Subcycle dynamics in the laser ionization of molecules

The time and momentum distributions of electron emission from a molecule during a single laser cycle are calculated by solving a two-dimensional time-dependent Schrödinger equation. The momentum distributions strongly depend on the orbital symmetry and orientation of the molecular axis. Field-induced internal dynamics of the molecule can shift electron emission and recollision times through a large part of the laser cycle, which leads to corresponding variations of high-harmonic emission times and to the appearance of even harmonics.

Figure 1

Time distribution of density of recolliding electrons on target as obtained with probe distances $z_0=10$ (solid line), 15 (dashed line), and $20\mathrm{a.u.}$ (dotted line). In (a) all recollision momenta are included; in (b) momenta are chosen such that all electrons reach $20\mathrm{a.u.}$ (c) shows the relation between recollision momentum and excursion amplitude for short trajectories.

Figure 2

Dependence of the electron momentum distributions at $z_0=15\mathrm{a.u.}$ on molecular orientation and initial state symmetry. Upper panels: parallel $(\Vert ,g)$ and perpendicular $(\perp ,g)$ momentum distributions with initial gerade states for parallel $\theta =0°$ (solid line), $\theta =45°$ (dashed line), and perpendicular $\theta =90°$ (dotted line) alignment of the molecular axis. Lower panels: same for an antisymmetric initial state.

Figure 3

Time distributions of electron emission and recollision (left panels), state populations (center), and harmonic emission (right) for laser intensities $5.6\times {10}^{13}$, $1.3\times {10}^{14}$, and $2.2\times {10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$ (from top to bottom). The dot-dashed lines indicate $\mid \mathcal{E}\left(t\right)\mid$. Left panels: emission (solid) and recollision (dashed) distributions scaled to maximal values of 1 with the probe located at $z_0=6\mathrm{a.u.}$, molecular alignment parallel to the laser field. Center panels: populations of ground state (dashed) and first excited state (solid), parallel alignment. Right panels: combined intensity of harmonics above 90% of the harmonic cutoff energy with parallel (solid) and perpendicular (dashed) alignment.