We have observed that the amount of photoionization from an atomic Rydberg state with a pulsed laser depends critically on the length of the pulse, whereas lowest-order perturbation theory implies that the yield depends only on the fluence (the integrated intensity) of the pulse. We have measured the yield for one-photon ionization with short (0.5–2.7-ps) high-intensity laser pulses from the 27d Rydberg state in barium. With equal fluence, the yield decreases for pulses shorter than the Kepler orbit time of the outer electron (2.2 ps) because not all of its wave function comes near the core during the length of the pulse. The observations and this classical picture are confirmed by quantum-mechanical calculations.
We have measured the ionization fields for Na Rydberg states of $n\ensuremath{\ge}26$ by a 79-GHz microwave field. At low
n, where the microwave frequency $\ensuremath{\omega}$ is one-third the classical Kepler frequency ${\ensuremath{\omega}}_{K}$, which is equal to the
1/n3 spacing between
n states, the ionization field is $\ensuremath{\approx}1/3{n}^{5}$, in atomic units. As
n is raised ${\ensuremath{\omega}}_{K}$ decreases, and the ionization field rises above
1/3n5, reaching a constant value when $\ensuremath{\omega}={\ensuremath{\omega}}_{K}$. At very high
n, where $\ensuremath{\omega}>1/2{n}^{2}$, single photoionization is possible, and the measured rate agrees with the calculated value. These measurements, together with those done previously, connect the processes of field ionization and photoionization in Na. At low
n and radio frequencies ionization is simply field ionization, and at very high
n it occurs by photoionization, single-photon absorption. These measurements also show that microwave ionization of Rydberg atoms at 79 GHz results in significant populations' being left in the very high-lying states.