Excitation of multilevel Rydberg states (atoms and diatomic molecules) is studied under an intense time-periodic perturbation that is instantaneously turned on and off. For a one-quantum excitation from a lower lying state, general expressions for the Laplace images of the population amplitudes of Rydberg states are obtained with regard to their decay characteristics. It is shown that the problem considered is reduced to the determination of the positions and widths of the energy levels of a quantum system in the field of monochromatic laser radiation of the same frequency and intensity as that in a pulse mode. To determine these quantities, an integral formulation is proposed for the eigenvalue problem for energies that is relatively simply solved with regard to the effect of the ionic core and its complex vibration-rotation structure on diatomic molecules. The specific features of the excitation of Rydberg states and the behavior of Rydberg wave packets are studied depending on the intensity and duration of laser radiation. A quantum phenomenon of rotational orientation of electron-excited diatomic molecules is considered.
The effect of a low-frequency laser field on photoionization and radiative recombination processes is studied theoretically, with particular attention paid to the combined influence of the electron-ion Coulomb interaction and the laser field on continuum states of the system. A procedure for representing the asymptotic states of the system, with both long-range interactions properly accounted for, is described. The transition rates are strongly affected by the existence of a Rydberg series of resonances. Methods developed for describing similar resonant processs that occur in the field-free case are generalized here to include the effects of the external laser field. A simple model, based on parametrizations of the type employed in quantum-defect theory, is used to study the modification of resonant structures in the cross section introduced by the external field. In the weak-field limit, a low-frequency approximation for bound-state Compton scattering is obtained, with resonance effects accounted for.
Introduction Series of atomic spectral lines had been studied by emission from excited vapors even before Balmer described the regularity of the visible lines of hydrogen in 1884. However, direct excitation had been restricted to the few lowest-lying levels, particularly those accessible by electric dipole transitions from the ground state using classical light sources. The advent of tunable lasers in the 1970s changed such spectroscopic limitations enormonsly, allowing excitation to a very much larger number of states. Atoms in highly excited states, typically with principal quantum numbers n ≥ 10, are called Rydberg atoms. They are oversize, fragile states of significant intrinsic interest. They are very rare in nature because of their size and fragility, but they are found in interstellar space (H I regions). One of the best sources for comprehensive descriptions of Rydberg atoms and references to the literature describing their exploration is Ref. [120]. It describes their characteristic properties and behavior both classically and quantum mechanically, and especially their strongly enhanced interactions with external fields. The first thing to note is that Rydberg atoms are usually characterized by a single, highly excited electron while all the other electrons remain in their original states. Thus their properties are very nearly classical, and the atoms are much closer to the ideal hydrogen-like model of Chap. 7, along with appropriate quantum defects (see Chap. 10). The spectroscopy of such states is especially interesting because the active electron is much less perturbed by the other electrons in the atom than when it is in lower states. Moreover, there are some special circumstances in which the energy of a Rydberg state is degenerate with an atomic state having two electrons excited to lower levels, and such configurations yield further new information. Second, Rydberg atoms do not usually decay back to the ground state by emission of characteristic radiation, so their study is mediated by other techniques that reveal new and interesting properties. Such properties arise because Rydberg atoms have a huge dipole moment and very low binding energy resulting from the large average distance between the excited electron and the nucleus-plus-core of the rest of the atom with net charge e . Third, transitions between Rydberg states are quite different from those connecting to the ground state for at least three reasons.
The influence of laser fluctuations on the optical excitation of Rydberg and continuum states close to a photoionization threshold is investigated within the framework of the phase diffusion model. Theoretical methods are developed for solving the relevant master equation of the atomic density operator which is averaged over the laser fluctuations. In the long-time limit this laser-induced excitation process exhibits nonexponential decay. This nonexponential time dependence reflects the fluctuation-induced diffusion of the Rydberg electron toward the ionization threshold.
Rydberg-state excitation of atoms in strong infrared laser fields provides a new complementary aspect of the perspective of atom--strong field interactions. In this article, we perform an experimental and theoretical study on the corresponding process of diatomic molecules,
N2 and
O2. We show that neutral molecules can also survive strong 800-nm laser fields in high Rydberg states, while their behavior is remarkably different in comparison with their companion atoms, Ar and Xe. The Rydberg excitation of
N2 generally behaves similarly to Ar, while that of
O2 is more significantly suppressed than the ionization compared to Xe in a high intensity region, which can be understood in the frame of a semiclassical picture, together with their different structures of molecular orbitals. However, distinct quantum features in the Rydberg excitation processes that are apparently beyond the semiclassical picture have been identified, i.e., the less suppressed probability of
O2 at low intensity and the oscillation behavior of the ratio between
N2 and Ar, indicating that our understanding of the relevant physics is still far from complete.