Resonance effects in elastic cross sections for electron scattering on pyrimidine: Experiment and theory.

We measured differential cross sections for elastic (rotationally integrated) electron scattering on pyrimidine, both as a function of angle up to 180(∘) at electron energies of 1, 5, 10, and 20 eV and as a function of electron energy in the range 0.1-14 eV. The experimental results are compared to the results of the fixed-nuclei Schwinger variational and R-matrix theoretical methods, which reproduce satisfactorily the magnitudes and shapes of the experimental cross sections.

Dynamics of the Dissociating Uracil Anion Following Resonant Electron Attachment.

We report a combined experimental and theoretical investigation of dissociative electron attachment (DEA) to the nucleobase uracil. Using ion momentum imaging experiments employing a DEA reaction microscope we have measured 3-dimensional momentum distributions of specific anionic fragments following DEA to uracil by 6 eV electrons. From the measured anion fragment kinetic energy we determine the possible dissociation pathways and the total kinetic energy release.

Time-resolved photoelectron spectra of CS2: dynamics at conical intersections.

We report results of the application of a fully ab initio approach for simulating time-resolved molecular-frame photoelectron angular distributions around conical intersections in CS2. The technique employs wave packet densities obtained with the multiple spawning method in conjunction with geometry- and energy-dependent photoionization matrix elements. The robust agreement of these results with measured molecular-frame photoelectron angular distributions for CS2 demonstrates that this technique can successfully elucidate, and disentangle, the underlying nuclear and photoionization dynamics around conical intersections in polyatomic molecules.

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy.

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy.

Time-resolved photoelectron spectroscopy of wavepackets through a conical intersection in NO2.

We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two (2)A' states of NO(2). The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies.

Comment on "Ring-breaking electron attachment to uracil: Following bond dissociations via evolving resonances" [J. Chem. Phys. 128, 174302 (2008)].

We point out that the assignment of pi(*) resonances to calculated features in a recent paper by Gianturco et al. [J. Chem.

Breakdown of the two-step model in K-shell photoemission and subsequent decay probed by the molecular-frame photoelectron angular distributions of CO2.

We report results of measurements and of Hartree-Fock level calculations of molecular-frame photoelectron angular distributions (MFPADs) for C 1s photoemission from CO2. The agreement between the measured and calculated MFPADs is on average reasonable. The measured MFPADs display a weak but definite asymmetry with respect to the O+ and CO+ fragment ions at certain energies, providing evidence for an overlap of gerade and ungerade final ionic states giving rise to a partial breakdown of the two-step model of core-level photoionization and its subsequent Auger decay.

Absolute electron scattering cross sections for the CF2 radical.

Using a crossed electron-molecular beam experiment, featuring a skimmed nozzle beam with pyrolytic radical production, absolute elastic cross sections for electron scattering from the CF2 molecule have been measured. A new technique for placing measured cross sections on an absolute scale is used for molecular beams produced as skimmed supersonic jets. Absolute differential cross sections for CF2 are reported for incident electron energies of 30-50 eV and over an angular range of 20-135 deg.

Interaction of low-energy electrons with the pyrimidine bases and nucleosides of DNA.

We report computed cross sections for the elastic scattering of slow electrons by the pyrimidine bases of DNA, thymine and cytosine, and by the associated nucleosides, deoxythymidine and deoxycytidine. For the isolated bases, we carried out calculations both with and without the inclusion of polarization effects. For the nucleosides, we neglect polarization effects but estimate their influence on resonance positions by comparison with the results for the corresponding bases.

Resonant channel coupling in electron scattering by pyrazine.

Detailed investigation of the three low-energy resonances seen in electron scattering by the diazabenzene molecule pyrazine reveals that the first two are nearly pure single-channel shape resonances, but the third is, as long suspected, heavily mixed with core-excited resonances built on low-lying triplet states. Such resonant channel coupling is likely to be widespread in pi-ring molecules, including the nucleobases of DNA and RNA, where it may form a pathway for radiation damage.

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: an ab initio study of time-resolved photoelectron spectra.

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation.

Interaction of low-energy electrons with the purine bases, nucleosides, and nucleotides of DNA.

The authors report results from computational studies of the interaction of low-energy electrons with the purine bases of DNA, adenine and guanine, as well as with the associated nucleosides, deoxyadenosine and deoxyguanosine, and the nucleotide deoxyadenosine monophosphate. Their calculations focus on the characterization of the pi* shape resonances associated with the bases and also provide general information on the scattering of slow electrons by these targets. Results are obtained for adenine and guanine both with and without inclusion of polarization effects, and the resonance energy shifts observed due to polarization are used to predict pi* resonance energies in associated nucleosides and nucleotides, for which static-exchange calculations were carried out.

