6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

Most of the energy deposited in cells by ionizing radiation is channeled into the production of abundant free secondary electrons with ballistic energies between 1 and 20 electron volts. Here it is shown that reactions of such electrons, even at energies well below ionization thresholds, induce substantial yields of single- and double-strand breaks in DNA, which are caused by rapid decays of transient molecular resonances localized on the DNA's basic components. This finding presents a fundamental challenge to the traditional notion that genotoxic damage by secondary electrons can only occur at energies above the onset of ionization, or upon solvation when they become a slowly reacting chemical species.

http://science.sciencemag.org/content/sci/287/5458/1658.full.pdf

Resonant Formation of DNA Strand Breaks by Low-Energy (3 to 20 eV) Electrons

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

https://iopscience.iop.org/article/10.1088/0953-8984/4/5/001/pdf

Surface-enhanced Raman scattering

Electronic excitation energies are determined using the CAM-B3LYP Coulomb-attenuated functional [T. Yanai et al. Chem. Phys. Lett. 393, 51 (2004)], together with a standard generalized gradient approximation (GGA) and hybrid functional. The degree of spatial overlap between the occupied and virtual orbitals involved in an excitation is measured using a quantity Lambda, and the extent to which excitation energy errors correlate with Lambda is quantified. For a set of 59 excitations of local, Rydberg, and intramolecular charge-transfer character in 18 theoretically challenging main-group molecules, CAM-B3LYP provides by far the best overall performance; no correlation is observed between excitation energy errors and Lambda, reflecting the good quality, balanced description of all three categories of excitation. By contrast, a clear correlation is observed for the GGA and, to a lesser extent, the hybrid functional, allowing a simple diagnostic test to be proposed for judging the reliability of a general excitation from these functionals--when Lambda falls below a prescribed threshold, excitations are likely to be in very significant error. The study highlights the ambiguous nature of the term "charge transfer," providing insight into the observation that while many charge-transfer excitations are poorly described by GGA and hybrid functionals, others are accurately reproduced.

/pdf/excitation-energies-in-density-functional-theory-an-ep38p1jml8.pdf

Excitation energies in density functional theory : an evaluation and a diagnostic test.

Study of triplet states and short-lived negative ions by means of electron impact spectroscopy

Publisher Summary This chapter discusses resonance and threshold phenomena in low-energy electron collisions with molecules and clusters. Low-energy collisions of electrons with atoms and molecules are among the most important elementary processes in gaseous environments such as discharges, arcs, gas lasers, gaseous dielectrics and the earth's atmosphere. The dynamics behavior of low-energy electron-molecule collisions is discussed. The dynamical behavior of slow electrons traversing gases is to a large extent determined by two effects: the energy dependent evolution of the scattering phases for the relevant partial waves and the influence of temporary negative ion states (resonances). Some aspects of resonance and threshold phenomena are discussed. The theoretical description of electron-molecule collisions generally requires an adequate description of electronic, vibrational and rotational degrees of freedom. However, if the typical collision time is short compared to the rotational period, the molecule can be treated as having a fixed orientation during the collision process, and the result for the cross-section can be averaged over orientations. Treatment of vibrational dynamics is usually more important and more challenging to the theory. In the electron energy region important for applications, many inelastic processes such as vibrational excitation and dissociative electron attachment are driven by negative-ion resonances. The theoretical description of vibrational dynamics in these cases is usually based on the nonlocal complex potential describing the nuclear motion in the intermediate negative-ion state.

/pdf/resonance-and-threshold-phenomena-in-low-energy-electron-42hkyjmsxl.pdf

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran were measured in the energy range 0.1–20 eV, extending existing measurements to lower energies. The elastic cross sections were measured as a function of scattering angle from 10° to 180° at energies of 2 eV, 6 eV, 10 eV and 20 eV, and as a function of electron energy at 45°, 90°, 135° and 180°. The agreement with previous measurements and with the published theoretical work was generally satisfactory. The Ramsauer–Townsend minimum was observed at low energies, down to 0.24 eV at 180°. Three additional minima were observed at 1.13, 4.74 and 15.3 eV in the 180° elastic cross section. Vibrational excitation cross sections are reported as a function of electron energy from the threshold to 16 eV. They reveal threshold peaks and broad bands at 6.2 and 10.8 eV, attributed to shape resonances, in agreement with theoretical predictions, although the calculated energies are generally somewhat higher. A broad enhancement of the CH2 scissoring vibration is observed around 2.6 eV, implying a low-lying (shape) resonance similar to that observed earlier in cyclopropane.

/pdf/absolute-angle-differential-elastic-and-vibrational-48rjzttbqu.pdf

Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran

Absolute differential elastic and vibrational excitation cross sections have been measured for N2 in the scattering angle ranges starting between 0° and 20° and extending to 180°, at energies between 0.8 and 5 eV. The results agree with many previous measurements, in particular for angles around 90°, but discrepancies were found in some cases for angles close to 0° and 180°. Integral and momentum-transfer cross sections have been derived and compared with previous beam and swarm measurements. The procedures for determining the instrumental response function and for assuring optimal beam overlap over wide ranges of angles and energies are discussed.

https://doc.rero.ch/record/6691/files/allan_med.pdf

Measurement of the elastic and v = 0 ? 1 differential electron?N2 cross sections over a wide angular range

We studied dissociative electron attachment to a series of compounds with one or two hydroxyl groups. For the monoalcohols we found, apart from the known fragmentations in the 6–12 eV range proceeding via Feshbach resonances, also new weaker processes at lower energies, around 3 eV. They have a steep onset at the dissociation threshold and show a dramatic D/H isotope effect. We assigned them as proceeding via shape resonances with temporary occupation of σ*O–H orbitals. These low energy fragmentations become much stronger in the larger molecules and the strongest DEA process in the compounds with two hydroxyl groups, which thus represent an intermediate case between the behavior of small alcohols and the sugar ribose which was discovered to have strong DEA fragmentations near zero electron energy [S. Ptasinska, S. Denifl, P. Scheier and T. D. Mark, J. Chem. Phys., 2004, 120, 8505]. Above 6 eV, in the Feshbach resonance regime, the dominant process is a fast loss of a hydrogen atom from the hydroxyl group. In some cases the resulting (M – 1)– anion (loss of hydrogen atom) is sufficiently energy-rich to further dissociate by loss of stable, closed shell molecules like H2 or ethene. The fast primary process is state- and site selective in several cases, the negative ion states with a hole in the nO orbital losing the OH hydrogen, those with a hole in the σC–H orbitals the alkyl hydrogen.

/pdf/electron-induced-chemistry-of-alcohols-jlzy1yu0nu.pdf

Michael Allan

Papers

Study of triplet states and short-lived negative ions by means of electron impact spectroscopy

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

Absolute angle-differential elastic and vibrational excitation cross sections for electron collisions with tetrahydrofuran

Measurement of the elastic and v = 0 ? 1 differential electron?N2 cross sections over a wide angular range

Electron-induced chemistry of alcohols