Breakdown of sequential tunnel ionization in ultrashort electromagnetic pulses

Abstract

We consider double ionization of negative bromine ion in intense low-frequency electromagnetic fields. By solving numerically the two-electron time-dependent Schr{\" o}dinger equation we demonstrate that while for pulses of a few tens of femtoseconds duration and longer the sequential single-electron approximation perfectly describes the ionization dynamics, for pulses as short as a few femtoseconds this picture breaks down entirely, and the electron-electron interaction suppresses the rate of ionization by roughly one order of magnitude. We also show clear signatures of the collective tunneling effect in the photoelectron density distribution. This counter-intuitive channel of ionization opens up due to the electron-electron repulsion in the direction lateral to the applied electric field.

Figures (5)

• Figure 1: Probability density $|\Psi(x_1,x_2,t)|^2$ for the 1D ion (a,b) and distribution $F(x_1,x_2,t)$ calculated along (\ref{['F']}) for the 2D ion (c,d) with the interacting (a,c) and non-interacting electrons (b,d). The distributions are shown for the center of the pulse ($t=T/2$).
• Figure 2: Total probability of double ionization (\ref{['W(t)']}) in the LPR (a) and SPR (b). Blue and red lines correspond to the 1D and 2D cases respectively. Solid lines show the case of the interacting electrons, dashed lines -- that of the non-interacting electrons. The field parameters: $E_0 = 0.035$, $T=40$fs and $T=13$fs (a), $T=2$fs (b).
• Figure 3: Time-dependent populations of singly (solid black lines) and doubly (dashed black lines) ionized atoms in the LPR (a) and SPR (b).
• Figure 4: Same as in Fig.1, but in the short pulse regime. The field parameters and the time instants are: $t=2.5$ fs (a,b), $t=2.0$ fs (c,d), $E_0 = 0.035$, $T=2$ fs.
• Figure 5: (a): The normalized spatial distribution $|\Psi(x, y_1, x, y_2)|^2$ at $x=16.5$, corresponding to the local maximum of the diagonal flux in Fig.\ref{['fig:1D2D_add_br_short_t=2.88fs']}(c) for $t=2.0$ fs; (b): the effective potential barrier (\ref{['U(x,y)']}).