Massively Parallel Ionization of Extended Atomic Systems

Massively parallel ionization of many atoms in a cluster or biomolecule is identified as a new phenomenon of light-matter interaction which becomes feasible through short and intense FEL pulses. Almost simultaneously emitted from the illuminated target the photo-electrons can have such a high density that they interact substantially even after photoionization. This interaction results in a characteristic electron spectrum which can be interpreted as a convolution of a mean-field electron dynamics and binary electron-electron collisions. We demonstrate that this universal spectrum can be obtained analytically by summing synthetic two-body Coulomb collision events. Moreover, we propose an experiment with hydrogen clusters to observe massively parallel ionization.

Figure 1

Sketch of $N$-fold photoionization of atoms in a cluster of radius $R$, leading to an ionic back ground potential (blue) with total charge $N$ and depth $V_0=3N/2R$ cf. Eq. (1).

Figure 2

Photo-electron spectrum for sudden massively parallel ionization. (a) ${\mathrm{Ar}}_{147}$ with icosahedral geometry. Two thirds of the atoms lose an electron from the $3p$ level with an excess energy of $E^{*}=0.4\mathrm{keV}$. (b) Coulomb complex with $N=100$ and an excess energy of ${\epsilon }^{*}=2$. The dashed lines represent the analytical mean-field result from Eq. (3). The full widths at half maximum of both distributions, denoted by $\Delta E$ and $\Delta \epsilon$ respectively, are indicated by the thick gray arrows.

Figure 3

(a) Final electron spectra for $N={10}^2$ electrons propagated with $W_{\mathrm{short}}$ (blue, solid) and $W_{\mathrm{long}}$ (red, dashed) potential. (b) Final spectrum from propagation with Coulomb potential (green, solid) and convolution of Yukawa and anti-Yukawa spectra (black, dashed) according to Eq. (5). Dotted vertical lines indicate the excess energy ${\epsilon }^{*}=2$.

Figure 4

Photo-electron spectra for propagated Coulomb complex (solid green/gray line) and BIS approximation (dashed black) according to Eq. (6) for $N=10$ (upper row), ${10}^2$ (middle) and ${10}^3$ (lower) on a linear (left column) and a logarithmic (right) scale, respectively. The gray arrows indicate the theoretical value of $\Delta \epsilon =3/4$ according to Eq. (4) for the full width at half maximum of the mean-field distribution shown by thin solid lines.

Figure 5

Photo-electron spectrum from a 2 nm ${\mathrm{H}}_2$ cluster exposed to a pulse with 75 eV photon energy, corresponding to $E^{*}=60\mathrm{eV}$ (dotted vertical line). The pulse had an intensity of $2.5\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ and a duration of 500 as. The inset shows the same spectrum on logarithmic scale.