Z. Phys. A - Hadrons and Nuclei 342, 455461 (1992) zo,,.o..,. Hadrons for Physik A and Nuclei 9 Springer-Verlag 1992 Measurement of the transverse Doppler shift using a stored relativistic 7Li + ion beam R. Klein 1, R. Grieser 1, I. Hoog 1, G. Huber 1, I. Klaft 1, P. Merz 1, T. Kiihl 2, S Schriider 2, M. Grieser 3, D. Habs 3, W. Petrich 3, and D. Schwalm 4 1Institut flit Physik, Universit/it Mainz, Postfach 39 80, W-6500 Mainz, Federal Republic of Germany 2 Gesellschaft ffir Schwerionenforschung GSI, Postfach 110541, W-6200 Darmstadt, Federal Republic of Germany 3 Max-Planck-Institut fiir Kernphysik, Postfach 1039 80, W-6900 Heidelberg, Federal Republic of Germany 4 Physikalisches Institut, Universit/it Heidelberg, Philosopbenweg 12, W-6900 Heidelberg, Federal Republic of Germany Received October 21, 1991 We have performed for the first time precision spectros- copy on a coasting fast 7Li+ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropa- gating laser beams acting on two different hyperfine tran- sitions sharing a common upper level (A-system). One laser was frequency locked to the a 3 127j 2 hfs frequency component established as a secondary frequency stand- ard at 514 nm. The second laser was tuned over the A-re- sonance, which was recorded relative to 127j2 hfs compo- nents. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed. PACS: 03.30. + p; 29.20.Dh; 32.90. + a 1. Introduction Combining the experimental control given by heavy ion storage rings with precision laser spectroscopy gives ac- cess to a new dimension in fundamental experiments [1 ]. The long storage times allow for excellent beam prepa- ration by various cooling techniques such as electron [2] and laser cooling [3] which reduce the momentum spread of the ions for the spectroscopic experiments. The high velocity of the stored ions leads to stronger relativistic effects - e.g. the relativistic time dilatation reflected in the transverse Doppler shift - as compared to laser ex- periments with atomic beams. In this paper we present first spectroscopic measurements of a A-system [4] in an electron cooled ion beam of 7Li +, moving with a velocity of 6.4% speed of light, and give an outlook on a precision experiment for verifying special relativity. The 7Li+ ions are especially well suited for this kind of experiments as about 20% of the stored ions can be prepared to be in their metastable 3S~ state, which can be easily excited by optical lasers to the 3P 2 state. More- over, both states show large hyperfine splitting (see Fig. 1), which ensures a clean separation of the 3SI ( F = 3/2)~3P2 (F= 5/2) and the 3S1(F= 5/2) ~3p2(F=5/2) transition forming the A-system from other hfs-transitions. The basic idea of the relativity experiment is to excite the A-system by a co- and a counter-propagating laser beam in a collinear geometry with the ion beam. We have chosen this A-sytem which is less sensitive to frequency shifts due to light pressure effects and to line broadening due to optical pumping. According to the theory of spe- cial relativity (SRT) the Doppler-shifted resonance fre- quency VDS required to excite a transition of frequency v 0 in the ions' rest frame is given by V0=VD s7 (fl2)(l__fl COS 0) (1) where fl = rion/c denotes the velocity of the ion (in units of the velocity of light) in the laboratory system, 0 is the angle between V~o n and the photon wave vector kDs , while the relativistic time dilatation factor y (f12), which also determines the transverse Dopplershift observed for 0 = 90 ~ is equal to 1 Y (/72) = ]/1 - fl~ (2) Thus the laboratory frequency v a required to excite the 3Sj (F= 3/2)--*3/'2 (F= 5/2) transition of frequency v~ with a counter-propagating laser beam and the laser frequency Vp needed to excite the 3S1 (F=5/2) 2 in the co-propagating laser --, 3P 2 (F= 5/2) transition v 0 geometry are given by %=v~y(fl~)(l + fl) v2= v ,,y (fl~)(1- fl ) (3) (4) respectively, if perfect alignment of the two lasers with the ion beam is assumed. Keeping the parallel exciting laser frequency vp fixed, it acts only on ions with a specific