Spectrochimica Acta Part B 58 (2003) 1077–1082 0584-8547/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547Ž03.00065-X Resonance ionization spectroscopy of fermium (Zs100)   , M. Sewtz *, H. Backe , C.Z. Dong , A. Dretzke , K. Eberhardt , S. Fritzsche , a, a b a c b C. Gruning , R.G. Haire , G. Kube , P. Kunz , J. Lassen , W. Lauth , G. Passler , c d a e e a e ¨ P. Schwamb , P. Thorle , N. Trautmann a c c ¨ Institut fur Kernphysik der Universitat Mainz, D-55099 Mainz, Germany a ¨ ¨ Fachbereich Physik, Universitat Kassel, D-34132 Kassel, Germany b ¨ Institut fur Kernchemie der Universitat Mainz, D-55099 Mainz, Germany c ¨ ¨ Oak Ridge National Laboratory, Oak Ridge, TN 37831-6375, USA d Institut fur Physik der Universitat Mainz, D-55099 Mainz, Germany e ¨ ¨ Received 13 January 2003; accepted 2 March 2003 Abstract Laser spectroscopy has been applied for the first time to measure resonant transition frequencies of fermium (Zs 100). A number of 2.7=10 atoms was electrodeposited on a Ta filament and covered with a 1 mm Ti layer. Fm 10 atoms were evaporated from the filament at a temperature of approximately 1000 8C into an optical cell filled with a buffer gas and were subsequently photoionized in a two-photon 1q19 process using a tunable excimer-pumped dye laser and the excimer (XeF) laser itself to produce the resonant (1) and non-resonant (19) photon frequencies, respectively. The created photoions were identified by mass-selective detection. We observed resonant transitions of Fm at dye-laser energies of 25 099.8(2) and 25 111.8(2) cm . A possible classification of these levels is given on y1 the basis of multiconfiguration Dirac–Fock calculations. We also interpreted the recorded time of ion drift through the buffer gas as a novel way to measure the mean ionic charge radius of the heavy elements.  2003 Elsevier Science B.V. All rights reserved. Keywords: Fermium; Resonance ionization spectroscopy; Buffer gas; Multiconfiguration Dirac–Fock calculations  This publication comprises parts of the doctoral thesis of M. Sewtz.  This paper was presented at the International Conference on Laser Probing (LAP-2002), held in Leuven, Flanders, Belgium, July 2002, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q49-6131-3925199; fax: q 49-6131-3922964. http:yywww.kph.uni-mainz.de. E-mail address: sewtz@kph.uni-mainz.de (M. Sewtz). 1. Introduction At present, the most advanced method for the investigation of the properties of heavy elements with charge numbers larger than Zs100 is chem- istry with single atoms in aqueous solution w1x and in the gas phase w2x. In such experiments adsorp- tion enthalpies and the formation of molecules are studied. By comparison with the lighter chemical homologues changes in the valence electron con- figurations due to relativistic effects are deduced.