R. W. Dunford

Optical production of metastable krypton

We have investigated a new scheme for excitation of the 5s, J = 2 metastable level of Kr (5s[3/2]J = 2) which can be readily extended to other rare gases. In the scheme, an ultraviolet (UV) lamp is used to create a population of Kr atoms in the 5s[3/2]J = 1 level in a gas cell. The excited atoms are then pumped to the 5p[3/2]J = 2 level, using 819 nm light from a Ti:sapphire laser, from which they decay to the metastable state with a branching ratio of 77%. We made two striking observations: (1) the laser power required to saturate the second step decreases markedly as a function of gas cell pressure, and (2) the UV photon flux is converted with very high efficiency (≈10%) to metastable atom flux. A Monte Carlo study of the scattering of UV photons in the cell reproduces the trends observed. The understanding achieved points to the design of a higher flux source of metastable atoms.

An x-ray probe of laser-aligned molecules

We demonstrate a hard x-ray probe of laser-aligned small molecules. To align small molecules with optical lasers, high intensities at nonresonant wavelengths are necessary. We use 95ps pulses focused to 40μm from an 800nm Ti:sapphire laser at a peak intensity of 1012W∕cm2 to create an ensemble of aligned bromotrifluoromethane (CF3Br) molecules. Linearly polarized, 120ps x-ray pulses, focused to 10μm, tuned to the Br 1s→σ* preedge resonance at 13.476keV, probe the ensemble of laser-aligned molecules. The demonstrated methodology has a variety of applications and can enable ultrafast imaging of laser-controlled molecular motions with Angstrom-level resolution.

K-edge x-ray-absorption spectroscopy of laser-generated Kr{sup +} and Kr{sup 2+}

Tunable, polarized, microfocused x-ray pulses were used to record x-ray absorption spectra across the K edges of Kr{sup +} and Kr{sup 2+} produced by laser ionization of Kr. Prominent 1s {yields} 4p and 5p excitations are observed below the 1s ionization thresholds in accord with calculated transition energies and probabilities. Due to alignment of 4p hole states in the laser-ionization process, the Kr{sup +} 1s {yields} 4p cross section varies with respect to the angle between the laser and x-ray polarization vectors. This effect is used to determine the Kr{sup +} 4p{sub 3/2} and 4p{sub 1/2} quantum state populations, and these are compared with results of an adiabatic strong-field ionization theory that includes spin-orbit coupling.

Electron bremsstrahlung in collisions of 223 MeV/ u He-like uranium ions with gaseous targets

The bremsstrahlung process in the domain of strong Coulomb fields has been investigated for and Ar target electrons colliding with He-like uranium ions at . The differential cross sections for bremsstrahlung were measured at laboratory observation angles of , , and . Substantial discrepancies were found between the experimental cross sections and the first-order Born approximation calculations. The reported data provide a new testing ground for non-perturbative treatment of the coupling between radiation and matter in the presence of strong fields.

A simple cross-correlation technique between infrared and hard x-ray pulses

We report a gas phase technique to establish the temporal overlap of ultrafast infrared laser and hard x-ray pulses. We use tunnel ionization of a closed shell atom in the strong field at the focus of an infrared laser beam to open a distinct x-ray absorption resonance channel with a clear fluorescence signature. The technique has an intrinsic response of a few femtoseconds and is nondestructive to the two beams. It provides a step-functionlike cross-correlation result. The details of the transient provide a diagnostic of the temporal overlap of the two pulses.

Characterization of the spatiotemporal evolution of laser-generated plasmas

We characterize the time evolution of ion spatial distributions in a laser-produced plasma. Krypton ions are produced in strong, linearly and circularly polarized optical laser fields (1014–1015 W/cm2). The Kr+ ions are preferentially detected by resonant x-ray absorption. Using microfocused, tunable x rays from Argonne’s Advanced Photon Source, we measure ion densities as a function of time with 10 μm spatial resolution for times ≤50 ns. For plasma densities of the order of 1014 cm−3, we observe a systematic expansion of the ions outward from the laser focus. We find the expansion timescale to be independent of the plasma density though strongly dependent on the plasma shape and electron temperature. The former is defined by the laser focus, while the latter is controlled by the laser polarization state. We have developed a fluid description assuming a collisionless quasineutral plasma, which is modeled using a particle-in-cell approach. This simulation provides a quantitative description of the observed...

Double K -shell photoionization of silver

We have investigated double K-shell vacancy production in x-ray photoionization of silver. Measurements were carried out with photon energies (50-90 keV) varying from below threshold to beyond the expected maximum of the double K-shell ionization cross section. The limit of asymptotically high energies was deduced from measurements of double K-shell ionization following K-electron capture by {sup 109}Cd nuclei. The photon energy dependence of the ratio of double to single K vacancies produced is compared to similar measurements in helium and models thereof. The dependence of that ratio on atomic number is assessed by combining these data with previous experimental and theoretical estimates. The results show a clear growth in the relative importance of the dynamical electron-electron scattering contribution in heavy atoms.

A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

Abstract A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.

Resonant transfer and excitation investigated via X-ray emission in U90+ → C collisions

Abstract Resonant transfer and excitation (RTE) followed by X-ray emission in highly charged uranium ions colliding with carbon atoms has been investigated by applying the X-ray-particle-coincidence technique. This renders possible a subshell differential investigation of the population and radiative stabilization of different excited intermediate states. Preliminary results for the measured RTE excitation function for U 90+ → C collisions are presented.

Photon angular distribution of Radiative Electron Capture into the M shell of He-like uranium ions at 110–140 MeV/u

Measurements of the photon angular distribution of Radiative Electron Capture into the M shell have been performed with He-like uranium ions in the range 110–140 MeV/u. In addition, L REC was studied at a projectile energy of 140 MeV/u. In both cases, the experimental data show an asymmetry around 90° and agree well with a fully relativistic theory.