The main known regimes of atom ionization by a strong laser field are reviewed: the Kramers–Henneberger (high-frequency) and interference stabilization of atoms, barrier-suppression ionization and the model of wave packet spreading. The physics and relations between them are discussed.
This book presents and describes a series of unusual and striking strong-field phenomena concerning atoms and free electrons. Some of these phenomena are: multiphoton stimulated bremsstrahlung, free-electron lasers, wave-packet physics, above-threshold ionization, and strong-field stabilization in Rydberg atoms. The theoretical foundations and causes of the phenomena are described in detail, with all the approximations and derivations discussed. All the known and relevant experiments are described too, and their results are compared with those of the existing theoretical models.An extensive ge
Excitation and photoionization of Rydberg wave packets are described. Interference of bound-free transitions from many coherently populated Rydberg levels is shown to affect the dependence of the photoionization probability on the duration of interaction. Strong-field interference effects in such a system are described also. The most important of these is the field-induced interference stabilization of Rydberg atoms in a strong ionizing field. The intensity range in which this effect can exist is determined. The results derived are used for a qualitative model describing supercontinuum emission in the process of multiphoton ionization.
Photoionization of highly excited atoms by a strong microwave field is considered. Secondary coherent population of some neighboring Rydberg levels in the process of ionization is shown to be a reason for interference of transitions from these levels to the continuum. Interference is shown to be a reason for some new effects: stabilization of the atom, suppression of its ionization, appearance of a multipeak structure in the photoelectron spectrum, field-induced narrowing of its peaks, narrowing and localization of quasi-energy atomic spectrum in a strong field, etc.
Photoionization of Rydberg atoms is considered in the framework of the quasiclassical (WKB) approach. Distribution of the photoelectron density in space and time is found and analyzed. This distribution is shown to consist of two main parts: the temporary population, which exists only as long as the light field is on; and the population corresponding to irreversible ionization. The irreversible-ionization wave packets are shown to experience the effect referred to as Coulomb narrowing. An origin of this effect and the conditions of its existence are discussed. In a strong field, rather simple analytical expressions are found for the partial probabilities of above-threshold ionization and for the total probability of ionization. In contrast to many earlier works, in the present theory neither the rotating-wave nor ``pole'' approximations are used, and the results are derived for an arbitrary pulse shape. Strong-field interference stabilization of Rydberg atoms is confirmed to occur at pulse durations shorter than the Kepler period. The arising regimes of interference stabilization are analyzed in comparison with the predictions of earlier works.
We use the Kramers-Henneberger approximation to investigate the phenomenon of stabilization of quantum systems with short-range potentials in a strong electromagnetic field. We identify the physical limits imposed on our investigation by the use of this approximation, and the ranges of parameter values of the system and external field that lead to stabilization. We analyze the dependence of the stabilization threshold on the frequency of the laser light. The conclusions obtained from our analytic investigation are confirmed by numerical calculations.
The dynamics of an atom in a strong single mode non-classical electromagnetic cavity field is studied. The reconstruction of the energy spectrum of atom and the effect of the ionization suppression in the presence of non-classical field are discussed. It is found that Kramers – Henneberger mechanism of the ionization suppression is realized in strong nonclassical fields also. For squeezed vacuum states the threshold of the stabilization is approximately three times lower than for classical (coherent) state of the same intensity.