  Cold, dense Rydberg gases produced in a cold-atom trap are investigated using
spectroscopic methods and time-resolved electron counting. On the discrete
Rydberg resonances we observe large trap losses and long lasting electron
emission from the Rydberg gas (>30ms). Our observations are explained by
quasi-elastic l-mixing collisions between Rydberg atoms and slow electrons that
lead to the population of long-lived high-angular-momentum Rydberg states.
These atoms thermally ionize slowly and with large probabilities, leading to
the observed effects.
