Coulomb Asymmetry in Strong Field Multielectron Ionization of Diatomic Molecules

We measure the angular distribution of an electron emitted by a strong elliptically polarized two-color laser field from exploding doubly charged molecular nitrogen. This angular distribution is vastly different for emission of the electron from the up-field core of the molecule as compared to that from the down-field core. The emission from the down-field core leads to a slight rotation with respect to the internuclear axis in the direction expected by the Coulomb effect of the remaining ion, while, for the emission from the up-field core, this direction is inversed. Our semiclassical simulations suggest that this unexpected angular distribution is caused by an initial longitudinal momentum of the electron freed by over-the-barrier ionization above the inner barrier in the molecule. The initial kinetic energy is in the range of the potential energy of the Stark-shifted orbital above the barrier.

Figure 1

(a) The sketch of the elliptically polarized two-color pulse and the angular streaking concept in our coordinate system. Its major axis is along $y$ and the minor along $z$. For the absolute phase of $\Delta \Phi =0$, an electron freed at a certain instantaneous field points to $+y$ (solid red arrow) and receives a final momentum after the pulse points to $-z$ (open blue arrow) in the polarization plane driving solely by the rotating vector of the laser field. (b) The angular distribution of the explored ${\mathrm{N}}^{+}+{\mathrm{N}}^{+}$ channel. (c) The field-dressed double well potential of the diatomic molecular ion.

Figure 2

(a)–(d) The ion sum-momentum distributions of the exploded doubly (${\mathrm{N}}^{+}+{\mathrm{N}}^{+}$) and triply (${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$) ionized ${\mathrm{N}}_2$. The corresponding angular distributions are plotted in (e) ${\mathrm{N}}^{+}+{\mathrm{N}}^{+}$ and (f) ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$. The intensities of the FW and SH pulses are estimated to be $7.6\times {10}^{14}$ and $3.4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, respectively. The elliptical two-color laser field and the orientation of the molecule are shown as the insets.

Figure 3

The ion sum-momentum distributions of the ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ channel when the emission direction of the doubly charged ion ${\mathrm{N}}^{2+}$ is gated. For $\Delta \Phi =0$ (or $\pi$), the third electron is freed from the (a) down-field [or (c) up-field] or (b) up-field [or (d) down-field] core of the diatomic molecular ion when the doubly charged ion ${\mathrm{N}}^{2+}$ emits to $-y$ ($p{y}_{\mathrm{N}2+}<0$) or $+y$ ($p{y}_{\mathrm{N}2+}>0$), respectively. The insets show the elliptical two-color laser field and the orientation of the molecule.

Figure 4

(a)–(d) The deconvoluted momentum of the third electron freed from different sides of the molecules for the ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ channel. The corresponding angular distributions are plotted in (e),(f). (g),( h) The simulated angular distributions of the third electron freed from ${\mathrm{N}}^{2+}+{\mathrm{N}}^{+}$ by a two-color elliptical pulse of $\Delta \Phi =0$, where an initial longitudinal momentum is (g) considered or (h) switched off, respectively. The insets show the elliptical two-color laser field and the orientation of the molecule.