Stereodynamical Control of D₃⁺ Formation from the Bimolecular Photoreaction in the D₂-D₂ Dimer

We report the stereodynamic control of D₃⁺ formation from the laser-induced bimolecular reaction in a weakly bound D₂-D₂ dimer via impulsive molecular alignment. Using a linearly polarized moderately intense femtosecond pump pulse, the D₂ molecules in the dimer were prealigned prior to the bimolecular reaction triggered by a delayed probe pulse. The rotationally excited D₂ in the dimer was observed to rotate freely as if it were a monomer. It was demonstrated that the yield of photoreaction product D₃⁺ is increased or decreased when the molecular axis of D₂ is parallel or perpendicular to the probe laser polarization, respectively. The underlying physics of this steric effect is the alignment-dependent bond cleavage of D₂⁺ in the dimer induced by a photon-coupled parallel transition.