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The previously unreported decay of " Pr to levels in " Nd has been studied from massseparated activity produced in the thermal neutron fission of ' 'U. The '"Pr half-life was measured to be 3.24+0. 19 s, and its decay scheme was constructed based on y singles and coincidence measurements.Excited states in " Nd at 72.6 (2+) and 236.6 (4+) were established and the E(4+)/E(2+) ratio of 3.26 is close to the rigid rotor value of 3.33. The implications of the above ratio for the systematics of deformation in this region are discussed.A y transition observed at 285.0 keV probably results from the P decay of '"Ce with a half-life of 3. 1+0.3 s, but that conclusion is tentative. The most likely J for the "Pr ground state is 3.

The lifetimes of the metastable 1s2s2 p 4 P 5/2 level of He Ϫ , as well as the lifetime of the average of the 4 P 3/2 and 4 P 1/2 levels, have been measured using a new type of ion trap that stores keV ion beams using electrostatic fields only. The use of a pure electrostatic field avoids the complication of magnetic-field-induced mixing effects, which can interfere with the measurement of the spontaneous decay. The measured lifetime for the 4 P 5/2 state, after correction for decay induced by blackbody radiation, is 343Ϯ10 s. This value is consistent with previous experiments, and in excellent agreement with the most recent theoretical calculations. The average lifetime of the 4 P 3/2 and 4 P 1/2 is 8.9Ϯ0.2 s, which is about 20% lower than the weighted theoretical value. ͓S1050-2947͑99͒09501-3͔

The rates of reaction of hydrogen atoms with silane, germane, and (for comparison) ethylene have been determined at 20 Torr in a discharge flow system. The apparent bimolecular rate constants are 8.5 X 10~12, 4.3 X 10"10, and 3.5 X 10"13 cm3 molecule"1 sec"1, respectively, which are substantially larger than previously reported values. Activation energies were calculated by the empirical methods of Johnston (BEBO) and Spirin and compared with the values estimated from the experimental rate constants. The experimentally estimated values are lower than those predicted by the BEBO method, suggesting that the activated complex for reaction of hydrogen atoms with silane and germane may differ from the simple linear model for hydrogen atom transfer. * To whom correspondence should be addressed at Washington University.netic study of reactions of atomic hydrogen with the methane analogs silane and germane has been published until recently.3Recently reactions of hydrogen atoms,4 methyl radical,5 and trifluoromethyl radical6 with silicon and germanium hydrides and alkylsilanes have been reported. The results

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Direct elemental fluorination of representative aromatic substrates, including PhH, PhCH3, PhF, PhCI, PhBr, PhN02, PhCN, and PhOCH3, has been investigated in inert solvents, e.g., CC13F and other fluorocarbons, over the temperature range - 154 to 40 OC. In order to achieve the necessary control of the extremely reactive electrophile, and to minimize unwanted modifications of the reaction environment, the fluorination has been carried out at extremely low rates and correspondingly low conversions, generally below 0.01%, using as a reagent gaseous mixtures of F2 highly diluted ( < I mol %) in N2 or Ar. The partial rate factors, calculated from a consistent set of relative reactivity and orientation data measured at -78 OC in CC13F, correlate smoothly with the u+ constants for all the substituents investigated, giving a p+ value of -2.45 for aromatic substitution by elemental fluorine with a correlation coefficient of 0.993. These results characterize F2 as a highly reactive, and correspondently unselective, reagent, and support a polar electrophilic substitution mechanism that is discussed and compared with other plausible fluorination pathways