Structure and dynamics of atoms and molecules in strong transient fields

The goal of the program is to understand the structure and dynamics of atoms and molecules using photons and relativistic electrons as probes. The current program focussed on studying inner-shell x-ray photo-ionization and photo-excitation of atoms and laser-dressed atoms. The work at the ALS centers on studies of inner-shell photo-ionization and makes use of both synchrotron radiation and the newly constructed 1.5 GeV electron and photon beam line at the ALS booster.

I. Photo-ionization and double-ionization at relativistic energies (A. Belkacem and B. Feinberg)

At relativistic energies, the cross section for the atomic photoelectric effect drops off as does the cross section for liberating any bound electron through Compton scattering. However, when the photon energy exceeds twice the rest mass of the electron, ionization may proceed via electron-positron pair creation. We set up a first experiment to study this photo-ionization process, vacuum assisted photo-ionization, at an intense cobalt Source at the Lawrence Berkeley National Laboratory. A beam of 2x10⁸ photons/second, and 1.17 MeV and 1.33 MeV energies, is produced through collimation of an intense cobalt source. A signature of vacuum-assisted photo-ionization is given by the simultaneous detection of a K-vacancy and a positron emitted. We measure a ratio of vacuum-assisted photo-ionization to pair production for an Au target to be 2 x 10^-3, a value that is a factor of five larger than the theoretical value. This preliminary number constitutes the first observation of a new ionization mechanism that is entirely due to the relativistic nature of the photo-ionization process.

We used the same experimental set up to measure the ratio of double K-vacancy to single K-vacancy creation in Au. Using a target dependence measurement technique and extrapolating to very thin targets we obtain a value of 0.5-1 x10^-4. This low value is at the limit of sensitivity of this set up and the result should be considered an upper limit. Our experimental value agrees reasonably well with the theoretical values.

In order to study in more detail photo-ionization at higher energies we built a new beam line at the ALS booster ring, branching off the transfer line between the booster and the ALS storage ring. The 7-m long beam line consists of a front-end magnet to extract the 1.5 GeV electron beam, a radiator-target that produces bremsstrahlung photons. We use a high-Z high-density radiator (tungsten) to suppress the emission of low-energy (below 1 MeV) photons through the Landau-Pomeranchuk effect. Downstream from the radiator, a vertical magnet separates the produced photons from the electron beam. The two-beams enter the end-station simultaneously with the electron beam located 5-cm below the photon beam. This configuration allows us to carry electron impact as well as photon impact experiments with the same set up. Both beams are very stable with tunable intensities ranging from 10³ to 10¹⁰ electrons/second, 2-3 mm diameter and a repetition rate of 11 Hz. The end-station contains a thin Au target and an electron-positron spectrometer consisting of a permanent magnet and an array of scintillator detectors set in the horizontal plane. The energy of the electrons and positrons is given by the position of the scintillator detector hit. We adjusted the spectrometer to cover the energy range of 10 MeV to 100 MeV, an energy range in which the cross sections of vacuum-assisted photo-ionization, photo-ionization through the Compton effect and photoelectric effect are comparable. A germanium detector, set 2.5 cm from the Au target, is used to detect a K x-ray resulting from the filling of the Au inner-shell vacancy. The first measurement of 1.5 GeV electron impact ionization of Au K-shell yields a value of 25 barns, which is somewhat smaller than the value of 34 barns predicted by theory. However our current beam calibration is not yet reliable to point to a discrepancy theory-experiment. We are currently measuring and analyzing the first data of vacuum-assisted photo-ionization.

