Unified Approach to Probing Coulomb Effects in Tunnel Ionization for Any Ellipticity of Laser Light

We present experimental data that show significant deviations from theoretical predictions for the location of the center of the electron momenta distribution at low values of ellipticity $\epsilon$ of laser light. We show that these deviations are caused by significant Coulomb focusing along the minor axis of polarization, something that is normally neglected in the analysis of electron dynamics, even in cases where the Coulomb correction is otherwise taken into account. By investigating ellipticity-resolved electron momenta distributions in the plane of polarization, we show that Coulomb focusing predominates at lower values of ellipticity of laser light, while Coulomb asymmetry becomes important at higher values, showing that these two complementary phenomena can be used to probe long-range Coulomb interaction at all polarizations of laser light. Our results suggest that both the breakdown of Coulomb focusing and the onset of Coulomb asymmetry are linked to the disappearance of Rydberg states with increasing ellipticity.

Figure 1

Experimentally measured electron momenta distributions in the plane of laser polarization at lower ($\epsilon =0.2$) and higher ($\epsilon =0.7$) values of ellipticity. The coordinates of the center are labeled in the right panel with $P_x$ and $P_y$.

Figure 2

Center of the electron momenta distributions $P_x$ and $P_y$ along the major and minor axis of polarization, respectively, obtained from experiment. Theoretical curves correspond to expected values in the absence of any Coulomb interaction for $P_y$ (green curve) and $P_x$ (horizontal black line).

Figure 3

Experimentally measured electron momenta distributions along the minor axis of polarization as a function of ellipticity. The blue line is given by Eq. (2). A bifurcation occurs near $\epsilon \approx \pm 0.1$.

Figure 4

Experimental values of $P_x$ compared with the perturbative calculation, given by Eq. (5), following the approach in Ref. [10]. The Coulomb effects on $P_x$ are negligible for $\epsilon \le 0.1$.