Using a multistep laser excitation of alkaline earth atoms in an atomic beam we are able to excite autoionizing states in which one of the electrons can be in a state of almost arbitrary n and 1. The multistep approach uses all single photon transitions in which one electron is excited at a time, as shown in Fig. 1. In addition to the technical simplification of requiring low energy photons, exciting one electron at a time simplifies the observed spectra and their interpretation.
Using a multistep laser excitation of alkaline earth atoms in an atomic beam we are able to excite autoionizing states in which one of the electrons can be in a state of almost arbitrary n and 1. The multistep approach uses all single photon transitions in which one electron is excited at a time, as shown in Fig. 1. In addition to the technical simplification of requiring low energy photons, exciting one electron at a time simplifies the observed spectra and their interpretation.
Using multistep visible laser excitation of autoionizing states, it is possible to excite atoms from the ground state to autoionizing states lying as much 10,000 cm−1 above the ground state. In addition to the fact that the approach allows one to easily reach states lying at high energies, it also considerably simplifies the interpretation of the observed spectra, becuase typically, one excitation is orders of magnitude stronger than the others. As a result, the Beulter‐Fano interference phoenomenon associated with the vuv excitation of autoionizing states is absent. We describe the multistep method and its application in a recent study of Ba 6pnd states, which lie ∼60,000 cm−1 above the ground state. In addition to illustrating the method, this study shows how the inherent simplicitiy of the approach allows one to observe the interaction between the 6p1/2nd and 6p3/2 series.
Multistep laser excitation makes it possible to excite one electron at at time in the excitation of doubly excited states. This suppresses the direct continuum excitation, removing the familiar Beutler–Fano interference profiles from the excitation of autoionizing states. Removing this complexity in the excitation process allows one to investigate inherently complex atomic systems, such as interacting autoionizing series. The ability to examine such systems experimentally has led to the development of theoretical techniques based on quantum-defect theory to interpret the experimental results. This powerful combination of complementary experimental and theoretical techniques has led to substantial advances in the study of autoionizing states.
The characteristics of the lowest 1 P autoionizing states of the Sr atom, which are excited by a weak electromagnetic field, and also the parameters, which determine the electronic and radiative decay (harmonic generation) of these states under multistep laser excitation through the intermediate discrete states 5s5d 1 D and 5p 2 1 D, are determined. The line profiles of the resonances in the ejected photon and electron spectra are obtained. The calculation is carried out in the diagonalization approximation, the state classification is based on the LS-coupling scheme and the coupling of photon and electron decay channels is allowed for. The theoretical results agree with the experimental data on the excitation of the 1 P autoionizing states obtained using linear and non-linear optics.
Traditional schemes for multistep resonance photoionization of atoms let every employed laser beam interact with the atoms simultaneously. In such a situation, analyses via time-dependent Schr\"odinger equation show that high ionization probability requires all the laser beams must be intense enough. In order to decrease laser intensity, we proposed a scheme that the laser beam used to pump the excited atoms (in a higher bound state) into an autoionization state does not interact with the atoms until all the population is transferred by the other lasers from a ground state to the bound state. As an interesting example, we examined three-step photoionization of 235U with our scheme, showing that the intensity of two laser beams can be lowered by two orders of magnitude without losing high ionization probability.
This paper is devoted to the autoionization process of large-angular-momentum Rydberg states. After a brief account of autoionization formalism in nonrelativistic theory and single-configuration approximation, we apply it to atomic states with one electron of large angular momentum. The dominant contribution is the direct one and the exchange contribution is ignored. In this framework, the autoionization width varies with the principal quantum number as ${n}^{\mathrm{\ensuremath{-}}3}$ and exponentially with the angular momentum. When two excited electrons interact with a spherically symmetric core, it is demonstrated that the interaction between the Rydberg electron and the core polarized by the valence electron is proportional to the interaction between the two active electrons. Using newly derived radial matrix elements, we obtain a generally good agreement between our results and experiments on barium and strontium. Large-l Rydberg states are shown to be stable versus autoionization as soon as l is about 7. Possible refinements of our method are reviewed.
In this work radiative properties of neutral elements with one or two electrons outside closed subshells have been investigated. Using laser techniques in connection with atomic beams selective excitation of Rydberg states was performed. Most of the studied states were populated with two-step excitation. Some states were reached directly from the ground state or from a metastable state and occasionally two-photon absorption was used. The excitation sometimes involved the use of frequency-doubling and Raman- shifting techniques. By time-resolved detection of the decay light from an excited state, primarily information on the radiative lifetime is obtained. With two different experimental arrangements radiative lifetimes in several Rydberg states have been determined. Using the quantum-beat technique time-resolved detection also provides information on energy splittings. This technique was used to measure hyperfine-structure splittings.To obtain information about hyperfine-structure constants and isotope shifts high- resolution spectroscopy on a collimated atomic beam was also employed. The behaviour of highly excited alkali atoms is mostly hydrogen- like and well predicted by theory. Group III A elements have one electron outside a closed subshell and are expected to behave like alkali atoms. The situation is however complicated by the presence of doubly excited states. Configuration interaction can then cause a partial mixing between singly and doubly excited states. In the alkaline-earth atoms (group II A elements), having two electrons outside closed shells, the situation is similar. In a Rydberg series of one excited electron the admixture of doubly excited states can cause large deviations from hydrogenic behaviour. Another complication in the alkaline-earth atoms is the mixture of singlet- and triplet states. - 4 - The presented results from measurements made on Na, Mg, Ca, Sr, Ba, Al, Ga and In will primarily serve as a basis for tests of modern atomic-physics models. Some of the lifetime values are also of astrophysical interest. Keywords: Laser spectroscopy, Rydberg states, time resolved, high resolution, two-step excitation, metastable state, two-photon absorption, frequency doubling, Raman shifting, radiative lifetime, quantum beat, hyperfine structure, isotope shift, Stark shift, alkali atoms, group III A elements, alkaline-earth atoms, configuration interaction, singlet-triplet mixing.