Alignment of H₂⁺

Dimers exposed to high-intensity laser radiation show very different behaviour depending on the angle between the molecular axis and the electric field of the laser [1]. This is explained with two completely different mechanisms,

1. Geometric alignment: The absorption of energy is enhanced for molecules parallel or nearly parallel to the field in comparison to the others.
2. Dynamic alignment: The laser couples to the permanent or induced dipole moment of the non-oriented molecules and thus generates an effective torque towards the laser axis.
   

In the movies shown below the vertical polarized laser is indicated by a white background color.

(a) Laser-induced Rotation of H2+

H2+ is exposed to a laser pulse with a wavelength of 266 nm, a pulse duration of 500 fs and an intensity of 6·1013 W/cm2. The molecule is turned to the laser axis. When the laser peaks, it is oscillating around the laser axis. After the laser pulse, it is freely rotating.

(b) Dynamical Alignment of H2+	H2+ is exposed to a laser pulse with a wavelength of 266 nm, a pulse duration of 200 fs and an intensity of 1.2·1014 W/cm2. The molecule rotates towards the laser axis and fragments. Thus, strongly aligned fragments result.
(c) Rigid rotator model

1. M. Uhlmann, T. Kunert, and R. Schmidt,
   Molecular alignment of fragmenting H₂⁺ and H₂ in strong laser fields,
   Phys. Rev. A 72, 045402 (2005).