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...

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The rates of several electric-dipole forbidden decays of 3p and 3d levels in Cl-, S-, P-and Si-like ions of Fe have been measured optically at a heavy-ion storage ring. In several cases, more than one decay contributes to a given decay curve, which complicates the analysis. The lifetime results, with a precision range from 0.6 to 20%, compare well with some theoretical predictions, but are more precise. They are also more precise than some experimental data from an electrostatic ion trap.

The decays of the high angular momentum, high spin 4F0 states of the electron configuration 3s3p3d in aluminium-like ions (with three electrons outside a closed shell) have been identified in delayed spectra of foil excited Ti, Fe and Ni ion beams. The line identifications were aided by HXR, MCHF, and MCDF calculations and are corroborated by lifetime measurements which demonstrate the differential longevity of the fine structure levels. A search for the related case of ls2p3d4Fo level decays in Li-like ions of Si, P and S failed.