Phase-controlled stair-step decay of autoionizing radial wave packets

Abstract

The time-dependent survival probabilities of autoionizing $4pNd$ wave packets in calcium have been measured. Femtosecond laser pulses induce sequential isolated-core excitations of singly excited $4sNd$ radial wave packets to create and probe the nonstationary doubly excited states. Stair step rather than exponential decay of the wave packets is observed. Moreover, we show that the autoionization lifetime can be varied by a factor of five by adjusting the relative phases of the constituent eigenstates in the packet. The stair-step decay structure and its dependence on the wave packet phase is interpreted using an intuitive time-domain picture. The experimental results are in excellent agreement with a K-matrix based, 28-channel quantum-defect theory calculation as well as with an approximate two-channel formulation.