Nonsequential double ionization of N${}_2$ in a near-single-cycle laser pulse

We present a comparative study of nonsequential double ionization (NSDI) of N${}_2$ and Ar exposed to near-single-cycle laser pulses. The NSDI process is investigated using carrier-envelope-phase-tagged electron-ion coincidence spectroscopy. The measured NSDI spectra of N${}_2$ and Ar exhibit a striking resemblance. In particular, the correlated two-electron momentum distribution arising from NSDI of N${}_2$ also displays a cross-shape very similar to that reported for Ar [Bergues et al., Nat. Commun. 3, 813 (2012)]. We interpret our results in terms of recollision-excitation with subcycle depletion and discuss how this mechanism accounts for the observed similarities and differences in the ionization behavior of the two species.

Figure 1

Measured CEP dependence of the recoil-ion momentum distribution along the laser polarization axis: (a) for Ar${}^{2+}$ and (b) for N${}_2^{2+}$. The ions were generated in the same laser focus at a peak intensity of $I_0=3.6\times {10}^{14}$ W cm${}^{-2}$. Invariance under the symmetry transformation $(p,\varphi )\to (-p,\varphi +\pi )$ was used to symmetrize the experimental images. (c) Calculated CEP-dependent $p_{\left|\right|}$ distributions of Ar${}^{2+}$ using the semiclassical model described in the text. The undetermined CEP offset in the experimental data was chosen such that measured and calculated doubly charged ion spectra oscillate in phase. (d) Measured momentum spectra of Ar${}^{+}$ (blue pluses), N${}_2^{+}$ (red crosses), Ar${}^{2+}$ (blue circles), and N${}_2^{2+}$ (red diamonds), averaged over CEP, shown together with predictions for the Ar${}^{+}$ spectrum [blue (gray) solid line] and the Ar${}^{2+}$ spectrum (black solid line). The maxima of the doubly charged ion spectra are normalized to 1, and the spectra of the singly charged ions are normalized to 0.6 for visual convenience. The statistical uncertainty of the measured data is of the order of the marker size.

Figure 2

CEP-dependent asymmetries for Ar${}^{+}$ (blue pluses), N${}_2^{+}$ (red crosses), Ar${}^{2+}$ (blue circles), and N${}_2^{2+}$ (red diamonds), shown together with predictions for the Ar${}^{+}$ asymmetry [blue (gray) solid line] and the Ar${}^{2+}$ asymmetry (black solid line). The undetermined CEP offset in the experimental data was chosen such that the asymmetries of the measured and the calculated Ar${}^{+}$ spectra oscillate in phase. The statistical uncertainty of the measured data is of the order of the marker size.

Figure 3

Experimental two-electron momentum distributions showing the correlated momenta along the laser polarization axis of the first ($p_1$) and second ($p_2$) electrons emitted from Ar${}^{2+}$ (top row) or N${}_2^{2+}$ (bottom row) and the corresponding predictions for Ar (middle row) at the peak intensity $I_0=3.2\times {10}^{14}$ W cm${}^{-2}$. The CEP-averaged results are displayed in the left column. In the second through fifth columns, the correlation spectra are shown for CEP values of $-{45}^{\circ }$, ${45}^{\circ }$, ${180}^{\circ }$, and ${270}^{\circ }$, respectively. The signal in each image is averaged over a CEP range of $\pm 22.5^{\circ }$ and normalized to its maximum value. The undetermined CEP offset in the experimental data was chosen such that measured and calculated doubly charged ion spectra oscillate in phase. Invariance under the symmetry transformation $(p,\varphi )\to (-p,\varphi +\pi )$ and the symmetry with respect to the $p_2=p_1$ diagonal were used to symmetrize the experimental images.

Figure 4

Contribution of (a) the lowest excited state of Ar${}^{+}$ with an excitation energy of 13.5 eV and two additional fictitious states with excitation energies of (b) 11.5 and (c) 9.5 eV to the correlation spectrum. (d) Sum of the contributions in (a), (b) and (c) normalized to 1. The excitation probability by an electron with a kinetic energy greater than the excitation energy is assumed to be equal for all three states. Comparison of the calculated CEP-resolved momentum spectra of the doubly charged ions (e) without and (f) with the contribution of the two lower-lying states. The corresponding CEP-averaged spectra and the asymmetry curves of singly and doubly charged ions are compared in (g) and (h) to the measured N${}_2^{2+}$ data (red diamonds). The black solid (dashed) lines in (g) and (h) are the calculation results obtained with (without) inclusion of the lower-lying states. The blue (gray) solid line and the red crosses in (h) denote, respectively, the calculated and measured asymmetry curves of the singly charged ions. Here, the undetermined CEP offset in the experimental data was chosen such that the asymmetries of the measured and the calculated Ar${}^{+}$ spectra oscillate in phase.