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

We report the observation of pronounced coherent population trapping and da rk resonances in Rydberg states of xenon. A weak two-photon coupling with r adiation of lambda (P) = 250 nm is induced between the 5p(6) S-1(0) ground state of xenon and state 5p(5)6p[1/2](0), leading to (2+1) resonantly enhan ced three-photon ionization. The state 5p(5)6p[1/2](0) is strongly coupled by radiation with lambda (D) similar or equal to 600 nm to 5p(5)ns[J(C)](1) or 5p(5)nd(J(C))(1) Rydberg states with principal quantum numbers n in the range 18 less than or equal to n less than or equal to 23 and with the rot ational quantum number of the ionic core J(C) = 1/2 or J(C) = 3/2. The ioni zation is monitored through observation of the photoelectrons with an energ y resolution DeltaE = 150 meV which is sufficient to distinguish the ioniza tion processes into the two ionization continua. Pronounced and robust dark resonances are observed in the ionization rate whenever lambda (D) is tune d to resonance with one of the ns- or nd-Rydberg states. The dark resonance s are due to efficient population trapping in the atomic ground state 5p(6 1)S(0) through the suppression of excitation of the intermediate state 5p(5 )6p[1/2](0). The resolution is sufficient to resolve the hyperfine structur e 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 c alculations 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 lambda (D) is gen erated from a laser with poor coherence properties. The maximum reduction o f the ionization signal clearly exceeds 50%, a value which is expected to b e 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 hig h lying electronic states of rare gas atoms.