Showing papers on "Ionization published in 2011"

Probing single-photon ionization on the attosecond time scale.

Abstract: We study photoionization of argon atoms excited by attosecond pulses using an interferometric measurement technique. We measure the difference in time delays between electrons emitted from the 3s(2) and from the 3p(6) shell, at different excitation energies ranging from 32 to 42 eV. The determination of photoemission time delays requires taking into account the measurement process, involving the interaction with a probing infrared field. This contribution can be estimated using a universal formula and is found to account for a substantial fraction of the measured delay.

Time-Resolved Holography with Photoelectrons

Abstract: Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow the measurement of electron motion in real time. We present experiments in which metastable xenon atoms were ionized with intense 7-micrometer laser pulses from a free-electron laser. Holographic structures were observed that record underlying electron dynamics on a sublaser-cycle time scale, enabling photoelectron spectroscopy with a time resolution of almost two orders of magnitude higher than the duration of the ionizing pulse.

Probe measurements of electron-energy distributions in plasmas: what can we measure and how can we achieve reliable results?

Abstract: An electric-probe method for the diagnostics of electron-distribution functions (EDFs) in plasmas is reviewed with emphasis on receiving reliable results while taking into account appropriate probe construction, various measurement errors and the limitations of theories. The starting point is a discussion of the Druyvesteyn method for measurements in weakly ionized, low-pressure and isotropic plasma. This section includes a description of correct probe design, the influence of circuit resistance, ion current and plasma oscillations and probe-surface effects on measurements. At present, the Druyvesteyn method is the most developed, consistent and routine way to measure the EDF. The following section of the review describes an extension of the classical EDF measurements into higher pressures, magnetic fields and anisotropic plasmas. To date, these methods have been used by a very limited number of researchers. Therefore, their verification has not yet been fully completed, and their reliable implementation still requires additional research. Nevertheless, the described methods are complemented by appropriate examples of measurements demonstrating their potential value.

Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+

Abstract: Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these observations are in the range (1.7+-1.3)x10^-16 s^-1

Efficient computer modeling of organic materials. The atom–atom, Coulomb–London–Pauli (AA-CLP) model for intermolecular electrostatic-polarization, dispersion and repulsion energies

Abstract: An atom–atom intermolecular force field with subdivision of interaction energies into Coulombic-polarization, dispersion (London) and repulsion (Pauli) terms is presented. Instead of using fixed interaction functions for atomic species, atom–atom potential functions are calculated for each different molecule on the basis of a few standard atomic parameters like atomic numbers, atomic polarizability and ionization potentials, and of local atomic point charges from Mulliken population analysis. The energy partitioning is conducted under guidance from the more accurate evaluation of the same terms by the PIXEL method, also highlighting some intrinsic deficiencies of all atom–atom schemes due to the neglect of penetration energies in Coulombic terms on localized charges. The potential energy scheme is optimized for H, C, N, O, Cl atoms in all chemical connectivities and can be extended to F, S, P, Br, I atoms with minor modifications. The scheme is shown to reproduce the sublimation heats of 154 organic crystal structures, to reproduce about 400 observed crystal structures without distortion, and to reproduce heats of evaporation and specific gravities of 12 common organic liquids. It is therefore suitable for both static and evolutionary (Monte Carlo) molecular simulation. Fine tuning of the four terms for specific systems can be easily performed on the basis of chemical intuition, by the introduction of one overall damping factor for each of them. The scheme is embedded in a suite of Fortran computer programs portable on any platform. For reproducibility and general use, source codes are available for distribution.

Fast gas heating in a nitrogen–oxygen discharge plasma: I. Kinetic mechanism

Abstract: A model of fast heating of nitrogen?oxygen mixtures excited by a gas discharge in a broad range of reduced electric fields E/N is presented. It is shown that in air at E/N ? 200?Td the main contribution to gas heating occurs due to dissociation reactions by electron impact of O2 molecules and due to processes of quenching of electronically excited N2(B?3?g, C?3?u, ) molecules by oxygen and excited O(1D) atoms by nitrogen. At E/N > 400?Td, dissociation reactions by electron impact of N2 molecules are dominant as well as the processes involving charged particles. The fraction of discharge energy converted to fast gas heating does not exceed 40%. An analysis of the experimental data on fast air heating in discharges at high reduced electric fields E/N is given.It was shown that, in a broad range of reduced electric fields, a fixed fraction of discharge power ?E spent on the excitation of electronic degrees of freedom, ionization and dissociation of molecules is converted to fast heating of nitrogen?oxygen mixtures. In air, the value of ?E is about 30 ? 3%. The value of ?E diminishes with decreasing share of oxygen in a mixture. The significant role of heat release in the pooling reactions of molecules for fast gas heating in pure nitrogen and in nitrogen with small admixtures of oxygen is demonstrated. The simulation results agree with experimental data at E/N < 200?Td within the range of oxygen content ? = 0?20%.

