LASER ASSISTED ELECTRON EMISSION PROCESSES IN ATOMIC AND NUCLEAR PHYSICS

Nuclear beta decay in the presence of an intense long-wavelength radiation field and above-threshold ionization of atoms are reviewed with particular consideration of the similarities of the two processes. Special emphasis is put on final state effects and the classical limit. For nuclear beta-decay, it is shown that large nonperturbative effects can be expected in differential decay rates such as angular distributions and energy spectra. These effects are essentially classical and are governed by the classical parameter ν = eA₀/mc with A₀ the amplitude of the vector potential of the external field. Modifications of the nuclear lifetime, on the other hand, are quantum effects. They are governed by the quantum mechanical parameter eħE₀/m²c³ (with E₀ the electric field amplitude of the external field) which is much smaller than ν. They follow perturbation theory up to very high intensities, i.e., only photon processes of very low order are involved. A general formalism is presented which allows to identify the quantum corrections to the total decay rate due to art increase of phase space and due to a modification of the selection rules for forbidden beta-decay. For above-threshold ionization, several different models are discussed where the external field, like in the theory of laser-assisted beta decay, largely couples only to the emitted electron. These are various models based on the Keldysh approximation, in particular the Keldysh-Faisal-Reiss (KFR) model and a two-step model which deals separately with the initial ejection of the electron into the continuum and the subsequent final-state interaction with the field. Special attention is given to the case of an extreme short-range binding potential. In analogy with laser-assisted beta-decay, the different intensity dependence of total ionization rates and electron energy spectra is discussed. Again, the former follow lowest order perturbation theory up to much higher intensities than the latter. The origin of these different behaviors, is, however, more difficult to locate than in the case of beta-decay. The classical limit of above-threshold ionization is discussed for the case of two different realizations: the long-wavelength limit and two-color ionization.