Dissociative Recombination Of NH~+

The dissociative recombination (DR) of NH+ion has been experimentallyinvestigated at the TSR storage ring, which is located at the Max Planck Insti-tute for Nuclear Physics in Heidelberg, Germany. The electron beam producedby photocathode electron target can be used for phase space cooling of storedion beam and provide electron for dissociative recombination process as targetwith very well energy resolution. Neutral DR fragments N and H are detecteddownstream of a fast merged electron and ion beam interaction zone. Trans-verse fragment distances were measured on a recently developed high count rateimaging detector, which enabled us to map the fragment imaging and branchingratios over collision energies Ed=0―12eV showing unprecedented detail. Frag-ments in ground states, N(4So)+H(2S), are almost not populated at all collisionenergies covered. At Ed<0.02eV, dominant product channels N(2Do)+H(2S)and N(2Po)+H(2S) are almost half and half. For Ed>0.02eV, additionalproduct channels become energetically accessible. The branching ratio of prod-uct channel N(4So)+H(n=2) is approximately up to80%at Ed≈0.12eV.The number of opening dissociation channels is less than seven for Ed<1eV.At even large collision energy, the energetically accessible product channels aremore than twenty, and their contribution are too indistinguishable to identifydominant product channels. Branching ratios of these product channels displayrich structure highly sensitive to collision energy. Once a new dissociation chan-nel opens, it rapidly dominate, then the corresponding population decreases ascollision energy increases.We independently measured total absolute DR merged beam rate coef-cient by silicon surface barrier detector with high count rate. The rate coef- fcient is maximum (2.0×106cm3s1) at0eV, then smoothly decreases to2.4×108cm3s1at0.06eV. With increasing of collision energy, the rate co-efcient curve oscillates, energetically accessible new product channels must beone reason. The minimum rate5.2×1010cm3s1appears at3.3eV. The ratecoefcient then gradually increases at even large collision energy. Combined themeasured branching ratios with total absolute DR merged beam rate coefcient,it can provide collision energy dependent partial rate coefcients towards partic-ular atomic product states. They show rich structure corresponding to collisionenergy.The covered high impact energies of～4―12eV extend over the range belowthe two lowest predicted dissociating states of NH+. Our focus has been on thefnal state populations in N and H atoms. The transverse fragment distancesdistributions allow detailed tracking of populations among groups of fnal statesas a function of the electron collision energy, revealing their correlation withdoubly excited states in the Franck―Condon region of the NH+ion. At low(～5eV) electron energy, identifed groups of fnal levels are nitrogen Rydbergstates together with ground state H. In a small electron energy interval near7eV, a signifcant part of the fnal state population is steered to hydrogen Rydbergatoms with N atoms in the frst excited (2D) term, showing the efect of Rydbergdoubly excited states below the predicted22Π ionic potential. The transversefragment distance distributions above～10eV are compatible with nitrogenRydberg states correlating to the doubly excited Rydberg state manifold belowthe ionic24Σ level.