The role of the quantum defect and high-order dispersion in Rydberg wave packets

Experimental and theoretical recurrence spectra of Rydberg electron wave packets are presented. Fractional revivals up to the seventh order are observed. In addition we observe a new interference effect: a forerunner wave packet in the first full revival, due to second-order dispersion. Also it is shown that the quantum defect of rubidium shifts the peaks in the interference pattern by the quantum defect modulo one times the classical orbiting period, while it leaves the envelope of the recurrence spectrum unaffected. All these quantum mechanical effects are explained by a Taylor expansion of the phase factors of the stationary states of which the wave packet is composed.