Laser Wakefield Acceleration Beyond 1 GeV using Ionization Induced Injection

Abstract: A series of laser wakefield accelerator experiments leading to electron energy exceeding 1 GeV are described. Theoretical concepts and experimental methods developed while conducting experiments using the 10 TW Ti:Sapphire laser at UCLA were implemented and transferred successfully to the 100 TW Callisto Laser System at the Jupiter Laser Facility at LLNL. To reach electron energies greater than 1 GeV with current laser systems, it is necessary to inject and trap electrons into the wake and to guide the laser for more than 1 cm of plasma. Using the 10 TW laser, the physics of selfguiding and the limitations in regards to pump depletion over cm-scale plasmas were demonstrated. Furthermore, a novel injection mechanism was explored which allows injection by ionization at conditions necessary for generating electron energies greater than a GeV. The 10 TW results were followed by self-guiding at the 100 TW scale over cm plasma lengths. The energy of the selfinjected electrons, at 3x10 18 cm -3 plasma density, was limited by dephasing to 720 MeV. Implementation of ionization injection allowed extending the acceleration well beyond a centimeter and 1.4 GeV electrons were measured.

EVIDENCE FOR ULTRA-FAST OUTFLOWS IN RADIO-QUIET ACTIVE GALACTIC NUCLEI. II. DETAILED PHOTOIONIZATION MODELING OF Fe K-SHELL ABSORPTION LINES

Abstract: X-ray absorption line spectroscopy has recently shown evidence for previously unknown Ultra-fast Outflows (UFOs) in radio-quiet active galactic nuclei (AGNs). These have been detected essentially through blueshifted Fe XXV/XXVI K-shell transitions. In the previous paper of this series we defined UFOs as those highly ionized absorbers with an outflow velocity higher than 10,000 km s–1 and assessed the statistical significance of the associated blueshifted absorption lines in a large sample of 42 local radio-quiet AGNs observed with XMM-Newton. The present paper is an extension of that work. First, we report a detailed curve of growth analysis of the main Fe XXV/XXVI transitions in photoionized plasmas. Then, we estimate an average spectral energy distribution for the sample sources and directly model the Fe K absorbers in the XMM-Newton spectra with the detailed Xstar photoionization code. We confirm that the frequency of sources in the radio-quiet sample showing UFOs is >35% and that the majority of the Fe K absorbers are indeed associated with UFOs. The outflow velocity distribution spans from ~10,000 km s–1 (~0.03c) up to ~100,000 km s–1 (~0.3c), with a peak and mean value of ~42,000 km s–1 (~0.14c). The ionization parameter is very high and in the range log ξ ~ 3-6 erg s–1 cm, with a mean value of log ξ ~ 4.2 erg s–1 cm. The associated column densities are also large, in the range N H ~ 1022-1024 cm–2, with a mean value of N H ~ 1023 cm–2. We discuss and estimate how selection effects, such as those related to the limited instrumental sensitivity at energies above 7 keV, may hamper the detection of even higher velocities and higher ionization absorbers. We argue that, overall, these results point to the presence of extremely ionized and possibly almost Compton-thick outflowing material in the innermost regions of AGNs. This also suggests that UFOs may potentially play a significant role in the expected cosmological feedback from AGNs and their study can provide important clues on the connection between accretion disks, winds, and jets.

Discharge physics of high power impulse magnetron sputtering

Abstract: High power impulse magnetron sputtering (HIPIMS) is pulsed sputtering where the peak power exceeds the time-averaged power by typically two orders of magnitude. The peak power density, averaged over the target area, can reach or exceed 10 7 W/m 2 , leading to plasma conditions that make ionization of the sputtered atoms very likely. A brief review of HIPIMS operation is given in a tutorial manner, illustrated by some original data related to the self-sputtering of niobium in argon and krypton. Emphasis is put on the current–voltage–time relationships near the threshold of self-sputtering runaway. The great variety of current pulse shapes delivers clues on the very strong gas rarefaction, self-sputtering runaway conditions, and the stopping of runaway due to the evolution of atom ionization and ion return probabilities as the gas plasma is replaced by metal plasma. The discussions are completed by considering instabilities and the special case of “gasless” self-sputtering.

