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At least 73 records · Page 4Linked to original sources
PubMed2000

Transmission Electron Diffraction at 200 eV and Damage Thresholds below the Carbon K Edge.

Transmission electron diffraction patterns from ultra-thin aromatic and aliphatic organic films at beam energies of 200 eV-1 keV have been recorded in a custom low energy electron transmission (LEET) chamber. A significant reduction of the molecular damage cross-section, measured by fading of diffraction spots, was found for thin films of the aromatic perylene when the beam energy was reduced from 400 to 200 eV. The corresponding measurements for the aliphatic tetracontane showed a smaller "threshold energy" and the differences are discussed. Electron beam damage from other aromatic materials has also been studied at low energy. Comparison of the carbon K shell ionization cross-section and the measured damage cross-sections show that carbon K-shell ionization is strongly correlated with the damage observed in aromatics at beam energies higher than 284 eV. Calculation of the minimum number of unit cells needed for imaging a single molecule, and comparison of calculated elastic with measured damage cross-sections both indicate new possibilities for imaging biomolecules with low energy electrons.

Journal Article↗
PubMed2006

10-100 eV Ar+ ion induced damage to D-ribose and 2-deoxy-D-ribose molecules in condensed phase.

We report that 10-100 eV Ar+ ion irradiation induces severe damage to the biologically relevant sugar molecules D-ribose and 2-deoxy-D-ribose in the condensed phase on a polycrystalline Pt substrate. Ar+ ions with kinetic energies down to 15 eV induce effective decomposition of both sugar molecules, leading to the desorption of abundant cation and anion fragments, including CH3+, C2H3+, C3H3+, H3O+, CHO+, CH3O+, C2H3O+, H-, O-, and OH-, etc. Use of isotopically labelled molecules (5- 13C D-ribose and 1-D D-ribose) reveals the site specificity for some of the fragment origins, and thus the nature of the chemical bond breaking. It is found that all of the chemical bonds in both molecules are vulnerable to ion impact at energies down to 15 eV, particularly both the endo- and exocyclic C-O bonds. In addition to molecular fragmentation, several chemical reactions are also observed. A small amount of O-/O fragments abstract hydrogen to form OH-. It is found that the formation of the H3O+ ion is related to the hydroxyl groups of the sugar molecules, and is associated with additional hydrogen loss from the parent or adjacent molecules via hydrogen abstraction or proton transfer. The formation of several other cation fragments also requires hydrogen abstraction from its parent or an adjacent molecule. These fragmentations and reactions are likely to occur in a real biomedium during ionizing radiation treatment of tumors and thus bear significant radiobiological relevance.

Argon↗
PubMed2004

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV.

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.

Journal Article↗
PubMed2005

Individual fundamental mode dependence of H2O vibrational excitation in the 6-8 eV resonance region by electron impact.

Resonance phenomena in the bending (010), symmetric (100), and asymmetric (001) stretching vibrational modes in water (H(2)O) molecules have been investigated by measuring differential cross sections (DCSs) at fixed energy losses of 0.43, 0.46, 0.49, and 0.51 eV while sweeping the impact energies from 1.6 to 10 eV, at angles 60 degrees and 90 degrees, using a crossed-beam method. No resonancelike enhanced features have been observed in the study of the (010) mode. In the DCSs for the combined (100+001) mode, a single broad hump was observed at each angle, whose position shifts towards the higher energy side with increasing scattering angle, a feature typical of the interference profiles previously observed in a study of the (301) vibrational mode. DCSs for the (001) mode exhibited a rather isotropic angular distribution with no evidence of resonances. However, enhancement of DCSs were indeed observed for the (100) mode, at the energy loss of 0.46 eV, which is attributable to the A(1) symmetry resonance that was studied by Seng and Linder [J. Phys. B 9, 2539 (1976)], with a typical s-wave characteristic angular distribution. Theoretical analysis based on the continuum multiple-scattering approach has been carried out to provide rationale to the experimental results.

Journal Article↗
PubMed2005

A theoretical study on the inner-valence photoelectron spectra lying between 21 and 26 eV of the O2 molecule.

