Ionization of atoms by dense and compact beams of extreme relativistic electrons

Abstract

Ionization is one of the basic physical processes, occurring when charged particles penetrate atomic matter. When atoms are bombarded by very dense and compact beams of extreme relativistic electrons, two qualitatively new -- and very efficient -- ionization mechanisms arise: the tunnel or over-barrier ionization and the coherent impact ionization, which are driven by the low- and high-frequency parts, respectively, of the beam field. In these mechanisms significant fractions of the beam electrons act coherently, strongly enhancing the ionization process. They are also very sensitive to the spatiotemporal structure of the beam that can be used for analysing the beam properties.

Figures (5)

• Figure 1: Sketch of the electron beam - atom collision. The vectors $\bm b$ and $\bm r$ denote the coordinates of the atomic nucleus and the atomic electron, respectively. $L$, $a_0$ and $\bm v = (0,0,v)$ are the beam length, radius and velocity, respectively. The $z$-axis is also the symmetry axis of the beam.
• Figure 2: Sketch of a beam whose intensity: a) "turns on and off" within $\Delta L / v$, b) has a spike with duration $\delta L / v$.
• Figure 3: Single ionization of He(1s$^2$) atoms by electron beams, as a function of the electron energy. $N_t = 2 \times 10^6$, $a_0 = 1$$\mu m$, $L = 0.05$$\mu m$ and $0.06$$\mu m$, as indicated. Solid curves: the coherent impact ionization. Dot curve: the cross section (\ref{['e7']}).
• Figure 4: Single ionization of He(1s$^2$) by beams with $N_t = 1.5 \times 10^9$, $L = 3$$\mu m$ ($L/v = 10$ fs), $a_0 = 20$$\mu m$. Solid and dash-dot curves: the coherent impact ionization. Dash curve: tunnel ionization. Dot curve: the cross section (\ref{['e7']}). For more explanations see text.
• Figure 5: The probability $P_{\rm ion}(b)$ for single ionization of He(1s$^2$) as a function of the position $b$ of the atom with respect to the beam axis (see fig. \ref{['figure1']}). $N_t = 2 \times 10^6$, $L = 0.06$$\mu$m, $a_0 = 1$$\mu$m. Dash-dot, dash-dot-dot and solid curves: coherent impact ionization by 100 MeV, 1 GeV and 10 GeV beams, respectively.