Damage and ablation thresholds of fused-silica in femtosecond regime

Abstract: We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7-300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

Evidence for Island Structures as the Dominant Architecture of Asphaltenes

Abstract: Laser desorption laser ionization mass spectra of 23 model compounds and 2 petroleum asphaltene samples are presented. These experiments involved desorption by irradiation with the 10.6 μm output of a CO2 laser followed by single-photon ionization with the 157 nm output of a fluorine excimer laser. The average molecular weight of the asphaltene samples agrees closely with that found previously using multiphoton ionization with the 266 nm output of a Nd:YAG laser. The fragmentation behavior as a function of ionization laser pulse energy is studied to evaluate which families of model compounds fragment differently from asphaltenes and, hence, can be excluded from being dominant in asphaltenes. All model compounds having one aromatic core with and without various pendant alkyl groups show little to no fragmentation, mimicking the behavior observed for the two asphaltene samples, whereas all model compounds having more than one aromatic core show energy-dependent fragmentation. These observations support the ...

All-optical cascaded laser wakefield accelerator using ionization-induced injection.

Abstract: We report on near-GeV electron beam generation from an all-optical cascaded laser wakefield accelerator (LWFA). Electron injection and acceleration are successfully separated and controlled in different LWFA stages by employing two gas cells filled with a $\mathrm{He}/{\mathrm{O}}_{2}$ mixture and pure He gas, respectively. Electrons with a Maxwellian spectrum, generated from the first LWFA assisted by ionization-induced injection, were seeded into the second LWFA with a 3-mm-thick gas cell and accelerated to be a 0.8-GeV quasimonoenergetic electron beam, corresponding to an acceleration gradient of $187\text{ }\text{ }\mathrm{GV}/\mathrm{m}$. The demonstrated scheme paves the way towards the multi-GeV laser accelerators.

Gate-controlled ionization and screening of cobalt adatoms on a graphene surface

Abstract: By varying the voltage on an isolated gate electrode beneath a graphene sheet, the ionization state of cobalt atoms on its surface can be controlled. This enables the electronic structure of individual ionized atoms, and the resulting cloud of screening electrons that form around them, to be obtained with a scanning tunnelling microscope.

Nonlinear Atomic Response to Intense Ultrashort X Rays

Abstract: The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of ${\mathrm{Ne}}^{9+}$ is observed at x-ray frequencies below the ${\mathrm{Ne}}^{8+}$, $1{s}^{2}$ absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of ${\mathrm{Ne}}^{8+}$ ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.

Slater half-occupation technique revisited: the LDA-1/2 and GGA-1/2 approaches for atomic ionization energies and band gaps in semiconductors

Abstract: The very old and successful density-functional technique of half-occupation is revisited [J. C. Slater, Adv. Quant. Chem. 6, 1 (1972)]. We use it together with the modern exchange-correlation approximations to calculate atomic ionization energies and band gaps in semiconductors [L. G. Ferreira et al., Phys. Rev. B 78, 125116 (2008)]. Here we enlarge the results of the previous paper, add to its understandability, and show when the technique might fail. Even in this latter circumstance, the calculated band gaps are far better than those of simple LDA or GGA. As before, the difference between the Kohn-Sham ground state one-particle eigenvalues and the half-occupation eigenvalues is simply interpreted as the self-energy (not self-interaction) of the particle excitation. In both cases, that of atomic ionization energies and semiconductor band gaps, the technique is proven to be very worthy, because not only the results can be very precise but the calculations are fast and very simple.

Gate-Controlled Ionization and Screening of Cobalt Adatoms on a Graphene Surface

Abstract: By varying the voltage on an isolated gate electrode beneath a graphene sheet, the ionization state of cobalt atoms on its surface can be controlled. This enables the electronic structure of individual ionized atoms, and the resulting cloud of screening electrons that form around them, to be obtained with a scanning tunnelling microscope.

Ionization by drift and ambipolar electric fields in electronegative capacitive radio frequency plasmas.

Abstract: Unlike � - and � -mode operation, electrons accelerated by strong drift and ambipolar electric fields in the plasma bulk and at the sheath edges are found to dominate the ionization in strongly electronegative discharges. These fields are caused by a low bulk conductivity and local maxima of the electron density at the sheath edges, respectively. This drift-ambipolar mode is investigated by kinetic particle simulations, experimental phase-resolved optical emission spectroscopy, and an analytical model in CF4. Mode transitions induced by voltage and pressure variations are studied.

