Nonsequential Double Ionization by Counterrotating Circularly Polarized Two-Color Laser Fields

We report on nonsequential double ionization of Ar by a laser pulse consisting of two counterrotating circularly polarized fields (390 and 780 nm). The double-ionization probability depends strongly on the relative intensity of the two fields and shows a kneelike structure as a function of intensity. We conclude that double ionization is driven by a beam of nearly monoenergetic recolliding electrons, which can be controlled in intensity and energy by the field parameters. The electron momentum distributions show the recolliding electron as well as a second electron which escapes from an intermediate excited state of ${\mathrm{Ar}}^{+}$.

Figure 1

The measured and simulated relative yield of double to single ionization is shown in column I. In column II, measured electron momentum distributions are shown for the field ratios $E_{390}/E_{780}$ of (b) 0.8:1, (e) 1.5:1, and (h) 3.5:1 and a maximum combined intensity of $5.0\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. In column III, the corresponding combined electric fields $E_y$ and $E_z$ (arbitrary units) as well as the vector potentials $A_y$ and $A_z$ (atomic units) are depicted for the same ratios $E_{390}/E_{780}$ (c) 0.8:1, (f) 1.5:1, and (i) 3.5:1. One maximum of the combined electric field is marked with a green dot. The helicities and the temporal development of the electric fields and the vector potentials are indicated with arrows. In column IV, the corresponding simulated energy distribution of the returning electrons is shown, which has been calculated also for the ratios $E_{390}/E_{780}$ (d) 0.8:1, (g) 1.5:1, and (j) 3.5:1 using CTMC simulations. The energy needed for double ionization (27.6 eV) and the relevant excitation energy (17.14 eV) are marked with dashed orange and red lines, respectively. The number of recollision events is normalized to $10^6$ single-ionization events in column IV. All events exceeding 17.14 eV are integrated, which is visualized as the shaded red area. This integral corresponds to the number of returning electrons having enough energy to excite ${\mathrm{Ar}}^{+}$ and strongly depends on the field ratio $E_{390}/E_{780}$.

Figure 2

Relative yield of double ionization is shown as a function of intensity in focus for linear polarization at 780 mm and for a CRTC field with $E_{390}/E_{780}=1.23$, which is close to the maximum in Fig. 1. For the CRTC field, a similar kneelike shape is observed as it is commonly known for linear polarization. The two first points for linear polarization on the ordinate indicate that no double-ionization events were measured within a reasonable amount of time.

Figure 3

Measured electron momentum distributions for (a) single and (b) double ionization for a ratio of $E_{390}/E_{780}=1.42$ and a combined intensity of $4.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The corresponding vector potential is shown as a black curve, and the phase that belongs to the maximum combined field is indicated by a green dot. The dashed white line serves as guide to the eye regarding the rotation of the low-energy structure (purple structure at momenta smaller than 0.3 a.u.). (c) The normalized difference $(A-B)/(A+B)$ where $A$ ($B$) is the number of counts in the corresponding bins normalized to the sum of all counts for double (single) ionization. (d) The tomographically reconstructed electron momentum distribution is shown for the condition that the first electron has been detected in the shaded area in (c). Electrons in this area have recollisionally excited the ion at the time of the pulse which is indicated by the yellow dot in (c). In (d), we show the momenta of the corresponding excited electron set free later in the pulse. The labels 1, 2, and 3 represent the first (1), second (2), and third (3) maxima of the field following the recollision and correspond to time delays of 433, 1300, and 2167 attoseconds after recollision.