The electric dipole intercombination decay of the 2s2p 3 P 0 1 level in the Be-like ion C 21 has been optically observed with ions circulating in a heavy-ion storage ring. The measured transition probability of ͑102.94 6 0.14͒ s 21 , corresponding to a lifetime of ͑9.714 6 0.013͒ ms, is in good agreement with recent theoretical calculations. This precision measurement thus resolves the longstanding discrepancy of data from a radiofrequency ion trap experiment with theoretical predictions.[S0031-9007 (97)03328-0] PACS numbers: 32.70.Cs, 31.50. + w, 39.90. + dSlow radiative decays of few-electron atomic systems with level lifetimes in the range of ms to s presently find much attention both in fundamental atomic structure studies and in astrophysical observations. Line intensities of spin or electric-dipole forbidden transitions represent important tools in the study of elemental abundances, electron densities, and temperatures in low-density terrestrial or astrophysical plasmas by atomic spectroscopy [1,2]. Increasingly precise calculations are available for slow forbidden decays in few-electron atomic systems [3]. On the other hand, versatile and precise methods of measuring such small decay rates are still lacking; in particular, lifetime measurements in low-energy ion traps so far suffer from large systematic corrections due to interactions with the residual gas.Heavy-ion storage rings-offering long storage times, small effects of collisions with the residual gas, separation of ion production and storage regions, and charge-state selectivity-were recently demonstrated to provide a favorable environment for such lifetime measurements which employed particle detection [4,5] or laser techniques [6,7] to obtain the corresponding decay curves. The method described here significantly differs from earlier measurements at storage rings by its versatile approach using optical fluorescence observation after simple collisional excitation, which works for many levels of interest independently of special observation schemes or the availability of lasers for optical excitation. By the clean decay signal and the related high precision, it opens up perspectives for the study of a variety of similar intercombination or forbidden transitions.A case of particular interest for astrophysics [2,8] is the ion C 21 of the Be isoelectronic sequence (Fig. 1). Recent compilations and theoretical discussions regarding the transition probabilities in this sequence [8][9][10][11][12][13][14][15][16] make clear that quite a number of theoretical studies (often at variance with each other) are opposed by rather few experimental tests. In particular, the intercombination decay rate of the 2s2p 3 P 0 1 level so far has been measured for only four ions: C 21 [17 -19], Fe 221 [20,21], Kr 321 [22], and Xe 501 [23]. The measurements on highly charged ions used fast, foil-excited ion beams, and now corroborate suitable relativistic calculations [24]. In the low-Z range, however, a discrepancy is found between the experiment and a number of recent the...

No abstract

Recombination of Au 25+ ions with free electrons has been investigated in a mergedbeams experiment at the UNILAC of GSI in Darmstadt. Special emphasis was put on the recombination at very low energies in the electron-ion centre-of-mass frame. At E rel = 0 eV we found an enormous recombination rate exceeding the theoretical expectations for radiative recombination by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

No abstract

No abstract