Molecular alignment of fragmenting $\mathrm{H}_2{}^{+}$ and ${\mathrm{H}}_2$ in strong laser fields

We present time-dependent ab initio calculations of fragmenting $\mathrm{H}_2{}^{+}$ and ${\mathrm{H}}_2$ molecules in strong laser fields, taking into account all nuclear and electronic degrees of freedom. In particular, very different and apparently contradicting experimental observations on the alignment of fragmenting hydrogen molecules are naturally explained and consistently interpreted. In addition, striking differences in the alignment of fragmenting $\mathrm{H}_2{}^{+}$ and ${\mathrm{H}}_2$ are predicted. Whereas ${\mathrm{H}}_2$ fragments show decreasing alignment with increasing laser intensity, the alignment of $\mathrm{H}_2{}^{+}$ fragments is nearly independent of the intensity. This is in contrast to existing intuitive expectations, but in accord with available experimental data.

Figure 1

(Color online) Ground-state potential curves for $\mathrm{H}_2{}^{+}$ (top) and ${\mathrm{H}}_2$ (bottom, calculated using the Hartree-Fock approximation). The horizontal lines illustrate the vibrational levels of $\mathrm{H}_2{}^{+}$. The vertical dotted lines indicate the Franck-Condon region. The broken lines refer to the first excited surface of $\mathrm{H}_2{}^{+}$ shifted down by the energy of one photon for two wavelengths.

Figure 2

Evolution of the angle $\Theta$ between laser polarization and molecular axis as a function of time for $\mathrm{H}_2{}^{+}$ for two vibrational levels $\nu =6$ and 9 and the laser parameters (given above) of the experiments of Sändig et al. [20] (left side) and Williams et al. [21] (right side).

Figure 3

Evolution of the angle $\Theta$ between laser polarization and molecular axis as a function of time for ${\mathrm{H}}_2$ (upper graphs, initially in the vibrational ground state) and $\mathrm{H}_2{}^{+}$ (lower graphs, initially in the sixth vibrationally excited state) with the laser parameters (given above) of the experiments of Thompson et al. [18] (left side) and Frasinski et al. [19] (right side).

Figure 4

(Color online) $⟨{\cos }^2\Theta ⟩$ distribution (top) and fragmentation probability (bottom) for ${\mathrm{H}}_2$ and $\mathrm{H}_2{}^{+}$ as a function of the laser intensity. The wavelength of the laser is 266 nm and the pulse duration of the ${\sin }^2$ pulse is 50 fs.