Selectivity in electron emission induced by ultrashort circularly polarized laser pulses

We theoretically investigate the ionization of neonlike atoms by an intense ultrashort circularly polarized laser pulse as a function of wavelength, spanning the regime from single-photon ionization to multiphoton and tunneling ionization. In order to determine the dependence of the ionization probabilities on the magnetic quantum number of the initial state, we perform ab initio numerical calculations of the corresponding time-dependent Schrodinger equation in the single-active-electron approximation. Specific emphasis of our analysis is given to the intermediate-wavelength regime of about 50 nm to 300 nm, in which the trend in the ionization probability of counterrotating electrons strongly differs from those for electrons in the other two initial magnetic sublevels. Theoretical analysis and numerical results indicate that the enhanced emission of counterrotating electrons occurs via photon absorption channels, which are only accessible for electrons rotating opposite to the rotation direction of the external electric field. Physical mechanisms behind the enhancement are identified as threshold effects, in which the emission into continuum states with low angular momentum quantum numbers is favored, and resonant enhanced ionization involving transitions via specific excited states in the atom. Numerical results showing the strong population in the relevant excited states, specifically those with low angular momentum quantum number, for the ionization of counterrotating electrons, and the overall similar trends in ionization and excitation for electrons with the other two initial magnetic quantum numbers support our interpretation. Overall, these effects lead to a change in the ionization ratio of co- to counterrotating electrons by two orders of magnitude from 10:1 to 1:10. This strong selectivity in the emission of electrons may lead to new opportunities in the generation of ultrashort spin-polarized electron pulses.