Michael G. White

Chemokines and receptors in HIV encephalitis

Background: Chemokines are involved in the migration of leukocytes and have been implicated in several inflammatory diseases of the central nervous system. Some of their receptors have been proposed to mediate HIV infection. Objective: To determine changes in chemokine and receptor expression in HIV encephalitis, and to determine whether upregulation leads to recruitment of infected monocytes across the blood‐brain barrier and participates in HIV neuropathology. Methods: Immunocytochemistry and double‐label immunofluorescent laser confocal microscopy was performed with antibodies to chemokines and their receptors on brain tissues from patients who died with or without HIV encephalitis. In vivo distribution was compared with in vitro cultures of human neuroglial cells. Results: The β‐chemokines monocyte chemotactic protein‐1, macrophage inflammatory protein‐1α, and RANTES were detected on brain macrophages. Their presence was associated with the histopathological signs of HIV encephalitis. The α‐chemokines IP‐10 (10 kDa inflammatory protein) and interleukin‐8 were expressed by astrocytes in all tissues, including controls. Presence of the CXC‐chemokine receptor (CXCR)‐4 was seen on brain macrophages/microglia, neurons, and astrocytes. CC‐Chemokine receptor (CCR)‐5 was detected only on macrophages/microglia. CCR‐3 and CCR‐1 were expressed by macrophages and endothelial cells. In vitro studies examining the presence of CCR‐3, CCR‐5, and CXCR‐4 on human brain cell cultures demonstrated abundant neuronal and microglial expression.Background:Chemokines are involved in the migration of leukocytes and have been implicated in several inflammatory diseases of the central nervous system. Some of their receptors have been proposed to mediate HIV infection.Objective:To determine changes in chemokine and receptor expression in HIV en

Rotationally resolved photoionization of O2+ near threshold

Abstract A delayed, pulsed field ionization technique is used in combination with a high-resolution VUV radiation source to measure the rotational structure associated with the X 2 Π g ←X 3 Σ g − (0, 0) ionizing transition in O 2 . The data have been fit to theoretical expressions derived by Buckingham, Orr and Sichel (1970) for rotationally resolved one-photon ionization cross sections. From the fit we obtain an accurate ionization potential for O 2 (97348±2 cm −1 ), as well as evidence that the outgoing l =3 continuum channel is important in the threshold ionization of O 2 .

Rotationally resolved threshold photoelectron spectrum of the methyl radical

We report the rotationally resolved, one‐photon threshold photoelectron spectrum of the methyl radical, CH3, produced by supersonic‐jet, flash pyrolysis. Only rotational transitions with ΔK=0, ±2 are observed and this result is shown to be consistent with photoionization selection rules in D3h symmetry. Assignment of the threshold photoelectron spectrum results in an adiabatic ionization potential of 79 349±3 cm−1.

Rotationally resolved photoionization of H2O

A rotationally resolved one‐photon threshold photoionization spectrum of jet‐cooled water (H2O and D2O) has been obtained by pulsed field ionization of extremely high‐n Rydberg states. Observed spectral intensities for both vibrationless (0,0,0) and vibrationally excited (1,0,0) water cation show a strong propensity for ΔN=0, ±1 transitions. In contrast to earlier work on O2 and HCl, the lack of large ΔN transitions suggests that ionization occurs with only small angular momentum transfers between the core and photoelectron. The presence of both type A and type C ‘‘symmetric top’’ transitions varies from the conclusions of a recent MQDT analysis of H2O photoionization, which predicts only type C transitions. Rotational analysis of the spectra yields improved ionization potentials for both H2O and D2O. The ionization potential of the (1,0,0) vibrational level provides a direct measurement of the symmetric stretch fundamental in H2O+ which is in excellent agreement with an earlier indirect determination. Ro...

