B. D. DePaola

IR-assisted ionization of helium by attosecond extreme ultraviolet radiation

Attosecond science has opened up the possibility of manipulating electrons on their fundamental timescales. Here, we use both theory and experi- ment to investigate ionization dynamics in helium on the attosecond timescale by simultaneously irradiating the atom with a soft x-ray attosecond pulse train (APT) and an ultrafast laser pulse. Because the APT has resolution in both energy and time, we observe processes that could not be observed without resolu- tion in both domains simultaneously. We show that resonant absorption is impor- tant in the excitation of helium and that small changes in energies of harmonics that comprise the APT can result in large changes in the ionization process. With the help of theory, ionization pathways for the infrared-assisted excitation and ionization of helium by extreme ultraviolet (XUV) attosecond pulses have been identified and simple model interpretations have been developed that should be of general applicability to more complex systems (Zewail A 2000 J. Phys. Chem. A 104 5660-94).

Reducing the uncertainty in laser beam size measurement with a scanning edge method

A modification in the analysis of a conventional laser beam spot size measurement method has been developed. The new analysis significantly decreases the uncertainty in the estimation of the beam-spot size. A conventional beam scanning approach was used in the measurement, but instead of differentiating the data and fitting the result to a Gaussian function, the data were fit to an analytical approximation to the complementary error function. As a result, fitted parameters were obtained that were consistent with the standard differentiation approach, but with considerably smaller uncertainty.

Binary encounter electron production at relativistic velocities

Relativistic binary-encounter electrons produced in very fast collisions of bare and H-like Ar with carbon foils are studied as a function of laboratory frame emission angle. It is found that the velocity of the binary-encounter peak is consistent with a relativistic billiard ball model. The cross section for binary-encounter electron production is not in good agreement with a simple Rutherford scattering model. It is also found that the ratio of H-like to bare projectile cross sections is 1 at 0 degrees and becomes less than 1 at large emission angles.

A zero-degree tandem electron spectrometer and rydberg analyzer for projectile electron studies in ion-atom collisions☆

Abstract An electron spectrometer system consisting of a tandem 45° parallel-plate spectrometer preceded by a Rydberg analyzer has been developed to study projectile electron capture, ionization and excitation by 0° spectroscopy. This system has the ability to resolve a large number of excited and continuum atomic states of the projectile and has a low kinematic line broadening. High-n Rydberg electrons are field-ionized and decelerated in an inhomogeneous-field Rydberg analyzer. The field-ionized electrons are then energy-dispersed according to n-value. The design and performance of the system is presented with some results of continuum (cusp), Rydberg, Coster-Kronig and highly resolved (ΔE/E = 0.1–0.2%) K-Auger electrons for 0.5–1.75 MeV/amu, Fq++He, Ne, H2 collisions.

Polarization spectroscopy of O5+ (1s23p) states produced in the collisions of O6+ with He and H2

In this paper we describe polarization spectroscopy measurements of O5+(1s23p 2P3/2), following single-electron capture from He and H2. A novel arrangement for measurements of the polarization degree has been developed to reduce the systematic error. These measurements are shown as a function of projectile velocity, which was varied from 0.34 to 0.55 au. A simple model for the results with H2 is presented by considering the velocity dependence of capture cross sections for each 4l state and the cascade from 4l to 3p. However, for the case of the He target, no such simple model is found to be consistent with the experimental results.

Recoil ion momentum spectroscopy using magneto-optically trapped atoms

A novel apparatus has been developed in which atoms in a magneto-optical trap are used as targets in ion-atom collision experiments. The apparatus is an extension of earlier methodology in which the momentum of the recoiling target is measured and used to deduce the collision Q value and projectile scattering angle. In the present work, the low temperature of the target atoms yields increased momentum resolution, which in turn leads to improved Q value and scattering angle resolution. In addition, because the trapping process leaves some fraction of the atoms in an excited state, the new methodology is ideal for the study of collisions with excited targets. The prototypical system presented is low energy charge transfer between singly charged alkali ions and trapped rubidium atoms in the ground and first excited state.

