A. Morris

Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 7.—The methyl radical

The HeI vacuum ultraviolet photoelectron spectrum of the methyl radical produced by pyrolysis of azomethane has been investigated. Three ionization potentials, corresponding to the formation of the 1A′1, 3E′ and 1E′ ionic states, have been observed with vertical ionization potentials of 9.84, 14.76 and 16.10 eV respectively. The observed band positions have been interpreted with the aid of both ab initio and semiempirical molecular orbital calculations.The band associated with the first ionization potential is the only one for which vibrational structure could be resolved. This has been investigated in both CH3 and CD3 using Hel and NeI radiation. From the Franck–Condon envelope of this band and the observed shifts on deuteration, it is concluded that the methyl radical is essentially planar in its ground electronic state. The factors which control the values of the out-of-plane deformation frequencies in the ground state of CH3 and CH+3 are discussed in detail. Jahn–Teller splitting has been detected in the band associated with the second ionization potential of CH3.

Lymphatic Clearance of the Brain: Perivascular, Paravascular and Significance for Neurodegenerative Diseases.

The lymphatic clearance pathways of the brain are different compared to the other organs of the body and have been the subject of heated debates. Drainage of brain extracellular fluids, particularly interstitial fluid (ISF) and cerebrospinal fluid (CSF), is not only important for volume regulation, but also for removal of waste products such as amyloid beta (Aβ). CSF plays a special role in clinical medicine, as it is available for analysis of biomarkers for Alzheimer’s disease. Despite the lack of a complete anatomical and physiological picture of the communications between the subarachnoid space (SAS) and the brain parenchyma, it is often assumed that Aβ is cleared from the cerebral ISF into the CSF. Recent work suggests that clearance of the brain mainly occurs during sleep, with a specific role for peri- and para-vascular spaces as drainage pathways from the brain parenchyma. However, the direction of flow, the anatomical structures involved and the driving forces remain elusive, with partially conflicting data in literature. The presence of Aβ in the glia limitans in Alzheimer’s disease suggests a direct communication of ISF with CSF. Nonetheless, there is also the well-described pathology of cerebral amyloid angiopathy associated with the failure of perivascular drainage of Aβ. Herein, we review the role of the vasculature and the impact of vascular pathology on the peri- and para-vascular clearance pathways of the brain. The different views on the possible routes for ISF drainage of the brain are discussed in the context of pathological significance.

Vacuum ultraviolet photoelectron spectroscopy of transient species: Part 11. The NH2(X 2B1) radical

The HeI photoelectron spectrum of the NH2(X 2 B 1) radical produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of NH2(X 2 B 1) to the X 3 B 1, 1 A 1 and 1 B 1 states of NH2 +. Measurement of the first two adiabatic ionization potentials of NH2 gave a value for the separation of the ground vibrational levels in NH2 +(X 3 B 1) and NH2 +(1 A 1) of 0·99 ±0·02 eV. Ionization energies of NH2 have been estimated using ab initio calculations which include the effects of electron correlation. These calculations indicate that two other bands arising from ionization of NH2, corresponding to formation of the 3 A 2 and 1 A 2 stages, should be observed in the HeI region but no clear evidence of these was obtained experimentally because of overlapping band problems. Spectra of the deuteriated radical were obtained and are consistent with these conclusions. Calculated and observed separations of the two lowest states of NH2 + are compared with those for the analogous ...

Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 4.—Difluoromethylene and ozone

He I and He II spectra are reported for the isoelectronic molecules CF2 and O3. New values are reported for the first and higher ionization potentials. The spectra are interpreted with the aid of semi-empirical molecular orbital calculations.

