Enhanced Recollisions for Antisymmetric Molecular Orbitals in Intense Laser Fields

The peculiarities of antisymmetric molecular orbitals are investigated in very intense linearly polarized laser pulses. For this purpose, the ionization-recollision quantum dynamics is evaluated theoretically beyond the dipole approximation. As opposed to the usual situation, the laser magnetic field component is found to strongly enhance recollision probabilities for particularly oriented antisymmetric molecular orbitals. Harmonic generation and related processes are thus allowed at high laser intensities without the common limitations by the laser magnetic field.

Figure 1

Schematic diagram of an antisymmetric molecular orbital subjected to a linearly polarized laser field with polarization vector $\mathbf{E}$ and propagation direction $\mathbf{k}$ parallel to the molecular axis $\mathbf{R}$. The dashed line symbolizes the nodal line. Within the dipole approximation, the ejected electrons cannot travel perpendicular to the molecular axis $\mathbf{R}$. The magnetically induced electron drift helps to transfer the electron towards the molecular core, as indicated by the small arrows in the lower part of the figure.

Figure 2

Harmonic spectrum (a) for $\Theta =90°$ and (b) for $\Theta =0°$ at a laser intensity of $1.4·{10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ and a laser wavelength of 248 nm. The black line depicts the spectrum of a computation accomplished in dipole approximation, while the red (gray) one shows the harmonic signal for a simulation performed without the dipole approximation. The arrows in (b) indicate a minimum and a maximum of a two-center interference structure.

Figure 3

Harmonic spectrum for $\Theta =90°$ at a laser intensity of $3.51·{10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ and a laser wavelength of 248 nm. The black and red (gray) lines represent the harmonic spectrum obtained with and without the application of the dipole approximation, respectively.

Figure 4

Contour plot of the density of the electronic wave function for $\Theta =90°$ and a laser intensity of $1.4·{10}^{17}\mathrm{W}/{\mathrm{cm}}^2$ (molecular axis along the $y$ axis): (a) in the dipole approximation and (b) beyond the dipole approximation.