We have measured the production of both Lyα and Hα fl uorescence from atomic H and D for the photodissociation of H 2 and D 2 by linearly polarized photons with energies between 24 and 60 eV. In this energy range, excited photofragments result primarily from the production of doubly excited molecular species which promptly autoionize or dissociate into two neutrals. Our data are compared with ab initio calculations of the dissociation process, in which both doubly excited state production and prompt ionization (non resonant) channels are considered. Agreement between our experimental data and that of earlier work, and with our theoretical calculations, is qualitative at best.

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We have measured the degree of helicity of fluorescent radiation from Ar+[3p4[3P]4p] 2P(o)1/2 formed by circularly polarized synchrotron radiation in the region of double excitations converging to Ar+3p4 nl satellite states. Angular momentum coupling allows the partitioning of the one unit of angular momentum brought into the system to be demarcated. We obtain a nonvanishing expectation value of the total spin of the residual ion-photoelectron system indicating significant relativistic interactions during the photoionization process.

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Relative yields of Ca and Ca ions from thermal-energy collisions between highly excited Ca* 4snd 'D Rydberg atoms and Ca ground-state atoms are measured as a function of n. The collisions take place within an eA'usive Ca atomic beam. The cross section for a collision resulting in a bound Ca and a ground-state Ca+ is expected to be large when the binding energy of the excited Rydberg state nearly matches the electron affinity of Ca. %'e observe a large increase in positive and negative ions for n =25 only. This result strongly suggests that the binding energy of Ca is 24 1.4 meV. PACS numbers: 35.10.Hn, 34.80.Qb Atoms in Rydberg states provide a unique tool for collisional spectroscopy of weakly bound negative ions. The close spacing of Rydberg levels allows for selective excitation of atoms into states with small and nearly arbitrarily selected binding energies. Atoms in a sufficiently high level can then transfer the Rydberg electron in collisions with atoms or molecules that have a small electron affinity, producing a pair of oppositely charged ions. In this experiment, we have observed the relative rate as a function of n for formation of positive and negative Ca ions in the collision: Ca (4snd 'D)+Ca(4s2'S) Ca (4s 4p P)+Ca+(4s S). (1) Recent calculations by Fabrikant [1] show an enormous cross section for process (1), on the order of 10 '0 cm2