A photoelectron spectroscopic study of the ground state of CF+ via the ionization process CF+(X 1Σ+)←CF(X 2Π)

The CF (X 2Π) radical has been investigated with vacuum ultraviolet photoelectron spectroscopy and ab initio molecular orbital calculations. Only one band was observed and it was assigned to the ionization CF+(X 1Σ+)←CF(X 2Π). For this band, the adiabatic and vertical ionization potentials were measured as (9.11±0.01) and (9.55±0.01) eV, respectively, and analysis of the vibrational structure in this band allowed we and re for CF+(X 1Σ+) to be determined as (1840±30) cm−1 and (1.148±0.005) A. Comparison is made with other first row diatomic fluorides that have also been studied with PES.

Vacuum ultraviolet photoelectron spectroscopy of transient species

The He(I) photoelectron spectrum of the FO(X 2Π i ) radical, produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of FO(X 2Π) to the X 3Σ-, 1Δ and 1Σ+ states of FO-. Analysis of the vibrational structure in each band gave estimates of , r e and D e in the corresponding ionic state.

Temperature programmed desorption of molecular hydrogen from a Si(111) surface: Theory and experiment

New experimental temperature programmed desorption (TPD) data have been obtained under carefully controlled conditions for atomic deuterium on single crystal Si(111). A wide range of initial coverages from Θ=1.25 to 0.05 ML was used. It was found that the results could only be satisfactorily interpreted in terms of a two‐site adsorption model in which it is suggested that two formally second‐order reactions involving the monohydride (deuteride) contribute to the well‐known β1 desorption peak at ≊810 K with the relative importance of these two reactions changing with initial coverage. The pre‐exponential factors for these reactions were found to be 2 cm2 s−1 and 1 cm2 s−1 with corresponding activation energies of 57.5±2 kcal mol−1 and 56.5±2 kcal mol−1, respectively, for deuterium desorption when the energy difference between the two sites was taken to be 2.5 kcal mol−1. The other desorption channel (β2) was also found to exhibit second‐order kinetic behavior involving the dihydride (deuteride). In this ca...

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

Abstract The first band in the vacuum ultraviolet photoelectron spectrum of the silyl radical, corresponding to the process SiH 3 (X 1 A′ 1 ) ← SiH 3 (X 2 A 1 ), has been observed with HeI radiation. Extensive vibrational fine structure associated with the SiH 3 + deformation vibration was observed in this band and analysis of the structure gave a value of ω = 820 = 40 cm -1 for the out-of-plane deformation mode in the ion. The vertical and adiabatic ionization energies were measured as 8.74 = 0.01 eV and 8.14 ± 0.01 eV respectively and use of the latter value together with the established heat of formation of the silyl radical allows an improved heat of formation of SiH 3 - . Δ H 298 0 (SiH 3 - ) to be derived as 980 ± 7 kJ mol −1 .

The Cerebrovascular Basement Membrane: Role in the Clearance of β-amyloid and Cerebral Amyloid Angiopathy

Cerebral amyloid angiopathy (CAA), the accumulation of β-amyloid (Aβ) peptides in the walls of cerebral blood vessels, is observed in the majority of Alzheimer’s disease (AD) brains and is thought to be due to a failure of the aging brain to clear Aβ. Perivascular drainage of Aβ along cerebrovascular basement membranes (CVBMs) is one of the mechanisms by which Aβ is removed from the brain. CVBMs are specialized sheets of extracellular matrix that provide structural and functional support for cerebral blood vessels. Changes in CVBM composition and structure are observed in the aged and AD brain and may contribute to the development and progression of CAA. This review summarizes the properties of the CVBM, its role in mediating clearance of interstitial fluids and solutes from the brain, and evidence supporting a role for CVBM in the etiology of CAA.

The vacuum ultra-violet photoelectron spectrum of the SiO(X1Σ+) molecule

The vacuum ultra-violet photoelectron spectrum of the SiO(X1Σ+) molecule has been recorded and interpreted with the aid of both ab initio molecular orbital and multiple-scattering Xα calculations. Three bands have been observed with vertical ionization potentials of 11·61, 12·19 and 14·80 eV, corresponding to ionization to the X2Σ+, A2Π and B2Σ+ states of SiO+ respectively. Estimates of r e, and D e for these ionic states have been made.