Temporary anion states in the chloromethanes and in monochloroalkanes

et al. 1996

Differential cross section measurements for the elastic scattering of electrons from CH 3 Cl at energies from 0.5 to 9.5 eV are reported for scattering angles of 30°and 100°. The angular scattering dependence is determined at selected energies over this range. At energies below 1.0 eV, the cross sections are in excellent agreement with calculations using the Born dipole approximation. At large angles and higher energies, the scattering is dominated by a 2 A 1 temporary negative ion state near 3.5 eV. Energy loss data at 3.5, 5.0, and 8.5 eV are reported and the relative contributions of various vibrational modes determined. Differential cross sections for vibrational excitation of the 3 ͑a 1 ͒ C-Cl and 4 (e) CH stretching modes have been measured. The latter reveals a broad shape resonance of 2 E symmetry peaking near 5.5 eV. Angular distributions for excitation of these same modes are also reported. Using fittings to the vibrational excitation functions, the resonance parameters have been extracted and used in a mixed semiempirical ab initio calculation to compute the relative strengths of the vibrational energy loss peaks. These compare favorably with the results of the experiment. Elastic cross sections integrated over angle are reported for low energies. They are substantially larger than the results from recent measurements of the total cross section.

Section: A Elastic Scattering: Energy Dependencementioning

confidence: 99%

Low energy electron scattering from CH3Cl

Shi

Chan

et al. 1996

“…The resulting VAE for the 2 A 1 anion state of CCl 4 is Ϫ1.10 eV. Guerra et al 20 have performed MS-X␣ calculations, also using stabilization, and predict a VAE of Ϫ0.50 eV, with no other adjustments to the experimental results. Both approaches find good agreement between theory and experiment for the temporary anion states of the chloromethanes, and the discrepancy for the stable 2 A 1 anion of CCl 4 is rather unexpected.…”

Section: Integrated Zero Peak Cross Sectionsmentioning

confidence: 99%

Dissociative electron attachment near threshold, thermal attachment rates, and vertical attachment energies of chloroalkanes

Aflatooni

Burrow

2003

The peaks appearing near zero energy in the dissociative electron attachment cross section of 18 chloroalkanes are studied by electron beam methods. Fits to the experimental data are made using model cross sections having appropriate energy dependences and inclusion of the broadening due to the electron energy distribution. The magnitudes of the zero peaks are found to be well correlated with the vertical attachment energies ͑VAE͒ associated with occupation of the lowest empty orbitals of the compounds. The magnitudes rise exponentially by more than five orders of magnitude as VAE decreases from 2 eV to a slightly negative value. This dependence is a consequence not only of the thermal population of vibrational levels, but also of an approximately linear relationship between VAE and the energy of the crossing between the neutral and anion potential curves. Franck-Condon factors for the transition to the anion curve are computed for model potential curves, and the nature of the attachment from vibrational levels with energies near that of the crossing point is explored in a local potential resonance picture. A substantial contribution arises from tunneling to the anion state from vibrational levels below the barrier. Thermal attachment rate constants are also computed from our data. These are also shown to vary exponentially with VAE.

“…[10][11][12][13] The widths of these features as they appear in the derivatives of the total scattering cross section increase monotonically with VAE in a manner consistent with the attachment of a p-wave electron. 13 From such a plot, the energy separation of the dip and peak, ⌬ dp , in the derivative signal may be found for a particular VAE.…”

Section: The * Resonancementioning

confidence: 82%

“…The associated temporary anion state is short lived and dissociative, leading to the production of Cl − by the DEA process. 9 The VAEs of monochloroalkanes [10][11][12][13] vary from 1.8 to 2.4 eV, excluding methyl chloride. Mixing of the * and * orbitals in I-VI therefore leads to a LUMO that is primarily of * character but one possessing an admixture of * that is a strong function of the separation between the two groups.…”

Section: Introductionmentioning

confidence: 99%

Remote bond breaking by interacting temporary anion states

Burrow

2006

The cross section for bond breaking at the site of a dissociative temporary negative ion state through the dissociative electron attachment process can be considerably enhanced by the presence of a second longer-lived temporary negative ion state elsewhere in the molecule, even one quite remote from the first. In a series of chloroalkenes possessing both C-Cl and C v C bonds separated by various distances, we show that the cross sections are determined by the lifetime of the lower anion state created by the mixing of the anion states of these two moieties, with the wave function's coefficients giving the probability that the electron is located at the dissociative site. Furthermore, the lifetime of the composite anion state can be expressed in terms of these same coefficients and the lifetimes of the unmixed resonances. We also discuss how these results may give insight into the means by which strand breaks are induced in DNA by the attachment of slow electrons.