Asymmetric angular distributions in multiphoton ionisation of helium by elliptically polarised light

Abstract

Within a non-perturbative treatment of the multiphoton multichannel ionisation of helium we show for the first time that the angular distributions of the electrons ejected by elliptically polarised light exhibit the twofold symmetry recently observed experimentally on noble gases and already predicted by the authors for hydrogen. Several fine structures and details of the experimental angular distributions are successfully reproduced; a physical model for such asymmetries is provided, which is also able to explain why several approximations fail in reproducing them.

Photoelectron momentum distributions of atomic and molecular systems in strong circularly or elliptically polarized laser fields

“…If we also neglect the initial momentum, as is often done in the SQC model, the asymptotic photoelectron momentum would point in the direction of the minor polarization axis. However, observed PMDs show peaks rotated away from the predicted direction [31][32][33][34][35][36][37][38][39][40][41][42][43].…”

“…Therefore, it is not very clear whether the deviations can be explained by a common mechanism. On the one hand, it was pointed out that the Coulomb interaction between the electron and the parent ion after ionization is responsible for the deviation in atomic [32][33][34][35][36][38][39][40][41]43] as well as molecular systems [46][47][48][49][50][51][52][53]. Hence, the deviations, which break the fourfold symmetry of the PMD expected by the SFA, are sometimes called Coulomb asymmetry.…”

Threshold and Coulomb effects in multiphoton ionization

“…I am aware of only two attempts to describe this efFect in the context of multiphoton ionization. The first is by Trombetta, Ferrante, and Basile [2], and that focused on angular distribution. The effect demonstrated there was that rescattering in the Coulomb final-state wave function (or it turns out [3] any other wave function} introduces profound changes in the angular distribution of the emerging electron for some laser polarizations.…”

“…This is approximated by a form suggested by previous work [2] and it is modified to contain a parameter which is determined variationally. The result, after a series of approximations dictated by the parameters of a typical experiment [11],is surprisingly simple [see (2.10) and (2.24)].…”

“…It is however, the simplest for demonstrating the effects to be discussed here. Trombetta, Ferrante, and Basile [2] have approximated the exact final state by a "Coulomb-Volkov" state given by 4cv=g (r, t)e '~'u~(r), where a is still given by (2.4) and p(r) is the classical momentum of the electron which evolves in the potential U~a nd starts at p(~} =q. The path in the line integral of the first term in (2.5) is just the classical path followed by this electron.…”