Dissociative attachment from the O\u3csub\u3e2\u3c/sub\u3e (a\u3csup\u3e1\u3c/sup\u3e Δ \u3csub\u3e\u3ci\u3eg\u3c/i\u3e\u3c/sub\u3e) state

The dissociative attachment cross section for production of O- by electron impact on the metastable O2 (a1 Δ g) state is studied. The cross section is found to be 4.6 ± 1.3 × 10-18 cm2 at its maximum. From the measured energy dependence, we infer that the dissociative attachment process takes place through the O2̅ (2 Пu) state as in the case of O- production from the O2 ground state. The information thus obtained is used to estimate the portion of the cross section for excitation of the a1 Δg state by electron impact which proceeds via the O2-(2 Пu) state. This mechanism is shown to account for the location and approximate magnitude of the maximum in the excitation cross section. Finally, information is obtained concerning the cross section for positive ionization of the O2 (a- Δg) state near threshold

Dissociative attachment from the O\u3csub\u3e2\u3c/sub\u3e (a\u3csup\u3e1\u3c/sup\u3e Δ \u3csub\u3e\u3ci\u3eg\u3c/i\u3e\u3c/sub\u3e) state

The dissociative attachment cross section for production of O- by electron impact on the metastable O2 (a1 Δ g) state is studied. The cross section is found to be 4.6 ± 1.3 × 10-18 cm2 at its maximum. From the measured energy dependence, we infer that the dissociative attachment process takes place through the O2̅ (2 Пu) state as in the case of O- production from the O2 ground state. The information thus obtained is used to estimate the portion of the cross section for excitation of the a1 Δg state by electron impact which proceeds via the O2-(2 Пu) state. This mechanism is shown to account for the location and approximate magnitude of the maximum in the excitation cross section. Finally, information is obtained concerning the cross section for positive ionization of the O2 (a- Δg) state near threshold

Electron attachment to dye-sensitized solar cell components: cyanoacetic acid.

The energies of electron attachment associated with temporary occupation of the lower-lying virtual orbitals of cyanoacetic acid (CAA), proposed as a possible component of dye-sensitized solar cells, and its derivative methyl cyanoacetate (MCA) are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecule, supplied by B3LYP/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The vertical and adiabatic electron affinities are also evaluated at the B3LYP/6-31+G(d) level as the anion/neutral total energy difference. Dissociative electron attachment spectroscopy (DEAS) is used to measure the total anion current as a function of the incident electron energy in the 0-4 eV energy range, and the negative fragments generated through the dissociative decay channels of the molecular anion are detected with a mass filter. In both compounds only two intense fragment anion currents are observed, that due to loss of a hydrogen atom from the molecular anion ([M-H]-) and to formation of CN . In CAA the former signal displays a very sharp feature at 0.68 eV, assigned to a vibrational Feshbach resonance arising from coupling between a dipole bound anion state and a temporary sigma* anion state

Resonant dissociation in N/sub 2/ by electron impact: a source of heating in the thermosphere and auroras

An electron impact resonant dissociation process, leading to superthermal atom production in molecular nitrogen is described. The maximum cross section for this process is found to be 2.5 x 10/sup -18/ cm/sup 2/ at 10 eV. Measurements of scattered electrons indicate a value of -65 to -90 MeV for the electron affinity of N. The possible role of resonant dissociation as a source of heating in the thermosphere and in auroras is discussed

Electron attachment to the aza-derivatives of furan, pyrrole and thiophene.

The temporary anion states of gas-phase furan, isoxazole, oxazole, pyrrole, pyrazole, imidazole, thiophene, isothiazole and thiazole are characterised by means of electron transmission spectroscopy. The measured energies of vertical electron attachment are compared with the virtual orbital energies of the neutral state molecules supplied by MP2 and B3LYP calculations with the 6-31G* basis set. The calculated energies, scaled with empirical equations, reproduce satisfactorily the experimental attachment energies. Replacement of a ring CH group with a nitrogen atom increases the electron-acceptor properties, although the stabilisation of the p* anion states is not as large as that of the p cation states, in line with the bond length variations caused by aza-substitution. In the spectra of thiophene and isothiazole the first p* resonances display sharp vibrational structure with energy spacing of about 80 meV. The spectrum of isothiazole presents clear evidence for a low-energy (1.61 eV) resonance ascribed to the lowest s* anion state

Electron attachment to trans-azobenzene.

The temporary anion states of gas-phase trans-azobenzene are characterised by means of electron transmission spectroscopy (ETS) in the 0-6 eV range. The measured energies of vertical electron attachment are compared with the energies of the pi* virtual orbitals of the neutral molecule supplied by HF (at MP2 optimized geometries) and B3LYP calculations. The calculated energies, scaled with empirical equations, reproduce quantitatively the energies of the corresponding spectral features and predict a positive vertical electron affinity of 0.83 eV. The total anion current at the walls of the collision chamber and the mass-selected molecular anion current are also reported as a function of the impact electron energy. In agreement with previous data, long-lived (> 1 micro s) parent molecular anions are detected at zero eV and near 1 eV. The close similarity of the electron transmission spectrum with the derivatives with respect to energy of the anion currents suggests strongly that shape resonances produced by electron capture into empty pi* orbitals are the initial step in formation of the long lived molecular anions. This appears to rule out mechanisms in which direct formation of core-excited anion states are invoked. However, according to DFT calculations, conversion of the shape resonances around 1 eV to longer-lived sigma-pi* core-excited doublet anion states is possible on energetic grounds

On the treatment of LUMO energies for their use as descriptors.

Calculated energies of lowest unoccupied molecular orbitals (LUMOs) are frequently employed as descriptors in studies of quantitative structure-activity relationships (QSAR) and linear free energy relationships (LFER) involving electron transfer. However, the quantum chemical programs with which these are carried out, whether Hartree-Fock (HF) or density functional theory (DFT), do not treat orbitals of different character, for example, C=C \u3c0* and C-Cl \u3c3*, consistently, nor is there consistency among different families of compounds. These problems can be ameliorated with the use of the experimental equivalent of the LUMO energy, the vertical attachment energy (VAE), or by shifting and scaling LUMO energies to a training set of available VAEs in similar compounds. Examples from the literature are used to illustrate these points

Are there pi* shape resonances in electron scattering from phosphate groups?

The temporary anion states of trimethyl phosphate and several compounds bearing the P=O group were explored using electron transmission spectroscopy and ab initio calculations to determine if these states have the characteristics of the pi* resonances usually associated with multiple bonds. No evidence was found for this in (CH3O)3PO and, by extension, we do not expect them to appear in the phosphate group of DNA. Cl3PO, on the other hand, does display such characteristics to some extent, and we show that they arise from the spatial properties of the σ* (P-Cl) orbitals rather than from multiple PO bonding. A novel computational means to explore effects due to the relative size of a molecular orbital and that of the angular momentum barrier responsible for confining the additional electron is presented