Tunnel Ionization of Molecules and Orbital Imaging

We study whether tunnel ionization of aligned molecules can be used to map out the electronic structure of the ionizing orbitals. We show that the common view, which associates tunnel ionization rates with the electronic density profile of the ionizing orbital, is not always correct. Using the example of tunnel ionization from the ${\mathrm{CO}}_2$ molecule, we show how and why the angular structure of the alignment-dependent ionization rate moves with increasing the strength of the electric field. These modifications reflect a general trend for molecules.

Figure 1

Tunnel ionization rate of a ${\mathrm{CO}}_2$ molecule for low and high strength of the ionizing dc electric field. Only the dimensionless alignment-dependent factor is shown.

Figure 2

Schematic of tunneling, with $z_{\mathrm{in}}$ and $z_{\mathrm{ex}}$ the entrance and the exit points and $z_0$ is the matching point.

Figure 3

Ionization of a ${\mathrm{CO}}_2$ molecule into the ground electronic state of the cation at the equilibrium geometry of the neutral. (a) Total ionization probability from numerical simulations including single ionization channel. (b) Angle-dependent factor $R({\theta }_L)$ from the analytical analysis. Contributions from both ${\Pi }_x$ and ${\Pi }_y$ channels are included.