POPULATION TRAPPING IN KR AND XE IN INTENSE LASER FIELDS

We have observed trapping of population in Rydberg states of atoms and ions exposed to intense laser radiation. The trapping occurs in nonstationary ''wave-packet'' superposition states, created, via ac Stark-shifted multiphoton resonances during the laser pulse. The formation of these highly excited bound states requires the nonresonant absorption of up to 20 or more photons; some of the ionic Rydberg states He nearly 32 eV above the ionic ground state. All of the trapped states having binding energies much less than the photon energy but remain stable against ionization even when the multiphoton ionization probability of the atom is saturated.