Multichannel contributions in the nonsequential double ionization of CO${}_2$

The intermediate molecular states in the double ionization of molecules by laser-driven electron recollision are encoded into the angular-dependent signature of the process. We identify through this signature the specific ionic states participating in the double-ionization mechanism. Using CO${}_2$ as an example we employ a 1350-nm strong field at $2\times {10}^{14}$ W cm${}^{-2}$ to ensure that nonsequential double ionization (NSDI) is the exclusive mechanism in CO${}_2^{2+}$ formation. Under these conditions we obtain two main results. First, the shape of the recolliding electron wave packet is inferred from measurements of the ellipticity dependence of the signal, and second, the inelastic recollision cross section in the molecular frame is extracted from the variation of the signal as a function of the angle between strong-field probe polarization and molecular axis. These results provide an angularly resolved electron-ion collision measurement in CO${}_2$ and demonstrate contributions to NSDI from both ground and first excited ionic states.

Figure 1

Pump-probe scheme for investigating the ellipticity and polarization dependence of NSDI in impulsively aligned CO${}_2$. Ions were detected via a TOF spectrometer operated in Wiley-McLaren mode. Inset: Half-revival of the rotational wave packet induced by the pump pulse under the employed experimental conditions.

Figure 2

CO${}_2^{2+}$ yield as a function of driving laser ellipticity $\epsilon$ comparing 800 and 1350 nm at an intensity of $2\times {10}^{14}$ W cm${}^{-2}$. $\theta$ denotes the angle between the molecular bond axis and the major axis of the laser polarization ellipse.

Figure 3

Total CO${}_2^{+}$ and CO${}_2^{2+}$ yields and their ratio as a function of the angle $\theta$ between the linear laser polarization and the molecular axis for 1350 nm and $2\times {10}^{14}$ W cm${}^{-2}$. Lines were obtained from polynomial fits.

Figure 4

Schematic of the three channels considered for NSDI in this experiment. Solid arrows denote tunneling ionization; dashed arrows, inelastic recollision. Note that the ionization continuum of the parent ion is reached at 23.5 eV with respect to the ionic ground state [20].

Figure 5

CO${}_2$ HOMO aligned at an angle ${\theta }_M$ with respect to the $x$ axis (set as the alignment axis) and probed by an electric field $\vec{E}$ at an angle ${\theta }_L$ with respect to alignment axis and $\alpha$ with respect to the molecular axis.

Figure 6

Polar plot (in degree) of $w^{-}$ (resp. $w^{+}$) distribution (defined by Eq. (A2)) (black dashed curve, resp. blue solid curve) for $n=10$ in (a) and $n=50$ in (b). The red dash-dot curve is the deconvolved single ionization yield $\Theta \left(\alpha \right).$

Figure 7

(a) Convolved $P_{\mathrm{tot}}(\alpha ,\epsilon )$ distribution with the molecular angular distribution at the half revival for $\mathrm{n}=10$. (b) Lineouts of (a) for ${\theta }_L=0^{\circ }$, ${45}^{\circ }$ and ${90}^{\circ }$. (c) Lineouts of the same distribution for $n=50$.