We have performed the first excitation of high-rt states of positronium using resonant, two-photon excitation applied to the «*1 to 2 and 2 to n transitions. Absolute values for the line positions were quantitatively determined for n ~14 and 15 and compared well with calculated predictions. The prediction of the n ~3 scaling of the relative transition rates was observed for n = 13 to 19.PACS numbers: 36.10.Dr, 32.80.Pj Since the discovery of positronium (Ps), spectroscopic studies have been performed on sublevels of the ground and n =2 energy levels of the system. These include the singlet-triplet splitting in the ground state, 1 the twophoton \S-2S transition, 2 and the 2S-2P splitting in the first excited state. 3 These measurements represent a sensitive test of quantum electrodynamics due to the precision of the experimental results, lack of importance of strong forces, and high calculational accuracy available from the theory. In this Letter, we report the first observation in Ps of the excitation of high-H states, « -13-15, a preliminary measurement of the energy of two of these levels, ^ = 14 and 15, and preliminary observations on the relative transition probabilities for n = 13-19. Excitation of high-rt states was performed by a resonant, twophoton excitation n = 1 to 2 and n sss 2to 13-19 and using an extension of our previous investigations into the production of an optically saturated population of n =2 Ps. 4 The basic quantum electrodynamic interactions in Ps, particularly energy shifts induced by perturbative external fields, may be tested in new ways by measurements of the energies and transition properties of high-/z states. Excitation to high-/i states results in a population of long-lived, neutral Ps because the deexcitation and annihilation lifetimes scale as n~3. Such excited populations of Ps can be used to study areas such as antihydrogen production or exotic many-body states of Ps. 5 We can also tune through the resonant excitation profile using narrow laser linewidths allowing us to map the velocity distribution of the ground-and (n =2)-state populations. Since these transitions leave the Ps system in a bound state, they can be of particular use in laser-cooling experiments. 5 In this experiment we excite high-rt states in Ps through a two-step excitation process using the n = 2 state as an intermediate state. The experiment was conducted using extensions of the techniques previously employed to observe the population of the n = 2 levels of Ps. 4 Ps and laser light interacted in the ultrahigh-vacuum, experimental chamber used with the intense, low-energy positron beam at the 100-MeV electron linac at Lawrence Livermore National Laboratory. Thermal Ps was formed by guiding a 1-keV pulsed positron beam with a 200-G magnetic field onto a hot, 1000-K, clean copper single crystal cut along the 100 face. The target was biased with respect to a grid to attract any reemitted positrons. Pulses of 15-ns duration typically contained 10 5 positrons which were converted into Ps with an overall effici...

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Publication costs assisted by the Atomic Energy Commission and Oakland University Radicals produced after electron attachment to 5-bromo-6-methyluracil and the 5-halouracils at 77°K in neutral (12 M LiCl) and alkaline (8 M NaOH) aqueous glasses have been investigated by esr spectroscopy. Electron attachment to 5-bromo-6-methyluracil in 12 M LiCl at 77°K results in an anion radical. The anion dehalogenates upon warming at 150°K to form the 6-methyluracil-5-yl radical. Esr spectra indicate that upon further warming this second species undergoes hydrolysis to form the 5-hydroxy-6-methyl-5,6dihydrouracil-6-yl radical. The results for the halouracils in 12 M LiCl suggest stable anions for bromo-, chloroand fluorouracil at 77°K. Warming the bromouracil anion to 155°K results in a spectrum attributed to the uracilyl radical. Warming to 165°K results in the hydrolysis of the uracilyl radical to form a radical analogous to that found for 5-bromo-6-methyluracil. The fluorouracil and chlorouracil radical anions remained stable throughout the temperature range investigated. In 8 M NaOD the anions were found to be somewhat less stable. The anions of 5-bromouracil and 5-bromo-6-methyluracil were not observed at 77°K; however, those of chloroand fluorouracil were observed. The difference in stability of the anions in the two glasses is attributed to the fact that the nitrogens are protonated in the neutral glass whereas they are not in the alkaline glass. In addition protonation of the anion at an oxygen in the neutral glass is also likely. This would produce a neutral radical and result in further stabilization.

The electron transfer reaction within various dinucleoside phosphate radical anions has been investigated by ESR spectroscopy and pulse radiolysis. In the ESR work electrons are produced by photolysis of KiFeCCNje in a 12 M LiCl glass at 77 K. Upon photobleaching the electrons react with the dinucleoside phosphate to form the anion radical. The anions of the four DNA nucleosides were also produced and their ESR spectra were appropriately weighted and summed by computer to simulate the spectra found for the dinucleoside phosphate anions. From the analysis the relative amounts of each of the nucleoside anions in the dinucleoside phosphate anion were determined. For example in thymidylyl-(3'-5')-2/-deoxyadenosine (TdA), where the bases are likely stacked so as to allow electron transfer to the base with the greatest electron affinity, the electron is found to localize on thymine. Whereas in equal mixtures of T and dA in which the molecules are isolated in the aqueous medium anions of both nucleosides are found in approximately equal amounts. These results and those found for the other dinucleosides studied (TdC, TdG, dAdC, TT) suggest the electron affinity of the pyrimidine bases are greater than the purine bases; however, the results are not sufficient to distinguish between the individual purine or pyrimidine. When dinucleoside phosphate anions containing thymidine are warmed protonation occurs only on thymine to produce the well known "thymyl" spectrum. Pulse radiolysis experiments on individual nucleotides (TMP, dAMP), mixtures of these nucleotides and the dinucleoside phosphate, TdA, in aqueous solution at room temperature show that in the TdA anion electron transfer occurs from adenine to thymine, whereas no electron transfer is found for mixtures of individual nucleotides. Protonation is found to occur only on thymine in the TdA anion in agreement with the ESR results.