Routes to formation of highly excited neutral atoms in the breakup of strongly driven H${}_2$

We present a theoretical quasiclassical treatment of the formation, during Coulomb explosion, of highly excited neutral H atoms (${\text{H}}^{*}$) for strongly driven ${\text{H}}_2$. This process, where after the laser field is turned off, one electron escapes to the continuum while the other occupies a Rydberg state, was recently reported in an experimental study [B. Manschwetus et al., Phys. Rev. Lett. 102, 113002 (2009)]. We find that two-electron effects are important in order to correctly account for all pathways leading to ${\text{H}}^{*}$ formation. We identify two pathways where the electron that escapes to the continuum does so either very quickly or after remaining bound for a few periods of the laser field. These two pathways of H${}^{*}$ formation have distinct traces in the probability distribution of the escaping electron momentum components.

Figure 1

Top row: Final energy distribution of the ${\text{H}}^{+}$ or ${\text{H}}^{*}$ fragments, in the ${\text{H}}^{*}$ formation channel. Bottom row: 2D electron momentum distribution of the escaping electron, expressed in units of $\sqrt{U_p}$ [$U_p=E_0^2/\left(4{\omega }^2\right)$].

Figure 2

Schematic illustration of the two routes leading to formation of ${\text{H}}^{*}$: (a) Pathway A, (b) pathway B. Shown is the time-dependent position along the laser field for electrons (black lines) and ions (gray dashed lines).

Figure 3

2D electron momentum distribution of the escaping electron expressed in $\sqrt{U_p}$ for pathways A and B. The white arrow in (a) and (e) denotes the direction of tunnel ionization of electron 1. We also plot the distribution of the laser field phase ${\phi }_0$ at the time when electron 1 tunnel ionizes in the initial state for both pathways. Accounting for tunnel ionization of electron 1 to the right as well and adding (a) and (b) yields Fig. 1c while adding (e) and (f) yields Fig. 1d.

Figure 4

(a) The mean interelectronic distance at $1.5\times {10}^{14}$ W/cm${}^2$ for pathway A (black line), pathway B (gray dotted line), and delayed pathway of NSDI (gray line). (b) The momentum distribution $p_x$ at $1.5\times {10}^{14}$ W/cm${}^2$; total ($*$), pathway A ($•$), and pathway B ($▸$).