C. Lewis Cocke

Observing the creation of electronic feshbach resonances in soft x-ray-induced O2 dissociation.

When an atom or molecule is ionized by an x-ray, highly excited states can be created that then decay, or autoionize, by ejecting a second electron from the ion. We found that autoionization after soft x-ray photoionization of molecular oxygen follows a complex multistep process. By interrupting the autoionization process with a short laser pulse, we showed that autoionization cannot occur until the internuclear separation of the fragments is greater than approximately 30 angstroms. As the ion and excited neutral atom separated, we directly observed the transformation of electronically bound states of the molecular ion into Feshbach resonances of the neutral oxygen atom that are characterized by both positive and negative binding energies. States with negative binding energies have not previously been predicted or observed in neutral atoms.

Precision control of carrier-envelope phase in grating based chirped pulse amplifiers.

It is demonstrated that the carrier-envelope (CE) phase of pulses from a high power ultrafast laser system with a grating-based stretcher and compressor can be stabilized to a root mean square (rms) value of 180 mrad over almost 2 hours, excluding a brief re-locking period. The stabilization was accomplished via feedback control of the grating separation in the stretcher. It shows that the long term CE phase stability of a grating based chirped pulse amplification system can be as good as that of lasers using a glass-block stretcher and a prism pair compressor. Moreover, by adjusting the grating separation to preset values, the relative CE phase could be locked to an arbitrary value in the range of 2pi. This method is better than using a pair of wedge plates to adjust the phase after the hollow-core fiber compressor. The CE phase stabilization after a hollow-core fiber compressor was confirmed by a CE-phase meter based on the measurement of the left-to-right asymmetry of electrons produced by above-threshold ionization.

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

Measurement of the Lamb-shift in hydrogen-like oxygen☆

Abstract Preliminary results have been obtained for the Lamb-shift in hydrogen-like oxygen by the anisotropy method. For these measurements, 32 MeV O5+ ions were obtained from the KSU EN tandem accelerator. The beam was excited by passage through a thin ( 2s 1 2 state. This beam was passed through a small prequenching magnetic field into a quenching chamber which has two proportional counter detectors viewing about 2 cm of the beam. One detector views parallel to a Stark mixing field and the other perpendicular to it. The Stark mixing field is obtained from the motional electric field as the beam ions pass through an applied magnetic field. The mean results from 35 separate determinations to date are: the anisotropy ratio R = 0.07246±0.00048, and the Lamb-shift S = 2192±15 GHz, in agreement with theory.

KSU project: CRYEBIS for producing slow, bare, heavy-ion beams

The design and construction of a cryogenic electron beam ion source is in progress at Kansas State University. In about two years this source should produce high yields of bare and few‐electron argon ions with low kinetic energies (∼keV–MeV). The design is similar to the latest French designs {CRYEBIS II [J. Arianer, C. Goldstein, H. Laurent, and M. Malard, IEEE Trans. Nucl. Sci. NS‐30, 2737 (1983)] and DIONE [B. Gastineau, J. Faure, and A. Courtois, Nucl. Instrum. Methods B9, 538 (1985)]}. However, several significant changes were incorporated in order to get a simpler and more flexible design. The key factors of the operation which are expected to limit the performance are discussed. The general design is outlined which allows significant future upgrading to obtain nongaseous as well as much heavier, highly ionized ions.