Synergistic Effect of Graphene Oxide/MWCNT Films in Laser Desorption/Ionization Mass Spectrometry of Small Molecules and Tissue Imaging

Abstract: Matrix-assisted laser desorption/ionization mass spectrometry has been considered an important tool for various biochemical analyses and proteomics research. Although addition of conventional matrix efficiently supports laser desorption/ionization of analytes with minimal fragmentation, it often results in high background interference and misinterpretation of the spatial distribution of biomolecules especially in low-mass regions. Here, we show design, systematic characterization, and application of graphene oxide/multiwalled carbon nanotube-based films fabricated on solid substrates as a new matrix-free laser desorption/ionization platform. We demonstrate that the graphene oxide/multiwalled carbon nanotube double layer provides many advantages as a laser desorption/ionization substrate, such as efficient desorption/ionization of analytes with minimum fragmentation, high salt tolerance, no sweet-spots for mass signal, excellent durability against mechanical and photoagitation and prolonged exposure to amb...

Impact of hollow-atom formation on coherent x-ray scattering at high intensity

Abstract: X-ray free-electron lasers (FELs) are promising tools for structural determination of macromolecules via coherent x-ray scattering. During ultrashort and ultraintense x-ray pulses with an atomic-scale wavelength, samples are subject to radiation damage and possibly become highly ionized, which may influence the quality of x-ray scattering patterns. We develop a toolkit to treat detailed ionization, relaxation, and scattering dynamics for an atom within a consistent theoretical framework. The coherent x-ray scattering problem including radiation damage is investigated as a function of x-ray FEL parameters such as pulse length, fluence, and photon energy. We find that the x-ray scattering intensity saturates at a fluence of $~$${10}^{7}$ photon/\AA{}${}^{2}$ per pulse but can be maximized by using a pulse duration much shorter than the time scales involved in the relaxation of the inner-shell vacancy states created. Under these conditions, both inner-shell electrons in a carbon atom are removed, and the resulting hollow atom gives rise to a scattering pattern with little loss of quality for a spatial resolution $g1$ \AA{}. Our numerical results predict that in order to scatter from a carbon atom 0.1 photon per x-ray pulse, within a spatial resolution of 1.7 \AA{}, a fluence of $1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ photons/\AA{}${}^{2}$ per pulse is required at a pulse length of 1 fs and a photon energy of 12 keV. By using a pulse length of a few hundred attoseconds, one can suppress even secondary ionization processes in extended systems. The present results suggest that high-brightness attosecond x-ray FELs would be ideal for single-shot imaging of individual macromolecules.

The Nature of LINER-like Emission in Red Galaxies

Abstract: Passive red galaxies frequently contain warm ionized gas and have spectra similar to low-ionization nuclear emission-line regions (LINERs). Here we investigate the nature of the ionizing sources powering this emission, by comparing nuclear spectroscopy from the Palomar survey with larger aperture data from the Sloan Digital Sky Survey. We find the line emission in the majority of passive red galaxies is spatially extended; the Halpha surface brightness profile depends on radius (r) as r^(-1.28). We detect strong line ratio gradients with radius in [N II]/Ha, [S II]/Ha, and [O III]/[S II], requiring the ionization parameter to increase outwards. Combined with a realistic gas density profile, this outward increasing ionization parameter convincingly rules out AGN as the dominant ionizing source, and strongly favors distributed ionizing sources. Sources that follow the stellar density profile can additionally reproduce the observed luminosity-dependence of the line ratio gradient. Post-AGB stars provide a natural ionization source candidate, though they have an ionization parameter deficit. Velocity width differences among different emission lines disfavor shocks as the dominant ionization mechanism, and suggest that the interstellar medium in these galaxies contains multiple components. We conclude that the line emission in most LINER-like galaxies found in large aperture (>100pc) spectroscopy is not primarily powered by AGN activity and thus does not trace the AGN bolometric luminosity. However, they can be used to trace warm gas in these red galaxies.

Timing the release in sequential double ionization

Abstract: An ‘attoclock’ that measures the relative release time of electrons during double ionization is now presented. The technique enables investigation of the subtle differences between sequential and non-sequential ionization when elliptically polarized light is used to excite two electrons from argon atoms.