Low-energy electron collisions with gas-phase uracil.

We have studied gas-phase collisions between slow electrons and uracil molecules with a view to understanding the resonance structure of the scattering cross section. Our symmetry-resolved results for elastic scattering, computed in the fixed-nuclei, static-exchange and static-exchange-plus-polarization approximations, provide locations for the expected pi* shape resonances and indicate the possible presence of a low-energy sigma* resonance as well. Electron-impact excitation calculations were carried out for low-lying triplet and singlet excitation channels and yield a very large singlet cross section.

Low-energy nondipole effects in molecular nitrogen valence-shell photoionization.

Observations are reported for the first time of significant nondipole effects in the photoionization of the outer-valence orbitals of diatomic molecules. Measured nondipole angular-distribution parameters for the 3sigma(g), 1pi(u), and 2sigma(u) shells of N2 exhibit spectral variations with incident photon energies from thresholds to approximately 200 eV which are attributed via concomitant calculations to particular final-state symmetry waves arising from (E1)multiply sign in circle(M1,E2) radiation-matter interactions first-order in photon momentum. Comparisons with previously reported K-edge studies in N2 verify linear scaling with photon momentum, accounting in part for the significantly enhanced nondipole behavior observed in inner-shell ionization at correspondingly higher momentum values in this molecule.

Low-energy electron scattering by deoxyribose and related molecules.

We apply first-principles computational methods to study elastic scattering of low-energy electrons by 2-deoxyribose and 2-deoxyribose monophosphate, which are of interest as components of the DNA backbone, and to tetrahydrofuran (THF), which has been studied as a deoxyribose analog. To investigate the dependence of the scattering process on the molecular conformation, we examine Cs and C2 conformers of THF as well as the planar C(2v) geometry imposed in earlier calculations. There is little difference between the elastic cross sections determined at the nonplanar geometries, but there are noticeable differences between those results and the cross sections computed using the planar ring.

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: wave-packet dynamics on effective potential surfaces of reduced dimensionality.

We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde.

Isotope-induced partial localization of core electrons in the homonuclear molecule N2.

Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction ('gerade' and 'ungerade') are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites.

Elastic electron scattering by C2F4.

Recent measurements [R. Panajotovic, M. Jelisavcic, R.

Low-energy electron collisions with sulfur hexafluoride, SF(6).

We report calculated cross sections for elastic and electronically inelastic collisions of low-energy electrons with sulfur hexafluoride, SF(6). Elastic cross sections are computed within the fixed-nuclei approximation, with polarization effects incorporated. Inelastic cross sections for nine low-lying states are computed in a few-channel approximation.

Vibrational branching ratios in photoionization of CO and N2.

We report results of experimental and theoretical studies of the vibrational branching ratios for CO 4sigma(-1) photoionization from 20 to 185 eV. Comparison with results for the 2sigma(u)(-1) channel of the isoelectronic N2 molecule shows the branching ratios for these two systems to be qualitatively different due to the underlying scattering dynamics: CO has a shape resonance at low energy but lacks a Cooper minimum at higher energies whereas the situation is reversed for N2.

Pump-probe photoionization study of the passage and bifurcation of a quantum wave packet across an avoided crossing.

The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra.

Probing wavepacket dynamics with femtosecond energy- and angle-resolved photoelectron spectroscopy.

Several recent studies have demonstrated how well-suited femtosecond time-resolved photoelectron spectra are for mapping wavepacket dynamics in molecular systems. Theoretical studies of femtosecond photoelectron spectra which incorporate a robust description of the underlying photoionization dynamics should enhance the utility of such spectra as a probe of wavepackets and of the evolution of electronic structure. This should be particularly true in regions of avoided crossings where the photoionization amplitudes and electronic structure may evolve rapidly with geometry.

Distribution of alleles of the methylenetetrahydrofolate reductase (MTHFR) C677T gene polymorphism in familial spina bifida.

Spina bifida cystica (SB) is one of the most common and disabling of birth defects. Folic acid supplementation in mothers during the periconceptional period has been shown to prevent more than 70% of neural tube defects (NTD) including SB. However, the mechanism is unknown.

Hereditary febrile seizures: phenotype and evidence for a chromosome 19p locus.

The occurrence of febrile seizures (FSs) in large autosomal dominant FS kindreds makes possible accurate delineation of the pure clinical phenotype of hereditary FS among secondary FS cases, and the identification of gene loci causing susceptibility to FS. Recently FS gene loci on chromosomes 8 and 19 were identified. We studied the phenotype of FS in four large families in which FS is an autosomal dominant trait.