Nonexponential decay of shock-wavepackets.

摘要

Short-pulse isolated core excitation (ICE) techniques have been used to create and observe autoionizing shock-wavepackets. Their dramatic, nonexponential, stair-step decay has been recorded, and is found to be well described by the formalism of Wang and Cooke [1].;Calcium atoms in a thermal beam are excited from their 4s 2 ground state to an intermediate 4s 4p state using a Hansch-style dye laser (lambda = 423nm). A second, similar dye laser (lambda = 393nm) promotes the 4p electron to a highly excited Rydberg state (25 ≤ n ≤ 32). Both nsec dye lasers have a sufficiently narrow band-width to ensure that the Rydberg electron is in a single-n eigenstate.;The shock-wavepacket is produced by a short-pulse ICE on resonance with the 4s nd → 4p nd transition (lambda ≅ 393nm). The short duration of this pulse ( FWHM ≤ 0.25ps) as compared to the Kepler period of the Rydberg electron (2ps ≤ TK ≤ 5ps depending on n), results in the creation of a shock wave front within the wavepacket. This shock front is a localized depletion of probability executing cyclical, radial excursions. Starting near the core, the wave front propagates through the radial wavepacket with a period equal to the Kepler period of the initial Rydberg state---"consuming" probability as it goes. After each orbit, the wavepacket resembles its original form, but with its overall amplitude greatly reduced. The periodic nature of this process is manifested in the decay rate of the shock-wavepacket as a series of stair-steps.;A second ICE (lambda ≅ 318nm), employed in a pump-probe fashion with the first, is used to measure the survival probability of the shock-wavepacket as a function of time. The observed decay is demonstratively non-exponential, and the predicted stair-steps are clearly present in the rate.;[1] Xiao Wang and W. E. Cooke, Phys. Rev. A 67, 976 (1991).