omega }_L=1.54\mathrm{e}\mathrm{V}$, and an optical delay of 250 fs. (a) The envelope of the laser field (gray) and the corresponding electron dipole amplitude, (b) the phase angle between the laser field and the dipole signal, (c) the total cluster ionization, and (d) the root-mean-square radius of the ion distribution. Note that the dipole phase angle passes $\pi /2$ as the rms radius is close to the critical value $R_c$ (dotted vertical line).

Figure 4

Calculated average ionic charge after dual-pulse excitation of the model system ${\mathrm{N}\mathrm{a}}_{55}$ as a function of the optical delay. Both pulses have a width of 50 fs at the intensities given in the figure. All ionization profiles show a strong delay dependence with a distinct peak. For overlapping pulses the constructive interference leads to a higher effective field strength which explains the signal enhancement at short delays for the lowest intensity.