Strong-field photoionisation of an initially excited hydrogen atom: Formation of Rydberg wavepacket, its structure and trapping of population at Rydberg levels

Strong-field photoionisation from high discrete levels of the hydrogen atom is considered. The secondary coherent population of Rydberg levels via Raman-type transitions is described. The space and time structure of the wavepackets arising is investigated. Sharp jumps of the semiclassical phase of the corresponding wave function are predicted. These features of the wavefunction are used to explain the reasons for its stability with respect to photoionisation in a strong ionising field. An experimental scheme with almost sudden laser excitation of atoms to Rydberg levels is proposed. This scheme is shown to be applicable for observation of the strong-field interference stabilisation of atoms.