Dynamics of charge clouds ejected from laser-induced warm dense gold nanofilms

We report a systematic study of the ejected charge dynamics surrounding laser-produced 30-nm warm dense gold films using single-shot femtosecond electron shadow imaging and deflectometry. The results reveal a two-step dynamical process of the ejected electrons under high pump fluence conditions: an initial emission and accumulation of a large amount of electrons near the pumped surface region, followed by the formation of hemispherical clouds of electrons on both sides of the film, which escape into the vacuum at a nearly isotropic and constant velocity with an unusually high kinetic energy of more than 300 eV. We also developed a model of the escaping charge distribution that not only reproduces the main features of the observed charge expansion dynamics but also allows us to extract the number of ejected electrons remaining in the cloud.

Figure 1

Schematic of ultrafast electron shadow imaging and electron deflectometry. The pump laser strikes the sample from the right side, and the vertical white line marks the original film position.

Figure 2

The radii of the hemispherical shadows at the front and rear sides as a function of time delays. The solid red line and dashed blue line are linear fits with slope $(1.3\pm 0.2)\times {10}^7$ m/s and $(5.8\pm 0.3)\times {10}^6$ m/s for the front and rear shadow sizes, respectively.

Figure 3

Deflection curves at different impact parameters (a) at the front and (b) at the rear sides of 30-nm gold film. The uncertainty of any given impact parameter is about 9 $\mu \mathrm{m}$. Inset: Charge cap size vs time delay.

Figure 4

Rescaled front deflection curve. Inset: The amount of ejected charge at the front surface as a function of absorbed fluences.