Temporal Effects in Multiphoton Ionization of Lithium

The temporal effects of multiphoton ionization are investigated using a two-color method which determines both the time and intensity at which the process occurs. We show that the total ionization efficiency depends strongly on the time at which an atom makes a transition to an excited state during an intense laser pulse. This result clearly shows that the ac-Stark-shifted bound-state resonances not only enhance the cross section for photoionization, but that the excited bound states provide temporary storage states for the atomic population during the photoionization process

Short-pulse Laser-Induced Stabilization of Autoionizing States

Atoms in doubly excited states above the first ionization limit can decay via autoionization in which an electron is emitted leaving an ion, or by photoemission which leaves the atom in a singly excited state. In this paper, it is demonstrated that interaction between the atoms and a laser pulse that is short compared to the autoionization lifetime can lead to large enhancement of the photoemission process by stimulating the atoms to emit a photon. Since the resultant singly excited atoms do not autoionize, this process can be viewed as an enhancement of the stabilization of the doubly excited atoms against autoionization. A simple theoretical model is outlined that shows good agreement with the experimental results

Shakeoff Measurement of the L = 3 States of Barium

Isolated core excitation was used to produce low-energy continuum electrons in the l = 3 angular momentum state of barium. Data were taken over a region of energy that coincided with the energy of the 6p3/2nf doubly excited states. Analysis of the data using multichannel quantum defect theory allowed the measurement of the widths of the 6p3/2nf states and the energy-dependent phase of the continuum electronic wave functions due to interaction with the doubly excited states. The phase of the continuum electrons is shown to vary continuously with energy, due to the anomalously broad widths of the double excited states

Direct Measurement of Oscillations between Degenerate Two-Electron Bound-State Configurations in a Rapidly Autoionizing System

In this paper we report a direct observation of the oscillation between bound-state configurations in a rapidly autoionizing system. Calcium atoms were excited to a pure 4p3/2nd two-electron configuration using a 500-fsec laser pulse. The initial 4p3/2nd doubly excited state is energy degenerate with the 4p1/2n\u27d states and several continuum channels. Because of the short-pulse excitation, the initial state of the atom is not an energy eigenstate, but a nonstationary wave packet. As a result, oscillations between the two bound configurations were produced. These oscillations were measured by scanning the timing of a second 500-fsec laser pulse tuned to drive the 4p1/2n\u27d ionic state back down to the 4sn\u27d singly excited configuration, which was subsequently detected using selective field ionization. A simple theoretical model was used to model the experimental results and produced good agreement with the data

Electric Field Controlled, Pulsed Autoionization in Two Electron Wave Packets

In this paper, control of the evolution of a two electron wave packet through the application of a static electric field is demonstrated. Specifically, application of a small electric field is used to produce pulsed autoionization events, the timing of which can be controlled on a picosecond time scale. The technique is demonstrated by exciting calcium atoms using a short-pulsed laser to the 4p3/219d doubly excited state, which is energy degenerate with the 4p1/2nk stark states. Evolution of the resultant wave packet is monitored through the application of a second short laser pulse, which stimulates the atoms to emit a photon producing singly excited Rydberg states which are detected using field ionization

Laser-Induced Stabilization of Autoionizing States

Stabilization of autoionizing states of barium by laser-induced, stimulated emission of light is demonstrated. Relative to purely flourescent stabilization, the data clearly show an enhancement of the stabilization process for laser pulses short compared to the flourescent lifetime of the autoionizing states. Shakeup spectra in which the principal quantum number of both electrons changes during the stimulated emission process are also clearly demonstrated

Shake-Off Measurements of Electron-Ion-Scattering Phase Shifts

Continuum electrons are produced in a specific l state with high-energy resolution using shake-off photoionization. This process is well characterized, so that the photoionization signal is an accurate measurement of the difference between the continuum electrons wave-function phase and that produced by a hydrogenic interaction. Measurements are reported showing the phase of a Ba++e- in a d wave, in cases where it is well behaved, and in cases where doubly excited resonances produce rapid phase variations

Electric-Field Enhancement of Dielectronic Recombination from a Continuum of Finite Bandwidth

A small electric field is shown to increase the dielectronic recombination, via autoionizing Rydberg states, of an electron from a continuum of finite bandwidth. The continuum of finite bandwidth is a broad autoionizing state which is part of a series converging to a higher limit, and the field enhancement of the rate occurs because the field converts the nl Rydberg states to nk Stark states, increasing the number of contributing recombination paths. The experimental results are in excellent agreement with the predictions of an isolated resonance approximation treatment and show clearly both the positive effect of Stark mixing and the negative effect of field ionization on dielectronic recombination

Evolution of Atomic Motion in an Intense Standing Wave

We have investigated the effect of the dipole force and its fluctuation on the motion of Li atoms in an intense, one-dimensional, near-resonant standing light wave. The duration of the interaction of the atoms with the standing wave was varied from several tens of spontaneous-emission lifetimes to several hundreds. For a standing-wave frequency blue detuned from resonance, diffusive heating can dominate the time-averaged dissipative dipole force so that there is no steady-state momentum distribution. However, for sufficiently large blue detunings, the rate of diffusion is so slow that the resulting distribution approaches a quasisteady state. For red detunings, the diffusion is balanced with the force and a true steady state is achieved. We apply a Monte Carlo method based on the density-matrix equations in the dressed-state representation to simulate the atomic motion. The dynamics of atom channeling is discussed

Intraconfiguration Interactions in Barium 6p\u3csub\u3e1/2\u3c/sub\u3enf Autoionizing States

Measurements are reported of the energies and linewidths of the J=2, 3, and 4 components of the 6p1/214f and 6p1/219f states of barium. These states are anomalously broad, having autoionization lifetimes of approximately 1/2 of a classical Rydberg orbit period, and previously measurements have discerned no differences among the various J components. This work shows that the differences can be observed, and are consistent with calculations. The variations between states would be large if the states were LS coupled, but the large 6p ionic fine-structure forces recoupling into jj configurations. The spherical symmetry of the 6p1/2 ionic state minimizes the differences between J states, but does not eliminate them