Rydberg-state excitation and ionization of argon atoms subject to elliptically polarized strong laser fields

We experimentally and theoretically investigate the ellipticity dependence of Rydberg state excitation (RSE) and ionization of Ar subject to strong laser fields. It is found that, at 800 nm, the ratio of RSE to ionization yields decreases monotonically with ellipticity for the intensities explored in this work, while, at 400 nm, the ratio decreases monotonically in the high-intensity regime but increases in the low-intensity regime. A dedicated semiclassical model where the influences of Coulomb potential and nonadiabatic effect have been considered can reproduce the experimental observations qualitatively at 800 nm and the high-intensity regime at 400 nm. However, in the low-intensity regime at 400 nm, our semiclassical model cannot reproduce the experimental measurements, indicating that the well-accepted semiclassical frustrated tunneling ionization mechanism is not valid any more for a Keldysh parameter significantly larger than unity. Our work provides a comprehensive understanding of the atomic RSE process and raises a challenge for the theoretical methods in the future.