Eur. Phys. J. D 17, 113–123 (2001) T HE EUROPEAN P HYSICAL JOURNAL D c  EDP Sciences Societ` a Italiana di Fisica Springer-Verlag 2001 Coherent population trapping involving Rydberg states in xenon probed by ionization suppression T. Halfmann a , K. B¨ ohmer, L.P. Yatsenko b , A. Horsmans, and K. Bergmann Fachbereich Physik der Universit¨at, Universit¨at Kaiserslautern, 67663 Kaiserslautern, Germany Received 7 May 2000 and Received in final form 25 June 2001 Abstract. We report the observation of pronounced coherent population trapping and dark resonances in Rydberg states of xenon. A weak two-photon coupling with radiation of λP = 250 nm is induced between the 5p 6 1 S0 ground state of xenon and state 5p 5 6p[1/2]0 , leading to (2+1) resonantly enhanced three-photon ionization. The state 5p 5 6p[1/2]0 is strongly coupled by radiation with λD ’ 600 nm to 5p 5 ns[JC]1 or 5p 5 nd[JC]1 Rydberg states with principal quantum numbers n in the range 18 ≤ n ≤ 23 and with the rotational quantum number of the ionic core JC =1/2 or JC =3/2. The ionization is monitored through observation of the photoelectrons with an energy resolution ΔE = 150 meV which is sufficient to distinguish the ionization processes into the two ionization continua. Pronounced and robust dark resonances are observed in the ionization rate whenever λD is tuned to resonance with one of the ns- or nd-Rydberg states. The dark resonances are due to efficient population trapping in the atomic ground state 5p 61 S0 through the suppression of excitation of the intermediate state 5p 5 6p[1/2]0 . The resolution is sufficient to resolve the hyperfine structure of the ns-Rydberg levels for odd xenon isotopes. The hyperfine splitting does not vary significantly with n in the given range. Results from model calculations taking the natural isotope abundance into account are in good agreement with the observed spectral structures. Pronounced dark resonances are also observed when the dressing radiation field with λD is generated from a laser with poor coherence properties. The maximum reduction of the ionization signal clearly exceeds 50%, a value which is expected to be the maximum, when the dip is caused by saturation of the transition rate between the intermediate and the Rydberg state due to incoherent radiation. This work demonstrates the potential of dark resonance spectroscopy of high lying electronic states of rare gas atoms. PACS. 42.50.Hz Strong-field excitation of optical transitions in quantum systems; multi-photon processes; dynamic Stark shift – 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states) – 32.10.Fn Fine and hyperfine structure 1 Introduction The interaction of strong, coherent radiation with mat- ter permits the manipulation of atomic and molecular systems and processes beyond the limits imposed by in- coherent excitation. Examples of processes that rely on coherent interactions are population transfer by Rapid Adiabatic Passage (RAP) [1,2], Stimulated Raman Scat- tering involving Adiabatic Passage (STIRAP) [3], Stark- Chirped Rapid Adiabatic Passage (SCRAP) [4], Electro- magnetically Induced Transparency (EIT) in otherwise opaque media [5,6], Coherent Population Trapping (CPT) and dark resonances [7] in radiative decay or fragmenta- tion processes [8,9] as well as coherent control of these processes [10]. a e-mail: halfmann@physik.uni-kl.de b Permanent address: Institute of Physics, Ukrainian Academy of Sciences, prospekt Nauki 46, Kiev-22, 252650, Ukraine. A typical example for a spectroscopic technique which relies on coherence is the preparation of dark resonances in a lambda-type or ladder-type level scheme [7,11,12]. A populated ground or metastable state is weakly coupled to an excited intermediate state by a probe laser field. The intermediate state is coupled to a target state by a strong dressing laser field. When the dressing laser is switched on, the transfer of population from the ground to the intermediate state by the probe laser is inhibited as in EIT. Unlike for STIRAP, where both radiation fields induce strong couplings, the population is trapped in the ground state rather than being transferred to the Rydberg state, without placing transient population into the inter- mediate level. The population of the intermediate state may be probed by observing decay channels such as fluorescence, (pre-) dissociation or ionization. A reduction of the in- termediate state population of up to 50% may also be caused by saturation, if an incoherent radiation field is strong enough to equilibrate the population between the