Resonances in the population of circular Rydberg states formed in beam-foil excitation

By directing fast moving 164 MeV F2656e12+ ion beam on 90 μg/cm2 thick carbon foil we have studied the unresolved decay of short-lived 2p (τ2p≈3.3 fs) and long-lived 2s (τ2s≈350.6 ps) states of H-like Fe in the time range 1.6×104-9.2×105τ2p, where, τ2p and τ2s are, respectively, radiative lifetimes of the 2p and 2s levels. At such large times four resonances have been observed as the humps riding on the decay curve of the beam-foil-excited 2s state. This unusual behavior is explained as consequence of the sequential cascading of circular Rydberg states (l=n−1) to 2p state, which modifies the time dependent photon intensity, I(t), of the 2p→1s transition from an exponential to hump-like structures for t»τ2p. From the detailed study of the cascading process the relative population of circular Rydberg states is determined. It is observed that certain circular Rydberg states are unexpectedly and profusely populated when fast H-like Fe ions emerge from the back surface of a thin solid foil.

► Resonances are observed in decay of beam-foil-excited state at very long times. ► The resonances are due to sequential cascading down of circular Rydberg states. ► Certain circular Rydberg states are unexpectedly populated in beam-foil-excitation.