Thermal desorption effects on fragment ion production from multi-photon ionized uridine and selected analogues.

Experiments on neutral gas-phase nucleosides are often complicated by thermal lability. Previous mass spectrometry studies of nucleosides have identified enhanced relative production of nucleobase ions (e.g. uracil+ from uridine) as a function of desorption temperature to be the critical indicator of thermal decomposition. On this basis, the present multi-photon ionization (MPI) experiments demonstrate that laser-based thermal desorption is effective for producing uridine, 5-methyluridine, and 2'-deoxyuridine targets without thermal decomposition. Our experiments also revealed one notable thermal dependence: the relative production of the sugar ion C5H9O4 + from intact uridine increased substantially with the desorption laser power and this only occurred at MPI wavelengths below 250 nm (full range studied 222-265 nm). We argue that this effect can only be rationalized plausibly in terms of changing populations of different isomers, tautomers, or conformers in the target as a function of the thermal desorption conditions. Furthermore, the wavelength threshold behavior of this thermally-sensitive MPI channel indicates a critical dependence on neutral excited state dynamics between the absorption of the first and second photons. The experimental results are complemented by density functional theory (DFT) optimizations of the lowest-energy structure of uridine and two further conformers distinguished by different orientations of the hydroxymethyl group on the sugar part of the molecule. The energies of the transitions states between these three conformers are low compared with the energy required for decomposition.

Electron- and photon-impact ionization of furfural.

The He(i) photoelectron spectrum of furfural has been investigated, with its vibrational structure assigned for the first time. The ground and excited ionized states are assigned through ab initio calculations performed at the outer-valence Green's function level. Triple differential cross sections (TDCSs) for electron-impact ionization of the unresolved combination of the 4a″ + 21a' highest and next-highest occupied molecular orbitals have also been obtained. Experimental TDCSs are recorded in a combination of asymmetric coplanar and doubly symmetric coplanar kinematics. The experimental TDCSs are compared to theoretical calculations, obtained within a molecular 3-body distorted wave framework that employed either an orientation average or proper TDCS average. The proper average calculations suggest that they may resolve some of the discrepancies regarding the angular distributions of the TDCS, when compared to calculations employing the orbital average.

Intermediate energy cross sections for electron-impact vibrational-excitation of pyrimidine.

We report differential cross sections (DCSs) and integral cross sections (ICSs) for electron-impact vibrational-excitation of pyrimidine, at incident electron energies in the range 15-50 eV. The scattered electron angular range for the DCS measurements was 15°-90°. The measurements at the DCS-level are the first to be reported for vibrational-excitation in pyrimidine via electron impact, while for the ICS we extend the results from the only previous condensed-phase study [P. L. Levesque, M. Michaud, and L. Sanche, J. Chem. Phys. 122, 094701 (2005)], for electron energies ⩽12 eV, to higher energies. Interestingly, the trend in the magnitude of the lower energy condensed-phase ICSs is much smaller when compared to the corresponding gas phase results. As there is no evidence for the existence of any shape-resonances, in the available pyrimidine total cross sections [Baek et al., Phys. Rev. A 88, 032702 (2013); Fuss et al., ibid. 88, 042702 (2013)], between 10 and 20 eV, this mismatch in absolute magnitude between the condensed-phase and gas-phase ICSs might be indicative for collective-behaviour effects in the condensed-phase results.

Intermediate energy electron impact excitation of composite vibrational modes in phenol.

We report differential cross section results from an experimental investigation into the electron impact excitation of a number of the low-lying composite (unresolved) vibrational modes in phenol (C6H5OH). The measurements were carried out at incident electron energies in the range 15-40 eV and for scattered-electron angles in the range 10-90°. The energy resolution of those measurements was typically ∼80 meV. Calculations, using the GAMESS code, were also undertaken with a B3LYP/aug-cc-pVDZ level model chemistry, in order to enable us to assign vibrational modes to the features observed in our energy loss spectra. To the best of our knowledge, the present cross sections are the first to be reported for vibrational excitation of the C6H5OH molecule by electron impact.

