Coherent population trapping involving Rydberg states in xenon probed by ionization suppression

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 P = 250 nm is induced between the 5p 61 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 D ' 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 = 2o rJC =3 =2. The ionization is monitored through observation of the photoelectrons with an energy resolution E = 150 meV which is sucient to distinguish the ionization processes into the two ionization continua. Pronounced and robust dark resonances are observed in the ionization rate whenever D is tuned to resonance with one of the ns- or nd-Rydberg states. The dark resonances are due to ecient 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 sucient to resolve the hyperne structure of the ns-Rydberg levels for odd xenon isotopes. The hyperne splitting does not vary signicantly 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 eld with D 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.