Orientation-dependent ionization rate of diatomic molecules

The ionization rate of molecules in strong laser fields depends on the orientation of the ionizing electric field relative to the molecular frame. The orientation dependence of the rate plays an important role in strong-field-based attosecond-scale and imaging methods, motivating us to investigate theoretically whether it can be extracted from the ionization yields or photoelectron momentum distributions obtained by ionization with laser pulses of various polarization forms. As a reference for comparison we use the orientation dependence of the static-field tunneling rate. Numerical solutions of the time-dependent Schrödinger equation for ionization of the ground and first excited states of a model <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msup><a:mrow><a:mi>HeH</a:mi></a:mrow><a:mo>+</a:mo></a:msup></a:math> molecule show that linearly or circularly polarized single-color pulses are of limited use for obtaining the orientation-dependent rate accurately. By adding a second harmonic with suitable polarization, i.e., using either linearly polarized two-color fields or counterrotating circularly polarized two-color fields, the agreement of the extracted signal with the exact tunneling rate is substantially improved.