Session M1: Focus Session: Photoionization Spectroscopy

X-ray absorption spectroscopy of photoionised plasmas at Z

Photoionised plasmas are found in astrophysical environments such as x-ray binaries, active galactic nuclei, and in the accretion disks of compact objects. The Z facility at Sandia National Laboratories is a powerful source of x-rays that enables us to produce and study in the laboratory photoionised plasmas relevant for astrophysics under well characterized conditions. We discuss an experimental and theory/modeling effort in which the intense x-ray flux emitted at the collapse of a z-pinch experiment conducted at Z is employed to produce a neon photoionized plasma. The broad-band x-ray radiation flux from the z-pinch is used to both create the neon photoionised plasma and provide a source of backlighting photons to study the atomic kinetics through K-shell line absorption spectroscopy. The plasma is contained in a cm-scale gas cell located at about 5 cm from the z-pinch, and the filling pressure is carefully monitored all the way to shot time since it determines the particle number density of the plasma. Time-integrated and gated transmission spectra are recorded with a TREX spectrometer equipped with two elliptically-bent crystals and a set of slits to record up to six spatially-resolved spectra per crystal in the same shot. The spectral resolution is approximately 1000. The transmission data shows line absorption transitions in several ionization stages of neon including Be-, Li-, He- and H-like Ne ions. Detailed modeling calculations of the absorption spectra are used to interpret and model the high-resolution transmission spectra recorded in the Z experiments with the goal of extracting the ion population distribution of the plasma. Furthermore, the analysis of the gated data provides a window into the dynamics of the photoionized plasma. The data analysis is performed with the aid of a novel application of genetic algorithms to plasma spectroscopy.   

Spectroscopic Analysis of K and L-shell emission with K-$\alpha $ lines for Cu

Significant X-ray emissions from moderately high atomic number plasmas such as Cu accompany substantial K-$\alpha$ radiation. K-emission spectroscopy is a convenient means to diagnose high temperature plasmas, as K-shell vacancies occur due to inner-shell excitation and ionization by hot electrons and photoionization. For plasmas in collisional equilibrium, K-$\alpha$ emission occurs usually from highly charged ions due to the high electron temperatures required for appreciable excitation of the K-$\alpha$ transitions. Our investigation will focus on K-$\alpha$ flux generated primarily by impacting electrons in thermal distribution. We will analyze the ionization dynamics and generate Cu spectrum using the temperature and density conditions obtained from non-LTE simulations and compare our results with recent K-shell experimental spectrum of Cu implosions on the Z machine at the Sandia national Laboratories. Our self-consistently generated atomic model employs an extensive atomic level structure and data to accurately model the plasma dynamics and the spectroscopic details of the emitted radiation. Spectroscopic modeling of these K- and L-shell emission should also provide quantitative diagnostics of plasma parameters.   

Low-Resolution Photodetachment Spectroscopy of the S$_{2}^{-}$ Ion

Numerous experiments have investigated the properties and dynamics of single- atom negative ions. Similar experiments can be conducted with molecular negative ions. Laser photodetachment spectroscopy of such ions is more complicated due to rotational and vibrational structure, and often yields spectroscopic benchmarks such as rotational constants. We have conducted low-resolution photodetachment spectroscopy of the S$_{2}^{-}$ ion. The ions are created in a Penning ion trap by a two-step dissociative attachment process. The photodetachment is achieved with a tunable ring-cavity titanium:sapphire laser. Our results yield a lower-limit estimate of the minimum detachment threshold energy and exhibit structure that may be due to rotational energy levels. Future experiments will focus on high-resolution detachment spectroscopy of these and other ions with an eye toward measurement of their molecular constants.   