Differential cross sections for electron impact excitation of the electronic bands of phenol.

We report results from a joint theoretical and experimental investigation into electron scattering from the important organic species phenol (C6H5OH). Specifically, differential cross sections (DCSs) have been measured and calculated for the electron-impact excitation of the electronic states of C6H5OH. The measurements were carried out at energies in the range 15-40 eV, and for scattered-electron angles between 10° and 90°. The energy resolution of those experiments was typically ∼80 meV. Corresponding Schwinger multichannel method with pseudo-potentials calculations, with and without Born-closure, were also performed for a sub-set of the excited electronic-states that were accessed in the measurements. Those calculations were conducted at the static exchange plus polarisation (SEP)-level using a minimum orbital basis for single configuration interaction (MOBSCI) approach. Agreement between the measured and calculated DCSs was typically fair, although to obtain quantitative accord, the theory would need to incorporate even more channels into the MOBSCI.

Triply differential (e,2e) studies of phenol.

We have measured (e,2e) triple differential cross sections (TDCS) for the electron-impact ionisation of phenol with coplanar asymmetrical kinematics for an incident electron energy of 250 eV. Experimental measurements of the angular distribution of the slow outgoing electrons at 20 eV are obtained when the incident electron scatters through angles of -5°, -10°, and -15°, respectively. The TDCS data are compared with calculations performed within the molecular 3-body distorted wave model. In this case, a mixed level of agreement, that was dependent on the kinematical condition being probed, was observed between the theoretical and experimental results in the binary peak region. The experimental intensity of the recoil features under all kinematical conditions was relatively small, but was still largely underestimated by the theoretical calculations.

Electron impact excitation of the ã (3)B(1u) electronic state in C2H4: an experimentally benchmarked system?

We report on differential and integral cross section measurements for the electron impact excitation of the lowest-lying triplet electronic state (ã (3)B(1u)) in ethylene (C(2)H(4)). The energy range of the present experiments was 9 eV-50 eV, with the angular range of the differential cross section measurements being 15°-90°. As the ground electronic state of C(2)H(4) is a (1)A(g) state, this singlet → triplet excitation process is expected to be dominated by exchange scattering. The present angular distributions are found to support that assertion. Comparison, where possible, with previous experimental results from the University of Fribourg group shows very good agreement, to within the uncertainties on the measured cross sections. Agreement with the available theories, however, is generally marginal with the theories typically overestimating the magnitude of the differential cross sections. Notwithstanding that, the shapes of the theoretical angular distributions were in fact found to be in good accord with the corresponding experimental results.

Low energy (e,2e) studies from CH4: results from symmetric coplanar experiments and molecular three-body distorted wave theory.

Low energy experimental and theoretical triply differential cross sections are presented for electron impact ionization of methane (CH(4)) for both the highest occupied molecular orbital (HOMO) and next highest occupied molecular orbital (NHOMO). The HOMO is a predominantly p-type orbital which is labeled 1t(2) and the NHOMO is predominantly s-type labeled 2a(1). Coplanar symmetric (symmetric both in final state electron energies and observation angles) are presented for final state electron energies ranging from 2.5 to 20 eV. The theoretical M3DW (molecular three-body distorted wave) results are in surprisingly good agreement with experiment for the HOMO state and less satisfactory agreement for the NHOMO state. The molecular NHOMO results are also compared with the ionization of the 2s shell of neon which is the isoelectronic atom.

Low-energy positron scattering from dihydropyran.

We report on total cross section measurements for positron scattering from dihydropyran (C(5)H(8)O), with the energy range of the present study being 0.15-48 eV. To the best of our knowledge, there are currently no other corresponding experimental data or theoretical computations against which we can compare our results. The effect of this species' important dipole moment and significant dipole polarizability on the scattering dynamics is considered, as is the opening of the positronium formation channel.

Investigation into the valence electronic structure of norbornene using electron momentum spectroscopy, Green's function, and density functional theories.