Ab initio calculations were carried out to study the structures of the photoelectron spectra of the O(2) molecule using complete active space self-consistent field and multireference single and double excitation configuration interaction methods. We focused our attention primarily on the inner-valence ionized states lying below 26 eV. Vibrational levels were calculated using adiabatic potential curves and vibrational wave functions were used to obtain Franck-Condon factors. We show that not only discrete vibrational levels of (3) (2)Pi(u), but also the continuum of nuclear motion in (1) (2)Sigma(u) (-) contribute to the bands of vibrational progression and the broad peak of the continuum between 22.5 and 26 eV. Other states with weak intensity lying between 19 and 26 eV are also discussed.

Journal Article↗
PubMed2005

Experiments and quantum-chemical calculations on Rydberg states of H2CS in the region 5.6-9.5 eV.

Absorption spectrum of H(2)CS in the region 5.6-9.5 eV was recorded with a continuously tunable light source of synchrotron radiation. After we subtracted absorption bands of CS(2), our spectrum clearly shows vibrational progressions associated with transitions (1)A(1)(pi,pi*)-X (1)A(1) and (1)B(2)(n,4s)-X (1)A(1) in the region 5.6-6.7 eV. A spectrum from which absorption of C(2)H(4) and CS(2) are subtracted shows several discrete bands in the region 6.9-9.5 eV. A Rydberg state (1)B(2)(n,4p(z)) lying below Rydberg state (1)A(1)(n,4p(y)) is confirmed, and the C-H symmetric stretching (nu(1)) and CH out-of-plane bending (nu(4)) modes for a transition (1)B(2)(n,4s)-X (1)A(1) are identified. New transitions to Rydberg states associated with excitation to 5s-11s, 5p(z)-7p(z), 5p(y)-7p(y), and 3d-6d are identified based on quantum defects and comparison with vertical excitation energies predicted with time-dependent density-functional theory (TD-DFT) and outer-valence Green's-function (OVGF) methods. For lower excited states predictions from these TD-DFT6-31+G calculations agree satisfactorily with experimental values, but for higher Rydberg states the OVGF method using aug-cc-pVTZ basis set augmented with extra diffuse functions yields more accurate predictions of excitation energies.

Journal Article↗
PubMed2005

Dissociation of energy-selected c-C2H4S+ in a region 10.6-11.8 eV: threshold photoelectron-photoion coincidence experiments and quantum-chemical calculations.

Dissociation of energy-selected c-C2H4S+ was investigated in a region of 10.6-11.8 eV with a threshold photoelectron-photoion coincidence technique and a synchrotron as a source of vacuum ultraviolet radiation. Branching ratios and average releases of kinetic energy in channels of formation of c-C2H4S+, CH3CS+, and HCS+ were obtained from well-resolved time-of-flight peaks in coincidence mass spectra. Measured average releases of kinetic energy for channel CH3CS+ + H of least energy are substantial and much greater than calculated with quasiequilibrium theory; in contrast, small releases of kinetic energy near the appearance onset for channel HCS+ + CH3 agree satisfactorily with statistical calculations. Calculations of molecular electronic structures and energetics of c-C2H4S+ and C2H3S+ isomers and various fragments and transition states were also performed with Gaussian 3 method to establish dissociation mechanisms. A predicted dissociation energy of 11.05 eV for c-C2H4S --> HCS+ + CH3 agrees with a linearly extrapolated threshold at 10.99+/-0.04 eV and a predicted dissociation mechanism that c-C2H4S+ isomerizes to CH3CHS+ before dissociating to HCS+ + CH3 supports the experimental results. The large releases of kinetic energy for channel CH3CS+ + H might result from a dissociation mechanism according to which c-C2H4S+ isomerizes to a local minimum CH3CSH+ and then dissociates through a transition state to form CH3CS+ + H.

Journal Article↗
PubMed2005

Fragmentation dynamics of condensed phase thymine by low-energy (10-200 eV) heavy-ion impact.

We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10-200 eV) Ar(+) irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH(+), HNC(3)H(4) (+), C(3)H(3) (+), OCNH(2) (+), [T-OCN](+), [T-OCNH(2)](+), [T-O](+), and [T+H](+) and the major anions as H(-), O(-), CN(-),and OCN(-). Cation fragment desorption appears at much lower threshold energies (near 15 eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H(-) and O(-) fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O(-)+[T-O](+), due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar(+) suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.

