Excitation of Hg(6 3P1) by low energy alkali ion impact: Optical polarization and cross sections for magnetic sublevels

Intensity and polarization of emitted light from collisions of ground state alkali ions and mercury atoms have been studied by a combined crossed-beam and optical technique. Both the intensity and polarization of the Hg(6 3P1→6 1S0)-253.7 nm line exhibit a pronounced structure as a function of ion energy in the investigated range (from apparent thresholds to 3 keV); in particular, the polarization fraction takes on in succession very large positive and negative values. From the emission cross sections for several transitions, after correction for cascade and anisotropy, absolute cross sections for the excitation process M++Hg(6 1S0) →M++Hg(6 3P1), where M+=Li+, Na+, and K+, have been obtained. Using emission and polarization data, cross sections for population of the magnetic sublevels, m/sub j/=0 and vertical-barm/sub j/vertical-bar=1, have also been obtained. These cross sections have apparent thresholds much larger than the endoergicities of the processes, but reach very high values (of the order of 10-16 cm2) in the investigated energy range; they also show a pronounced oscillatory structure. Although it appears that the primary excitation mechanism involves short-range repulsive forces, the final evolution of the collision system towards products is best depicted by reference to the adiabatic potential energy curves for the HgM+ molecules. In particular the prominent structure in the cross sections for the magnetic sublevels, which oscillate in antiphase, is discussed in terms of quantum mechanical phase interference between the quasimolecular terms dissociating to the 3P1 state of mercury