Time‐of‐flight photoelectron spectroscopy of gases using synchrotron radiation

A gas-phase time-of-flight (TOF) photoelectron spectrometer has been developed for use with synchrotron radiation. The excellent time structure of the synchrotron radiation at the Stanford Positron Electron Accelerator Ring (SPEAR) has been used as the time base for the TOF measurements. The TOF analyzer employs two multichannel plates (MCPs) in tandem as a fast electron multiplier with a matched 50-Omega anode to form an electron detector with a timing resolution of </=70 ps. The spectrometer is presently capable of analyzing electrons over a wide energy range (1-50 eV) at medium energy resolution (</=5%) and high angular resolution (+/-3 degrees ).

Rotationally resolved threshold photoelectron spectra of OH and OD

The results of combined experimental and theoretical studies of the rotationally resolved photoelectron spectra of OH and OD following single‐photon ionization are presented. The measured zero‐kinetic‐energy (ZEKE) spectra were obtained using pulsed field ionization in conjunction with a vacuum ultraviolet laser source. The OH^+ and OD^+ (X ^3Σ^−, v^+=0) rotational distributions were studied over the range 95.0–95.4 nm. Agreement between the observed and calculated spectra is very encouraging. Improved values for the ionization potentials of OH and OD (104 989 and 105 085 ± 2 cm^(−1), respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed.

MPI photoelectron spectroscopy of ungerade excited states of acetylene: Intermediate state mixing and ion state selection

Three photon resonant, four photon (3+1) ionization spectroscopy and photoelectron spectroscopy have been used to study the ungerade excited states of acetylene in the energy range from 74 500 to 90 000 cm−1. Sharp bands from the nR (π3u nsσg) and 1Φu (π3u ndδg) Rydberg series dominate the MPI spectrum. A large number of Rydberg and valence states which are prominent in VUV absorption spectra are absent or weak in MPI studies. These weak bands are only observable under high power conditions, which suggests that nonradiative decay is rapid enough to depopulate these states before ionization occurs. The photoelectron results provide further insight into the nature of the excited states. Ionization through the sharp bands occurs via Δν=0 Franck–Condon transitions, resulting in ions in a single vibrational state. Ionization through bands which are mixed results in complicated ion vibrational distributions including excitation of both cis and trans bends.

Anomalous branch intensities in the threshold photoionization of HCl

The rotationally resolved threshold photoionization spectrum of HCl has been observed for ionization into both spin‐orbit components of the ground electronic state of HCl+. The data indicate an extreme asymmetry in the apportioning of angular momentum between the escaping photoelectron and the ion core. Observed transitions in which the core rotation decreases are found to be heavily favored compared to those where the core rotation increases by an equal amount. A mechanism of increased negative branch intensity due to field‐ or dipole‐induced mixing of Rydberg series converging to higher ion rotational levels is proposed as a possible explanation. Direct observation of the transition HCl (X 1Σ+,J‘=0)→HCl+ (X 2Π3/2,J+= (3)/(2) ) yields an ionization potential of 102 802.8±2 cm−1 for HCl.

Adsorbate-Driven Morphological Changes of a Gold Surface at Low Temperatures

Using STM, infrared absorption reflection spectroscopy experiments and density functional calculations we show that low temperature adsorption of CO on gold surfaces modified by vacancy islands leads to morphological changes and the formation of nanosized Au particles. These results demonstrate a dynamic response of a surface during adsorption with consequences for the surface reactivity.

Compact vacuum ultraviolet source for photoelectron spectroscopy

A simple source chamber is described which enables the introduction of laser‐generated, vacuum ultraviolet (VUV) radiation below 102 nm into most existing photoelectron and photoion spectrometers. A long capillary light guide provides a conductance barrier between the VUV source and experimental chambers, and also provides a small, well‐collimated VUV beam at the interaction region. A platinum photodiode is found to provide a useful measure of the VUV intensity with minimal interference from the unconverted UV light. The performance and capabilities of the source are demonstrated by several applications using photoelectron spectroscopy.