Cations↗
PubMed1993

Measurement of DNA damage by electrons with energies between 25 and 4000 eV.

All ionizing radiations deposit energy stochastically along their tracks. The resulting distribution of energies deposited in a small target such as the DNA helix leads to a corresponding spectrum in the severity of damage produced. So far, most information about the probable spectra of DNA lesion complexity has come from Monte Carlo studies which endeavour to model the relationship between the energy deposited in DNA and the damage induced. The aim of this paper is to establish methods of determining this relationship by irradiating pBR322 plasmid DNA using low energy electrons with energies comparable with the minimum energy thought to produce critical damage. The technique of agarose gel electrophoresis has been used to ascertain the fraction of DNA single- and double-strand breaks induced by monoenergetic electrons with energies as low as 25 eV. Our data show that the threshold electron energy for induction of single-strand breaks is < 25 eV, and for double-strand breaks between 25 and 50 eV.

DNA↗
PubMed2006

Properties of the 4.45 eV optical absorption band in LiF:Mg,Ti.

The optical absorption (OA) and thermoluminescence (TL) of dosimetric LiF:Mg,Ti (TLD-100) as well as nominally pure LiF single crystal have been studied as a function of irradiation dose, thermal and optical bleaching in order to investigate the role of the 4.45 eV OA band in low temperature TL. Computerised deconvolution was used to resolve the absorption spectrum into individual gaussian bands and the TL glow curve into glow peaks. Although the 4.45 eV OA band shows thermal decay characteristics similar to the 4.0 eV band its dose filling constant and optical bleaching properties suggest that it cannot be associated with the TL of composite peaks 4 or 5. Its presence in optical grade single crystal LiF further suggests that it is an intrinsic defect or possibly associated with chance impurities other than Mg, Ti.

Computer Simulation↗
PubMed2000

Technique for direct eV-scale measurements of the Mu and tau neutrino masses using supernova neutrinos

Early black hole formation in a core-collapse supernova will abruptly truncate the neutrino fluxes. The sharp cutoff can be used to make model-independent time-of-flight neutrino mass tests. Assuming a neutrino luminosity of 10(52) erg/s per flavor at cutoff and a distance of 10 kpc, Super-Kamiokande can detect an electron neutrino mass as small as 1.8 eV, and the proposed OMNIS detector can detect mu and tau neutrino masses as small as 6 eV. We present the first technique with direct sensitivity to eV-scale mu and tau neutrino masses.

Journal Article↗
PubMed2002

Mechanisms of photo double ionization of helium by 530 eV photons.

We have measured fully differential cross sections for photo double ionization of helium 450 eV above the threshold. We have found an extremely asymmetric energy sharing between the photoelectrons and an angular asymmetry parameter beta approximately 2 and beta approximately 0 for the fast and slow electrons, respectively. The electron angular distributions show a dominance of the shakeoff for 2 eV electrons and clear evidence of an inelastic electron-electron scattering at an electron energy of 30 eV. The data are in excellent agreement with convergent close-coupling calculations.

Journal Article↗
PubMed2003

Electron impact ionization of sodium Rydberg atoms below 2 eV.

We have observed electron impact ionization of highly excited sodium Rydberg atoms in ns and nd states, n=35-51, below E=2 eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from sigma(35d) approximately 7 x 10(-10) to sigma(50d) approximately 4 x 10(-9) cm(2). The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and sigma(n) follows a power law in n. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.

Journal Article↗
PubMed2004

Vibrations of micro-eV energies in nanocrystalline microstructures.

The phonon density of states of nanocrystalline bcc Fe and nanocrystalline fcc Ni3Fe were measured by inelastic neutron scattering in two different ranges of energy. As has been reported previously, the nanocrystalline materials showed enhancements in their phonon density of states at energies from 2 to 15 meV, compared to control samples composed of large crystals. The present measurements were extended to energies in the micro-eV range, and showed significant, but smaller, enhancements in the number of modes in the energy range from 5 to 18 microeV. These modes of micro-eV energies provide a long-wavelength limit that bounds the fraction of modes at milli-eV energies originating with the cooperative dynamics of the nanocrystalline microstructure.