Photoionization of Li

The time-dependent close-coupling approach to multiple photoionization of lithium is presented. Double photoionization of lithium can be treated as a two-electron ejection process where the outgoing electrons move in the field of a ``frozen-core" Li$^{2+}$ $1s$ state. Recent calculations of this process have resulted in total and triple differential cross sections that are in good agreement with other close-coupling approaches. The time-dependent approach can also be extended to treat the interaction of all three lithium electrons, as is required if triple photoionization is examined, that is, the simultaneous ejection of all three electrons from lithium. The most detailed information about this process is found in the fully angular and energy differential cross sections, which provide information as to how the ionized electrons leave the atom. We present our formulation of the fully differential cross section expression, and provide some convergence studies of the angular distributions.   

Photoelectron Angular Distributions from Xe in a DC Electric Field

Velocity map photoelectron imaging provides a powerful and efficient means to record both high resolution photoelectron spectra and photoelectron angular distributions simultaneously. As it is often implemented, however, the electric field in the interaction region is non-negligible, ranging from ~50 - 1000 V/cm. We have used this aspect of the technique to study the decay of autoionizing Stark states of Xe in fields between 50 and 700 V/cm. Specifically, we have used two-photon excitation to prepare the Xe 6p'[1/2]$_{0}$ state, and probed one-photon transitions from that state to autoionizing resonances converging to the $^{2}$P$_{1/2}$ spin-orbit excited state of Xe$^{+}$. The polarization of the probe is aligned to excite only the M = 1 and -1 states. At low fields, the expected ns' and nd' Rydberg series are observed, but at higher external DC fields the np' series appear, as well the hydrogenic manifold of Stark states. Although the external field breaks the cylindrical symmetry required for the usual image reconstruction methods, the raw images provide insight into the nature of the decay mechanism and the Stark mixings.   

Electron Correlation and Molecular Dynamics in Ethylene after Photo Double Ionization

As the simplest molecule with a carbon double bond, ethylene is a benchmark system for studying intershell and intrashell photo ionization processes and track down electron correlation. Moreover, non-adiabatic effects such as the transition through two conical intersections present in ethylene play an important role in understanding the molecular dynamics of this system. Therefore, we have investigated the electron correlation and the influence of molecular dynamics upon photo double ionization of ethylene molecules with a single photon (41 eV) in a kinematically complete experiment (COLTRIMS technique). The different pathways leading to dissociation could be identified including the symmetric break-up, deprotonation, and molecular rearrangement. Surprisingly, a stable dication of ethylene was not observed, raising the question what inhibits the release of two electrons from the pi orbital or the stability of the dication.   

Updated square-well potential model for $A$@C$_{60}$ photoionization

The empirical square-well potential model for photoionization of atoms $A$ associated with endo-fullerenes $A$@C$_{60}$ [1] has played an important role in providing the initial understanding of their spectra. The model approximates the C$_ {60}$ cage by a square-well potential of a certain inner radius $r_{o}$, width $\Delta$, and depth $U_{o}$. Their values were determined by approximately matching the thus theoretically calculated and experimentally measured photoionization cross section of \textit{empty} C$_{60}$ [2]. The recent, first of its kind, experiment [3] on $A$@C$_{60}$ (namely, Xe@C$_{60}^{+} $) photoionization has revealed \textit{quantitative} discrepancies between predictions of the model and experimental data. To us, this implies that $r_{o}$, $\Delta$, and $U_{o}$ for \textit{empty} C$_{60}$ might not be the same as for an \textit{endo-fullerene} $A$@C$_{60}$. We thus find, and report on, updated $r_{o}$, $\Delta$, and $U_{o}$ which bring on a better quantitative agreement between the model and experiment. \\[4pt] [1] V. K. Dolmatov, Adv. Quant. Chem. \textbf{58}, 13 (2009).\\[0pt] [2] Y. B. Xu \textit{et.\ al.}, PRL \textbf{76}, 3538 (1996).\\[0pt] [3] A. L. D. Kilcoyne \textit{et.\ al.}, PRL \textbf{105}, 213001 (2010).