MOTRIMS as a generalized probe of AMO processes

Magneto-optical trap recoil ion momentum spectroscopy (MOTRIMS) is one of the newest offshoots of the generalized TRIMS approach to ion–atom collisions. By using lasers instead of the more usual supersonic expansion to cool the target, MOTRIMS has demonstrated two distinct advantages over conventional TRIMS. The first is better resolution, now limited by detectors instead of target temperature. The second is its suitability for use in the study of laser-excited targets. In this presentation we will present a third advantage: The use of MOTRIMS as a general-purpose probe of AMO processes in cold atomic clouds of atoms and molecules. Specifically, the projectile ion beam can be used as a probe of processes as diverse as target dressing by femtosecond optical pulses, photo-association (laser-assisted cold collisions) photo-ionization, and electromagnetically-induced transparency. We will present data for the processes we have investigated, and speculations on what we expect to see for the processes we plan to investigate in the future.

Electron capture from elliptic Rydberg states: impact perpendicular to the minor axis

The total cross section, , for electron capture by Na+ ions from oriented coherent elliptic states (CES) of Li with principal quantum number n = 25 was studied experimentally for impact perpendicular to the minor axis of the elliptic orbit. The remaining geometrical parameters of the CES, which are the eccentricity, e , and the angle, , between the major axis and the beam direction, were varied in the course of the experiments, as was the reduced impact velocity v r = nv , where v is the projectile velocity in atomic units. Several representative cuts were chosen within the parameter space (v r ,e , ) = (0.74-2.09, 0-1, 0-2 ). The velocity range includes the region of matching velocities (v r 1) where for given geometrical parameters attains its maximum value and the high-velocity region where is a strongly decreasing function of v r . The cross section depends sensitively on each geometrical parameter, and the dependences change dramatically as v r is varied. The spatial distribution of the CES apparently governs in some region of parameter space (v r ,e , ) and in another the momentum distribution prevails.

The n-dependence of electron capture from Rydberg states

The cross section, σn, for electron capture by Na+ ions from coherent elliptic states (CES) of Li was studied experimentally. The dependences on the principal quantum number, n, the eccentricity, e, and the spatial orientation of the CES were determined at the two reduced ion velocities vr = 1.20 and 1.68 (vr≡v/ve, where v is the ion speed and ve = 1/n the mean electron speed of the CES in atomic units). The ion velocity, , was perpendicular to the minor axis of the CES throughout the experiment, so the orientation is fully specified by the angle, , between and the electric dipole moment, , of the CES. According to a generalized correspondence principle, the reduced cross section, = lim n→∞σn/n4, is given accurately by classical mechanics, and it is a universal function of the scaled parameters of the collision, which in the present experiment are vr, e and . It is assumed that this result can be extrapolated to the range of n-values, 20-35, covered in this paper, i.e. σn = np where and p4 depend only weakly on n. The experimental data support the assumed existence of a universal cross section function and the expected n4-dependence is also confirmed at vr = 1.20, but at vr = 1.68 the n-dependence is closer to n3.

Dynamics of Rydberg states in crossed E and B fields: coherent elliptic states

The dynamics of the n = 25 shell of Li atoms in weak electric, E, and magnetic, B, fields was studied experimentally as a function of the rate of change of the electric field strength, , the strength, B, of a constant magnetic field and the angle between the two fixed field directions, . Prior to the variation of E, it was held constant at with strong enough to dominate the Stark-Zeeman manifold for the shell. The uppermost state, which is a coherent elliptic state (CES), was populated selectively by pulsed laser excitation. The field was subsequently varied without rotation from to at the constant rate . It is shown experimentally, in accordance with theory, that the dynamics is described by a single dimensionless parameter given by in atomic units. When the variation of E is so slow that the electron has time to adjust and the population then remains in the uppermost state of the manifold (adiabatic transformation), for smaller the population shifts to an interval of states centred near the middle of the manifold and at the wavefunction is almost frozen and the lowermost part of the manifold populated (diabatic transformation). The experimental findings for near-adiabatic evolution of the CES agree with theoretical results obtained in a non-relativistic, hydrogenic model but discrepancies are seen when non-hydrogenic states are populated for .