Enhanced laser-induced alignment and orientation of molecules using quantum-state-selection

We demonstrate the use of strong inhomogeneous electric field to spatially disperse rotational states of a molecule. In the experiment a supersonic beam of iodobenzene molecules is formed and subsequently dispersed in an electric field. The molecules are then aligned using a focused ns YAG laser or oriented using a combination of the laser and a static electric field. The molecules are ionized with an intense fs laser pulse and the ionic fragments are detected using a velocity map image spectrometer. The recoil direction provides information about the spacial orientation of the molecules. By moving the laser focus it is possible to probe different rotational states. The state selection uses the fact that different rotational states experience different Stark shifts. The rotational ground state of iodobenzene will experience the largest Stark shift and will therefore be most strongly deflected in the electric field. The deflected profile of the molecular pulse has been measured and agrees well with trajectory simulations with a temperature of 1K. By selecting molecules in the lowest rotational states a higher degree of alignment can be obtained even at modest laser fields. The combination of a laser field and a weak static electric field lifts the degeneracy of states pointing up and down through the interaction with the molecular dipole moment. The quantum-state selection gives a large advantage with more than 70% of the molecules being oriented the same way. In conclusion the use of an electrostatic deflector enables us to select the lowest lying rotational quantum-states. This allows creation of unprecedented alignment and orientation by laser and static fields.