Asymmetry in orientation-resolved strong-field ionization of polar molecules

We present calculations of orientation-dependent ionization yields from aligned <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msup> <a:mrow> <a:msub> <a:mi mathvariant="normal">H</a:mi> <a:mn>2</a:mn> </a:msub> </a:mrow> <a:mo>+</a:mo> </a:msup> </a:math> and oriented polar <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msup> <c:mrow> <c:mi>HeH</c:mi> </c:mrow> <c:mrow> <c:mn>2</c:mn> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:math> and CO molecules by simulating the three-dimensional time-dependent Schrödinger equation. For the polar molecules, we observe asymmetries in orientation-resolved total ionization yields: The yields are not reflection symmetric across the orientation angle <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi>β</d:mi> <d:mo>=</d:mo> <d:msup> <d:mn>90</d:mn> <d:mo>∘</d:mo> </d:msup> </d:mrow> </d:math> with respect to the axis of the linear polarization of the laser field. A simple ionization model, applicable in the tunneling ionization regime, explains the asymmetry in ionization yields based on structural factors from weak-field asymptotic tunneling theory and asymmetries in the external field.