Results of a study of the valence electronic structure of norbornene (C(7)H(10)), up to binding energies of 30 eV, are reported. Experimental electron momentum spectroscopy (EMS) and theoretical Green's function and density functional theory approaches were utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all 19 valence orbitals of norbornene. This experimentally validated model was then used to extract other molecular properties of norbornene (geometry, infrared spectrum). When these calculated properties are compared to corresponding results from independent measurements, reasonable agreement is typically found. Due to the improved energy resolution, EMS is now at a stage to very finely image the effective topology of molecular orbitals at varying distances from the molecular center, and the way the individual atomic components interact with each other, often in excellent agreement with theory. This will be demonstrated here. Green's Function calculations employing the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than about 22 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet emission and newly presented (e,2e) ionization spectra. Finally, limitations inherent to calculations of momentum distributions based on Kohn-Sham orbitals and employing the vertical depiction of ionization processes are emphasized, in a formal discussion of EMS cross sections employing Dyson orbitals.

Norbornane: an investigation into its valence electronic structure using electron momentum spectroscopy, and density functional and Green's function theories.

We report on the results of an exhaustive study of the valence electronic structure of norbornane (C(7)H(12)), up to binding energies of 29 eV. Experimental electron momentum spectroscopy and theoretical Green's function and density functional theory approaches were all utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among all the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all of the 20 valence orbitals of norbornane. This experimentally validated quantum chemistry model was then used to extract some chemically important properties of norbornane. When these calculated properties are compared to corresponding results from other independent measurements, generally good agreement is found. Green's function calculations with the aid of the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than 22.5 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet photoemission and newly presented (e,2e) ionization spectra, except for the band associated with the 1a(2) (-1) one-hole state, which is probably subject to rather significant vibronic coupling effects, and a band at approximately 25 eV characterized by a momentum distribution of "s-type" symmetry, which Green's function calculations fail to reproduce. We note the vicinity of the vertical double ionization threshold at approximately 26 eV.

Chorioretinitis sclopetaria from BB ex memoria.

Chorioretinitis sclopetaria presents a characteristic pattern of choroidal and retinal changes caused by a high velocity projectile passing into the orbit, in close proximity to the globe. While it is unlikely that a patient should completely forget the trauma causing such damage, preserved or compensated visual function may blur the patient's memory of these events over time. Characteristic physical findings help to clarify the antecedent history. Despite the lack of an acknowledged history of ocular trauma or surgery, in our case, the characteristic ocular findings discovered at presentation allowed for recognition of the underlying etiology. Because of good visual function, the patient had completely forgotten about the trauma that occurred 12 years earlier. Strabismus surgery was performed for treatment of the presenting symptomatic diplopia. The pathognomonic findings in chorioretinitis sclopetaria are invaluable in correctly diagnosing this condition, especially when a history of ocular trauma is unavailable.

Anti-idiotype-induced, lipopolysaccharide-specific antibody response to Pseudomonas aeruginosa. II. Isotype and functional activity of the anti-idiotype-induced antibodies.

We recently developed a murine anti-idiotypic mAb that functioned as a molecular mimic of the O-specific polysaccharide side chain (Ps) of Pseudomonas aeruginosa LPS in vitro, and which induced Ps-specific antibodies in syngeneic BALB/c ByJ mice. In the current studies, we demonstrate that these anti-Id-induced, Ps-specific antibodies fix complement to the bacterial cell surface, and protect neutropenic mice from fatal P. aeruginosa sepsis. The isotypic distribution of the anti-Id-induced antibodies, however, resembles the restricted pattern (IgM and IgG3) seen after administration of purified Ps to mice. The immunogenicity and number of isotypes of Ps-specific antibody produced could be enhanced by conjugating the anti-Id to keyhole limpet hemocyanin. Finally, the anti-Id administered before immunization with purified Ps, primed BALB/c ByJ mice for production of other Ps-specific antibody isotypes (IgG1). These studies show that this anti-Id induces functional anti-Ps antibodies in syngeneic mice, and when used in conjugate form or as a priming agent before Ps immunization, yields an antibody response consistent with "T cell dependence." These immunization strategies may be useful for the induction of polysaccharide-specific antibodies in man.