Nonadiabatic tunneling in circularly polarized laser fields: Physical picture and calculations

Abstract: We consider selectivity of strong-field ionization in circularly polarized laser fields to the sense of electron rotation in the laser polarization plane in the initial state. We show that, in contrast to the textbook examples of one-photon ionization and bound-state excitations with increase in the electron angular momentum, and also in contrast to the well-studied ionization of Rydberg atoms in microwave fields, which all prefer corotating electrons, optical tunneling selectively depletes states where the electron initially rotates against the laser field. We also show that key assumptions regarding adiabaticity of optical tunneling may quickly become inaccurate in typical experimental conditions.

Surface layer accretion in conventional and transitional disks driven by far-ultraviolet ionization

Abstract: Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from {approx}0.01 {mu}m sized grains to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01-0.1 g cm{sup -2}, behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii {approx}> 1-10 AU. Surface layer accretion driven by FUV ionization can reproduce the trend of increasing accretion rate with increasing hole size seen in transitional disks. At radii {approx}<1-10 AU, FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance, and unless turbulent mixing of plasma can thicken the MRI-active layer, an additional means of transport is needed. In the case of transitional disks, it could be provided by planets.

Double-core-hole spectroscopy for chemical analysis with an intense X-ray femtosecond laser

Abstract: Theory predicts that double-core-hole (DCH) spectroscopy can provide a new powerful means of differentiating between similar chemical systems with a sensitivity not hitherto possible. Although DCH ionization on a single site in molecules was recently measured with double- and single-photon absorption, double-core holes with single vacancies on two different sites, allowing unambiguous chemical analysis, have remained elusive. Here we report that direct observation of double-core holes with single vacancies on two different sites produced via sequential two-photon absorption, using short, intense X-ray pulses from the Linac Coherent Light Source free-electron laser and compare it with theoretical modeling. The observation of DCH states, which exhibit a unique signature, and agreement with theory proves the feasibility of the method. Our findings exploit the ultrashort pulse duration of the free-electron laser to eject two core electrons on a time scale comparable to that of Auger decay and demonstrate possible future X-ray control of physical inner-shell processes.

Femtosecond Nonlinear Fiber Optics in the Ionization Regime

Abstract: By using a gas-filled kagome-style photonic crystal fiber, nonlinear fiber optics is studied in the regime of optically induced ionization. The fiber offers low anomalous dispersion over a broad bandwidth and low loss. Sequences of blueshifted pulses are emitted when 65 fs, few-microjoule pulses, corresponding to high-order solitons, are launched into the fiber and undergo self-compression. The experimental results are confirmed by numerical simulations which suggest that free-electron densities of ∼10(17) cm(-3) are achieved at peak intensities of 10(14) W/cm(2) over length scales of several centimeters.

Non-equilibrium plasma in liquid water: dynamics of generation and quenching

Abstract: In most cases, the electric breakdown of liquids is initiated by the application of high electric field on the electrode, followed by rapid propagation and branching of plasma channels. Typically plasmas are only considered to exist through the ionization of gases and typical production of plasmas in liquids generates bubbles through heating or via cavitation and sustains the plasmas within those bubbles. The question arises: is it possible to ionize the liquid without cracking and void formation?To answer this question we used a pulsed power system with 32–220 kV pulse amplitude, 0.5–12 ns pulse duration, 150 ps rise time. The discharge cell had a point-to-plate geometry with a tip diameter of 100 µm. These parameters allowed us to observe non-equilibrium plasma generation. The measurements were performed with the help of a 4Picos ICCD camera. It was found that the discharge in liquid water forms on a picosecond time scale. The increase of emission intensity and plasma formation took 200–300 ps. The diameter of the excited region near the tip of the high-voltage electrode was ~1 mm. After this initial stage emission rapidly decreased and the plasma region became almost invisible after 500 ps. The absence of emission during the rest of the pulse is explained by a decrease of the electrical field on the boundary of the conductive zone.Thus we have demonstrated the possibility of formation of non-equilibrium plasma in the liquid phase and investigated the dynamics of excitation and quenching of non-equilibrium plasma in liquid water.

The nature of the warm/hot intergalactic medium. I. numerical methods, convergence, and O VI absorption

Abstract: We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium (WHIM). The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z ~ 0.5, declining to 35%-40% at z = 0. We construct samples of synthetic O VI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (1013 cm?2 N O VI 1015 cm?2). Models that include only collisional ionization provide better fits for high column density absorbers (N O VI 1014 cm?2). The distribution of O VI in density and temperature exhibits two populations: one at T ~ 105.5 K (collisionally ionized, 55% of total O VI) and one at T ~ 104.5 K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O VI provides an important tool for a WHIM baryon census.