Journal Article↗
PubMed2006

Experimental evidence for modulations in the relative double-photoionization cross section of C60 from threshold up to 280 eV.

The relative double-photoionization cross section of neutral C60 clusters was investigated using monochromatized synchrotron radiation between 18 and 283 eV. Our measurement of the double-to-single photoionization ratio reveals two modulating components that are superimposed on a smooth ratio curve from threshold (19.0 eV) up to 280 eV, when inner-shell excitations become possible. The maxima in the modulation can be related to geometrical dimensions of the C60 cluster.

Journal Article↗
PubMed2026

Defective EV-mediated transport of SHH alters neural fate specification in EPM1 epilepsy.

The extracellular milieu, including extracellular vesicles (EVs), plays a pivotal role in brain development. In this study, we sought to elucidate the pathogenesis of progressive myoclonus epilepsy type 1 (EPM1), a disease caused by mutations in the CSTB gene, using cerebral organoids (COs) derived from patient cells. The results demonstrate that EPM1 COs display increased electrophysiological activity and disrupted excitatory/inhibitory (E/I) balance. Single-cell RNA sequencing analysis of ventral EPM1-COs revealed an abnormal specification of progenitor fate, with a shift toward dorsal neuron identities. We demonstrated that this misspecification is driven by a functional alteration of the ventral signaling niche, resulting from impaired EV dynamics and altered protein cargo. Mechanistically, we identified Sonic Hedgehog (SHH) as a direct physical interactor of CSTB and demonstrated that CSTB deficiency leads to reduced SHH content and secretion. Our findings establish CSTB as a safeguard of ventral patterning and identify the CSTB-SHH-EV axis as a potential therapeutic target for mitigating the E/I imbalance associated with EPM1.

Hedgehog Proteins↗
PubMed1983

Nucleotide sequence of the long terminal repeat and flanking cellular sequences of avian endogenous retrovirus ev-2: variation in Rous-associated virus-0 expression cannot be explained by differences in primary sequence.

A fragment of chicken DNA containing the left long terminal repeat of endogenous retrovirus ev-2 and flanking cellular sequences has been molecularly cloned and analyzed. Comparison with sequence data from the analogous regions of ev-1 and Rous-associated virus-0 viral DNA reveals similarities among flanking regions of the integrated proviruses and among all three long terminal repeats. From the latter finding, we conclude that the difference in level of expression of ev-2 and its progeny Rous-associated virus-0 provirus cannot be due to sequence differences in their upstream long terminal repeats.

Animals↗
PubMed1999

Single- and double-strand breaks in solid pBR322 DNA induced by ultrasoft X-rays at photon energies of 388, 435 and 573 eV.

We measured strand breaks of pBR322 plasmid DNA irradiated with ultrasoft X-rays using monochromatic synchrotron radiation as a light source. Three photon energies, 388, 435 and 573 eV, a value below and above the nitrogen K-edge and above the oxygen K-edge, respectively, were chosen for the irradiation experiments as they have an equivalent photon transmittance of the sample. Irradiated DNA was analyzed by agarose gel electrophoresis and the numbers of single- and double-strand breaks (ssb and dsb) were determined by measuring the band intensity on the gel after ethidium bromide staining. The action cross-sections for the ssb and dsb slightly increased with the photon energy. The ratio between 388 and 573 eV was about 1.5 for both forms of strand breaks. The absorbed energy required for a strand break was about 60 eV for ssb and 1 keV for dsb, less than one fifth of the values obtained previously in the 2 keV region. On the other hand, the absorbed energies per strand break, as well as the ratio of the action cross-section for the ssb to that for the dsb, were constant regardless of the photon energy used. The K-shell photoabsorption on carbon, nitrogen and oxygen atoms in the DNA molecule, followed by an Auger cascade, induced DNA strand breaks with a constant efficiency in terms of the absorbed energy. These results indicate that the strand breaks of the DNA molecule in the solid state are mainly caused by the photo- and Auger-electrons and the efficiency of the strand breaks little depends on the atoms ejecting these secondary electrons.

DNA↗