New techniques for making time-resolved studies of atoms in highly excited (Rydberg) states will be developed. In particular, the absorption or emission of radiation by Rydberg-state atoms will be monitored on a nanosecond time scale during and after irradiation by various laser pulse sequences. The signals thus produced (known as coherent transient effects) will be studied in order to provide both information about collisional relaxation processes in Rydberg atoms and other spectroscopic data.
New techniques for making time-resolved studies of atoms in highly excited (Rydberg) states will be developed. In particular, the absorption or emission of radiation by Rydberg-state atoms will be monitored on a nanosecond time scale during and after irradiation by various laser pulse sequences. The signals thus produced (known as coherent transient effects) will be studied in order to provide both information about collisional relaxation processes in Rydberg atoms and other spectroscopic data.
New techniques for making time-resolved studies of atoms in highly excited (Rydberg) states were investigated. In particular, attempts were made to observe coherent transient effects produced via CO2 laser irradiation, from a high density atomic Rydberg vapor produced by stepwise dye laser excitation, but no such effects were observable. It was found that this result was due to a very rapid collisional decay process which occurs in a Rydberg gas at the densities required for transient experiments (approx 10 to the 12th power atoms/cm3). The decay process was studied using ion collection and fluorescence techniques. (Author)
The short-pulse dynamics of excitation and stimulated emission between highly excited states (15<n<60) is studied with picosecond far-infrared pulses (40--60 $\ensuremath{\mu}\mathrm{m}$) originating from a free electron laser. We measured the excitation of a Rydberg wave packet in rubidium around n=40 excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary n=40 state we found that upon irradiating with a short far-infrared laser pulse, adjacent Rydberg states are populated by resonant Raman transfer via a deeper (n=20) bound state. Finally, we discuss the dynamics of wave-packet excitation at a Cooper minimum in lithium, where sign change of the excitation matrix elements causes excitation at large distances from the atomic core. Possible measurement methods for this effect are discussed and found to yield different information.
The goal of this work is to study and understand dynamics of atomic systems interacting with strong fields. To this end, a method referred to as strong-field spectroscopy is used, which measures the absorption spectrum of atoms interacting with such fields. In the scope of this work, a technique which allows the simultaneous measurement of absorption spectra of attosecond pulses and their reference signal is developed, which significantly increases the sensitivity to absorption changes. Additionally, a reconstruction method is introduced which retrieves the time-dependent evolution of the dipole response of an atomic system directly from a single absorption spectrum. Strong-field spectroscopy combines these techniques to examine the nonlinear strong-field dynamics in atomic and molecular systems and is applied to study the strongly correlated doubly excited states in helium. Furthermore, a measurement approach is introduced which uses the interaction with a high-intensity laser pulse to impose a time gate on the evolution of the coherent response of a system, which has previously been excited by an ultrashort laser pulse. With this approach the buildup of resonance phenomena in a wide range of physical systems can be studied. As a first application, the time-dependent buildup of a Fano resonance as well as the buildup of a whole Rydberg series are investigated.
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.
Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained.\nThe experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions).\nResults are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.
We have developed an all-optical method for measuring the lifetimes of nS and nD Rydberg states and demonstrate its capabilities with measurements on a dilute cloud of ultracold 87Rb atoms in a cryogenic environment. The method is based on the time-resolved observation of resonant light absorption by ground-state atoms and selective transfer of Rydberg atoms into the ground state at varying delay times in order to reconstruct Rydberg decay curves. Our measurements of the 87Rb30S1/2 state indicate an increase of the lifetime at lowered environment temperatures, as expected due to decreased blackbody radiation. For the 38D5/2 state with an attractive dipole-dipole interaction, ionization and lifetime reduction due to collisional effects are observed.