P. A. Heimann

Creation and diagnosis of a solid-density plasma with an X-ray free-electron laser

Matter with a high energy density (>105 joules per cm3) is prevalent throughout the Universe, being present in all types of stars and towards the centre of the giant planets; it is also relevant for inertial confinement fusion. Its thermodynamic and transport properties are challenging to measure, requiring the creation of sufficiently long-lived samples at homogeneous temperatures and densities. With the advent of the Linac Coherent Light Source (LCLS) X-ray laser, high-intensity radiation (>1017 watts per cm2, previously the domain of optical lasers) can be produced at X-ray wavelengths. The interaction of single atoms with such intense X-rays has recently been investigated. An understanding of the contrasting case of intense X-ray interaction with dense systems is important from a fundamental viewpoint and for applications. Here we report the experimental creation of a solid-density plasma at temperatures in excess of 106 kelvin on inertial-confinement timescales using an X-ray free-electron laser. We discuss the pertinent physics of the intense X-ray–matter interactions, and illustrate the importance of electron–ion collisions. Detailed simulations of the interaction process conducted with a radiative-collisional code show good qualitative agreement with the experimental results. We obtain insights into the evolution of the charge state distribution of the system, the electron density and temperature, and the timescales of collisional processes. Our results should inform future high-intensity X-ray experiments involving dense samples, such as X-ray diffractive imaging of biological systems, material science investigations, and the study of matter in extreme conditions.

Generation of subpicosecond X-ray pulses using RF orbit deflection

Abstract A technique is proposed for producing high average intensity X-ray radiation from a storage ring for studies of the ultrafast phenomena on a subpicosecond time scale. Two RF cavity accelerating structures excited in the E 110 mode can be installed in a storage ring to create vertical displacements of electrons correlated with their longitudinal position in the bunch. The magnitude of these displacements can be sufficient for the X-ray radiation of the electron bunch between accelerating structures to be viewed as produced by a large number of independent sources, each of a subpicosecond duration.

The soft x-ray instrument for materials studies at the linac coherent light source x-ray free-electron laser.

The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480-2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser.

Ultrafast x-ray diffraction using a streak-camera detector in averaging mode

We demonstrate an apparatus for measuring time-dependent x-ray diffraction. X-ray pulses from a synchrotron are diffracted by a pair of Si(111) crystals and detected with an x-ray streak camera that has single-shot resolution of better than 1 ps. The streak camera is driven by a photoconductive switch, which is triggered by 100-fs laser pulses at a repetition rate of 1 kHz. The laser and the streak camera are synchronized with the synchrotron pulses. In the averaging mode, trigger jitter results in 2-ps temporal resolution. We measured the duration of 5-keV pulses from the Advanced Light Source synchrotron to be 70ps.

Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity.

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

High resolution vacuum ultraviolet pulsed field ionization photoelectron band for OCS+(X 2Π): An experimental and theoretical study

The vacuum ultraviolet pulsed field ionization photoelectron (PFI-PE) band for OCS+(X 2Π) in the energy region of 11.09–11.87 eV has been measured using high resolution monochromatized synchrotron radiation. The ionization energies (IEs) for the formation of the (0,0,0) X 2Π3/2 and (0,0,0) 2Π1/2 states of OCS+ are determined to be 11.1831±0.0005 and 11.2286±0.0005 eV, respectively, yielding a value of 367±1.2 cm−1 for the spin–orbit splitting. Using the internally contracted multireference configuration interaction approach, three-dimensional potential energy functions (PEFs) for the OCS+(X 2Π) state have been generated and used in the variational Renner–Teller calculations of the vibronic states. The energies of all vibronic states (J=P) for J=1/2, 3/2, 5/2, and 7/2 have been computed in the energy range of ≈4000 cm−1 above the IE[OCS+(X 2Π3/2)] for the assignment of the experimental spectrum. By a minor modification of the ab initio PEFs, good correlations are found between the experimental and theoreti...

VUV high resolution and high flux beamline for chemical dynamics studies at the advanced light source

An undulator beamline, consisting of VUV high resolution and high flux branch lines, devoted to chemical dynamics experiments has been designed at the Advanced Light Source. The radiation source is an undulator having a 10-cm period, and the fundamental in the energy range from 6 to 30 eV is utilized by the experiments. The higher harmonics of the undulator due to the operation at high K values is suppressed by a novel gas filter. In the first branch, high-flux 2% bandwidth radiation is directed toward an end station for photodissociation. A photon flux calculation predicts 1016 photon/s at this end station. In the second branch, highly monochromatized radiation is sent to an end station for photoelectron spectroscopy and photoionization studies. For this purpose a vertical dispersion 6.65-m off-plane Eagle mounting was chosen for the monochromator in anticipation of achieving a resolving power of ∼ 5 × 104 with a 1200 grooves/mm grating and 1 × 105 to 2 × 105 with a 4800 grooves/mm grating.

