No abstract

No abstract

We present the first quantitative results for the ionization of hydrogen at relatively high frequencies, from low intensities to superintensities in excess of the atomic unit /o = 3.51 x 10 16 W/cm 2 . We find that at superintensities the atom has a tendency to stabilize against multiphoton ionization, even at low frequencies. We show, however, the existence of very short lifetimes for the ground state, at intensities just below those needed for stabilization.PACS numbers: 32.80.Rm Physical quantities characterizing the interaction of atoms with purely monochromatic radiation may be derived from a Floquet analysis of the Schrodinger equation of the problem. By applying this analysis to the "space-translated" version of the equation, Gavrila and Kaminski have developed an iterative approximation scheme, valid at sufficiently high frequencies co. l In the high-frequency limit the theory predicts stabilization of the atom against decay by multiphoton ionization (MPI), i.e., the existence of stationary, albeit distorted, atomic states. 2 At finite, though sufficiently large co, the theory allows for MPI. 3 So far only the distortion of low-lying states of H has been studied accurately for linear polarization. 4 We have meanwhile applied the theory to the ionization of H and are presenting here the first (nonperturbative) results to be obtained at relatively high frequencies for the decay rates and lifetimes. Besides, this is the first calculation allowing us to follow continuously the evolution of the lifetime from low intensities to superintensities, which have now become available experimentally. (Our results are subject to relativity and retardation corrections, which may become sizable under these circumstances.) At lower frequencies, where a full Floquet analysis is needed, Potvliege and Shakeshaft have also calculated lifetimes, 5 but only up to several times 10 ] 3 W/cm 2 .The values of the lifetimes play an essential role in assessing the possibility of exposing the (neutral) atom to superintense fields (for a discussion of this issue, see Lambropoulos, Ref. 6). As these are produced in the form of very short, picosecond or subpicosecond, laser pulses, in order that the atom can survive to feel the peak intensity in the pulse, its lifetime should exceed the rise time of the latter at all intensities passed. Our results answer the question of the atomic survival in the realm . C ln -V n (a 0 ,T)~-z-I e in *V(T + a 0 (e\cos))d

Atomic stabilization is a highlight of superintense laser–atom physics. A wealth of information has been gathered on it; established physical concepts have been revised in the process; points of contention have been debated. Recent technological breakthroughs are opening exciting perspectives of experimental study. With this in mind, we present a comprehensive overview of the phenomenon. We discuss the two forms of atomic stabilization identified theoretically. The first one, ‘quasistationary (adiabatic) stabilization’ (QS), refers to the limiting case of plane-wave monochromatic radiation. QS characterizes the fact that ionization rates, as calculated from single-state Floquet theory, decrease with intensity (possibly in an oscillatory manner) at high values of the field. We present predictions for QS from various forms of Floquet theory: high frequency (that has led to its discovery and offers the best physical insight), complex scaling, Sturmian, radiative close coupling and R-matrix. These predictions all agree quantitatively, and high-accuracy numerical results have been obtained for hydrogen. Predictions from non-Floquet theories are also discussed. Thereafter, we analyse the physical origin of QS. The alternative form of stabilization, ‘dynamic stabilization’ (DS), is presented next. This expresses the fact that the ionization probability at the end of a laser pulse of fixed shape and duration does not approach unity as the peak intensity is increased, but either starts decreasing with the intensity (possibly in an oscillatory manner), or flattens out at a value smaller than unity. We review the extensive research done on one-dimensional models, that has provided valuable insights into the phenomenon; two- and three-dimensional models are also considered. Full three-dimensional Coulomb calculations have encountered severe numerical handicaps in the past, and it is only recently that a comprehensive mapping of DS could be made for hydrogen. An adiabatic variation of the laser-pulse envelope keeps the system in the Floquet state associated with the initial state, that allows calculation of the ionization probability in terms of the corresponding rate. A nonadiabatic variation can excite other Floquet states, either discrete (‘shake-up’) or continuous (‘shake-off’), with considerable consequences for DS. A unitary interpretation of these aspects of DS is presented in terms of ‘multistate Floquet theory’. We then comment on the points of contention raised in connection with DS. Further, we review the extent to which the classical approach has been successful in describing DS. We next examine the concern that nonrelativistic (NR) predictions for stabilization may be inadequate in superintense fields, because relativistic corrections would invalidate them. It turns out that, although the relativistic corrections do limit stabilization, there is an ample ‘window’ of intensities for which the NR predictions remain valid. Finally, we discuss the experimental evidence in favour of stabilization. For lack of adequate ...