Route to direct multiphoton multiple ionization

We address the concept of direct multiphoton multiple ionization in atoms exposed to intense, short-wavelength radiation and explore the conditions under which such processes dominate over the sequential. Their contribution is shown to be quite robust, even under intensity fluctuations and interaction volume integration, and reasonable agreement with experimental data is also found.

Figure 1

Ionization of Ne at 93 eV under a deterministic pulse with durations of (a) 30 fs and (b) 5 fs. Solution of Eqs. (1a) in the presence of sequential channels alone (dashed lines) and with both direct and sequential processes included (solid lines).

Figure 2

Ionization of Ne at 93 eV under chaotic pulses 30 fs in duration and a coherence time of 6 fs. (a) Ion yields obtained by solving Eqs. (1a) for randomly chosen $F(t)$, in the presence of sequential (dashed) and sequential $+$direct (solid) channels. The presented yields are averaged over ${10}^4$ realizations. (b) Same as in panel (a) with volume expansion effects included.

Figure 3

Ionization of Ne under chaotic pulses of 30 fs in duration and a coherence time 6 fs. (a) Data obtained by [14]. (b)–(d) Ion yields at three different intensities, as obtained by Fig. 2b. The thin (black) bars correspond to sequential ionization whereas the broad (colored) bars refer to the case of open direct and sequential ionization channels.