Evidence for rescattering in intense, femtosecond laser interactions with a negative ion

It is now well established that energetic electron emission, nonsequential ionization, and high harmonic generation, produced during the interaction of intense, femtosecond laser pulses with atoms (and atomic positive ions), can be explained by invoking rescattering of the active electron in the laser field, the so-called rescattering mechanism. In contrast for negative ions, the role of rescattering has not been established experimentally. By irradiating F ions with ultrashort laser pulses, F ion yields as a function of intensity for both linearly and circularly polarized light have been measured. We find that, at intensities well below saturation for F production by sequential ionization, there is a small but significant enhancement in the yield for the case of linearly polarized light, providing the first clear experimental evidence for the existence of the rescattering mechanism in negative ions. DOI: 10.1103/PhysRevLett.93.223001 PACS numbers: 32.80.Gc, 32.80.Rm, 34.80.Dp The liberation of an electron from an atom immersed in a strong electric field was observed experimentally soon after the discovery that lasers could made to generate moderately intense pulses by q-switching them. It was quickly realized that ionization was proceeding by a nonlinear multiphoton absorption process which was easily described within the framework of perturbation theory. However, in the last 20 years the development of ultrashort (femtosecond) pulse lasers has opened up intensity regimes where perturbation theory is no longer valid. In this regime the photon density is so high that ionization proceeds via tunneling of an electron through the barrier created from the superposition of the atomic potential with the macroscopic electric field potential of the laser pulse. The transition from the multiphoton or perturbative to the tunneling or nonperturbative regimes is usually presented in terms of the value of the Keldysh parameter � [1] which is the ratio of the frequency of the