Emission of ions and charged clusters due to impulsive Coulomb explosion in ultrafast laser ablation of graphite

The dynamics of ultrafast laser ablation of graphite have been investigated by femtosecond time resolved autocorrelation measurements at $800\mathrm{nm}$. The results indicate that multiply charged atomic ions and ionized carbon clusters are repelled by a transient electric field developed on the surface during the laser pulse, the process known as impulsive Coulomb explosion. From time resolved measurements, we estimate that the decay time constant of the surface field is $125\pm 10\mathrm{fs}$.

Figure 1

Velocity distribution spectra of $C_1^{+}$, $C_1^{2+}$, and $C_1^{3+}$ ions obtained from pump-probe measurements on graphite at zero delay time, $\Delta \tau =0\mathrm{fs}$, between the two nearly identical pulses of the duration of $100\mathrm{fs}$ and the fluence of $100\mathrm{mJ}∕{\mathrm{cm}}^2$. The arrows indicate the mean velocities of the species.

Figure 2

(Color online) Atomic ion emission measurements on graphite as a function of pump-probe delay time. (a) Normalized yield of $C_1^{+}$, $C_1^{2+}$, and $C_1^{3+}$ as a function of the delay between two nearly identical pulses of the duration of $100\mathrm{fs}$ and the fluence of $100\mathrm{mJ}∕{\mathrm{cm}}^2$. $C_1^{+}$ is represented by the squares and dot-dashed line, $C_1^{2+}$ by the circles and dashed line, and $C_1^{3+}$ by the triangles and dotted line. The solid line is the reconstructed autocorrelation signal for $C_1^{3+}$ obtained via the iterative convolution method. (b) Experimentally derived mean momentum ratio for (i) $C_1^{2+}∕C_1^{+}$, (ii) $C_1^{3+}∕C_1^{+}$, and (iii) $C_1^{3+}∕C_1^{2+}$ as a function of delay time. The horizontal dotted lines represent the theoretical limit of the momentum ratio predicted by the impulsive Coulomb explosion model.

Figure 3

Experimentally determined first derivative of the mean velocity with respect to fragment mass, $dv∕dm$, above ion emission threshold laser fluence, $F=171\mathrm{mJ}∕{\mathrm{cm}}^2$. The inset shows the experimental mean velocity as a function of cluster order $n$. The solid line is the behavior expected when all clusters have the same momentum; the dashed line is the behavior expected when all clusters have the same kinetic energy (see text).