II. Laser-synchrotron two-color experiments (T. E. Glover and A. Belkacem)

Multiphoton processes driven by combined synchrotron (x-ray) and laser (optical) radiation provide a basis for novel scientific directions. From a fundamental perspective, synchrotron x-rays can probe unique states of matter formed while a gaseous or solid target is exposed to intense electromagnetic (laser) radiation. We performed a two-color experiment on a Si (111) photo-emission. Experiments are performed at the Advanced Light Source using ~700 eV synchrotron light and 800 nm (1 Khz, 100 fs - 40 ps) laser light. The laser intensities were varied from 10¹⁰ to 10¹⁴ W/cm2. A hemispherical analyzer records x-ray photoelectron spectra (XPS) in the vicinity of the Si 2p photo-emission peak and the arrival time of laser pulses (relative to the x-ray pulses) is varied using an optical delay arm. The ALS is operated in two-bunch mode and electronic gating is used to collect photoelectrons produced by ALS pulses temporally overlapped (or nearly overlapped) with laser pulses. We observe two modifications to the XPS when pulses are overlapped. First, the Si 2p peak is shifted to lower binding energy (up to 1 eV at high laser intensities). Second, the peak is broadened. These laser-induced modifications are transient. We note two possible mechanisms for the observed effects. First, laser dressing of the continuum can produce a lower amplitude, broader peak (via scattering of laser photons) while also shifting the XPS to lower binding energies (via AC stark shift). A second possibility, the 2p binding energy may be shifted as a result of highly transient excited states of Si created by the laser. Experiments are still in progress to shed some light on the mechanisms involved and possibly distinguish between the two possibilities.

III. Dynamics of ionization mechanisms in relativistic collisions involving heavy and highly charged ions. (A. Belkacem, D. Ionescu, and A. H. Sorensen)

The dynamics of mechanisms associated with the ionization of inner-shell electrons in relativistic collisions involving heavy and highly charged ions is investigated within a nonperturbative approach formulated in the time domain. The theoretical approach is based on the exact numerical solution of the time-dependent Dirac equation for two-Coulomb centers on a lattice in momentum space. We used the T3E supercomputer of the National Energy Research Supercomputer Center (NERSC) to solve numerically the time-dependent Dirac equation in relativistic heavy ion atomic collisions. By investigating the time development of the wave function associated with an initially bound target electron we obtain a direct and complete visualization of the dynamics in ion-atom collisions.

In figure1 we display snapshots of the electron probability density Y †(r,t) Y(r,t) in coordinate space at times t=22 and t=27, in natural units. The system shown is for 100 MeV/n U⁹²⁺ on Au⁷⁸⁺. Note that in the 3-D plots the projectiles moves from left to right and in the contour plots the projectile moves from down to up. In order to unravel the specific features of the different excitation mechanisms, we eliminated contributions associated with transitions to the target K-shell in the contour plots shown in Fig. 1. This is achieved by the coherent subtraction of the target 1s-state, weighted by the complex-valued 1s-transition amplitude, from the complete time-evolved wave function. Of particular interest are structures appearing in the contour plots in the region located halfway between the target and the projectile. In addition to the local maxima at the target and at the projectile positions, the probability density displays a third local maximum located in the middle. In an animation of the collision we see this maximum split dynamically into an electron density moving symmetrically and perpendicular with respect to the beam direction. This is interpreted as an ionization mechanism in which electrons are emitted with relatively high transverse velocities with respect to the projectile trajectory. Note that in contrast to the saddle point emission mechanism known from slow ion-atom collisions, these electrons have velocities higher than that of the projectile ion.

III. Future plans

Finish the measurements of the cross section of vacuum-assisted photo-ionization for Au and Ag targets. Modify the detection technique for the inner-shell from fluorescence to the detection of Auger electrons to extend the measurement to lower Z-targets. In the next step we plan to use a gas target (Ne, Ar and Xe) and a COLTRIMS to discriminate between the two competing mechanisms (pair creation on the nucleus and pair creation on the electron) that contribute to the vacuum-assisted photo-ionization cross section.

At relativistic energies the transverse fields induced by the electrons on a target are multiplied by the Lorentz factor which in this case is 3 x 10³. Different from the situation at low energies, at relativistic energies, the corresponding magnetic field has the same amplitude as the electric field. Thus the relativistic electron is equivalent to a very intense photon pulse with a duration of 10-21 s. We plan to use the COLTRIMS at the 1.5 GeV electron beam line to study the orientation effects in the dissociation of simple molecules by this highly transient and strong transverse field.

We plan to continue the measurements of the modifications of 2p Si photo-electron spectrum as a function of the time delay between the femtosecond laser and the x-ray on the ALS beam line 5.3.1. Parallel to this we will install a gas jet to perform gas phase studies in Argon using the COLTRIMS. These new studies will focus on the modifications of x-ray ionization and x-ray excitation of atoms and molecules dressed with femtosecond high-power laser light.