Excitation of laser state-prepared Na*(3p) to Na*(3d) in low-energy collisions with Na+: experiment and calculations of the potential curves of Na2+

The authors report experimental results on Na*(3p) to Na*(3d) excitation by Na + impact for collision energies E cm =20-47.5 eV together with calculated potential energy curves for the Na 2 + molecular ion for internuclear distances R=3-40 au. The state-to-state angular differential cross section for p to d collisional excitation has been measured. They find a pronounced maximum for the differential cross section at a reduced scattering angle tau =260 eV deg. Rotational coupling is responsible for this non-adiabatic collision. The largest excitation cross section is observed when the E vector of the exciting linearly polarised laser light is almost parallel to the velocity of the incoming Na + ion thus preparing a 3p sigma orbital asymptotically. Pseudopotential calculations are performed for the lowest twelve Sigma , ten Pi and six Delta states, and crossing points are characterised. Simple classical trajectory calculations, where the p to d excitation proceeds via a 3p Sigma -3d Pi transition at the crossing point theoretically predicted at R c =5.68 au and V c =5.72 eV, result in a reduced scattering angle tau =250 eV deg, which is in close agreement with experiment.