A high-resolution vacuum ultraviolet photoionization, photoelectron, and pulsed field ionization study of CS2 near the CS2+(X 2Π3/2,1/2) thresholds

High-resolution photoionization efficiency (PIE) and pulsed field ionization photoelectron (PFI-PE) spectra for CS2 have been measured using coherent vacuum ultraviolet (VUV) laser radiation in the energy range of 81 050–82 100 cm−1. The PIE and threshold photoelectron (TPE) spectra for CS2 in the energy range of 80 850–82 750 cm−1 have also been obtained using synchrotron radiation for comparison with results of the VUV laser study. The analysis of the PIE spectra reveals three Rydberg series converging to the excited CS2+(2Π1/2) spin–orbit state. These series, with quantum defects of 1.430, 1.616, and 0.053, are associated with the [2Π1/2]npσu, [2Π1/2]npπu, and [2Π1/2]nfu configurations, respectively. The Stark shift effect on the ionization threshold of CS2 has been examined as a function of dc electric fields (F) in the range of 0.65–1071 V/cm. The observed F dependence of the Stark shift for the ionization onset of CS2 is consistent with the prediction by the classical adiabatic field ionization form...

High-resolution photoelectron spectroscopy using multibunch synchrotron radiation:: rotational-resolved photoelectron bands of O2+(b 4Σg−, v+)1

Abstract We report rotational-resolved single-photon threshold photoelectron and pulsed field ionization zero kinetic energy (PFI–ZEKE) photoelectron (PE) spectra of O2 in the energy range of 18.1–20.2 eV measured using high-resolution monochromatized multibunch undulator synchrotron radiation. The PFI–ZEKE PE bands for O2+(b 4Σg−, v+=0–9) have been simulated using the Buckingham–Orr–Sichel model derived for rotationally resolved single-photon ionization cross-sections. Only the ΔN=−2, 0 and +2 (or O, Q and S) rotational branches are observed for these PFI–ZEKE PE bands, indicating that the outgoing electron continuum channels with angular momenta l=1 and 3 dominate in the threshold ionization transitions O2+(b 4Σg−, v+=0–9, N+)←O2(X 3Σg−, v″=0, N″). The relative rotational branch intensities for O2+(b 4Σg−, v+=4 and 5) are found to be drastically different from those for O2+(b 4Σg−, v+=0–3, 6 and 7). Considering that the energies of O2+(b 4Σg−, v+=4 and 5) are close to the dissociation limit of O+(4S)+O(3P) and that the crossing location of the O2+(b 4Σg−) and d 4Σg+ potential curves is shown to be in the vicinity of O2+(b 4Σg−, v+=4 and 5), we suggest that the latter observation is the result of predissociative perturbations by the d 4Σg+ state. Within the uncertainties of this experiment, the ionization energies for the formation of O2+(b 4Σg−, v+=0–9, N+=1) can be satisfactorily characterized using a Morse potential. The effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=0, 2–5) prepared in the present experiment are nearly constant with values in the range of 1.8–2.0 μs. The observation that the effective lifetimes for high-n Rydberg states converging to O2+(b 4Σg−, v+=4 and 5) are significantly longer than the known dissociative lifetimes of the corresponding ionic states is in accordance with the expectation that the couplings between the O2+ ion core and the high-n Rydberg electron involved are negligibly small.

A high resolution pulsed field ionization photoelectron study of O2 using third generation undulator synchrotron radiation

We have improved a newly developed experimental scheme for high resolution pulsed field ionization photoelectron (PFI-PE) studies [Hsu et al., Rev. Sci. Instrum. (in press)] using the high resolution monochromatized multibunch undulator synchrotron source of the Chemical Dynamics Beamline at the Advanced Light Source. This improved scheme makes possible PFI-PE measurements with essentially no contamination by background electrons arising from direct photoionization and prompt autoionization processes. We present here a preliminary analysis of the rotationally resolved PFI-PE spectrum for O2 obtained at a resolution of 0.5 meV (full-width-at-half-maximum) in the photon energy range of 18.1–19.4 eV, yielding accurate ionization energies for the transitions O2+(b 4Σg−, v+=0–9, N+=1)←O2(X 3Σg−, v=0, N=1).