A series of different alkyl vinyl ethers is investigated to decipher the possible reaction channels upon photoexcitation to the π3s-Rydberg and the ππ*-valence state at 200 nm using time-resolved photoelectron spectroscopy and on-the-fly time-dependent density functional theory dynamics simulations. The results indicate two possible relaxation pathways: (1) a radiationless decay through the ππ*-state back to the ground state via torsion of the C═C double bond, in accordance with the dynamics found in ethylene; and (2) a fast dissociation of the C-O bond between the alkyl and the vinoxy group in the πσ*-state. The latter state can be accessed only after excitation to the π3s-Rydberg state (quantum yield of ∼50% according to the dynamics simulations). Additionally, the excited state barrier leading to formation of a vinyl radical was found to be too high to be crossed. These results indicate that the dynamics of ethers crucially depend on the excitation wavelength and that the πσ*-state constitutes an important competitive reaction channel that leads to dissociation of the molecules.

Studies of the direct mechanism for dissociative recombination of HCO+ and HOC+ are presented. The calculations involve wave-packet propagation in three dimensions on electronically resonant states of HCO and HOC with the potential-energy surfaces and autoionization widths obtained from ab initio electron scattering and electronic structure calculations. The total cross section and branching ratios for the two molecules and their deuterated isotopologues are calculated and compared to available experiments. The effect of vibrational excitation in DCO+ has been studied as well.

The radiationless deactivation mechanisms of photo-excited thiophene have been studied using the multi-reference second-order perturbation theory and linear response coupled cluster methods. The electronic spectrum has been established and various minimum energy structures and conical intersections involving the ground and lowest singlet excited states have been characterized. Simplified reaction paths connecting the optimized geometries have been calculated as well. Based on these investigations, several deactivation mechanisms have been identified leading from the lowest bright 1ππ∗ states back to the electronic ground state. The excited state depletion in each case is possible due to the existence of low-lying conical intersections formed by either cleavage of one of the CS bonds or out-of-plane deformations of the aromatic ring. The deactivation mechanisms suggested in this work should provide some very efficient decay channels after excitation into the first UV absorption band of thiophene, and are good candidates to explain why this compound is non-fluorescent.

Possible mechanisms for the radiationless deactivation of photo-excited furan have been investigated using high-level electronic structure methods. Two different conical intersections between the S0 and S1 electronic states have been characterized, both involving various degrees of CO bond cleavage. One of these corresponds to a planar ring-opened structure and the other to an asymmetric ring-puckered structure. Calculations have been performed in order to establish the vertical electronic spectrum and to investigate the behaviour of the potential energy surfaces as the intersections are approached. The present results indicate that both crossings can be accessed through exothermic and barrierless processes after vertical excitation into the optically bright S2(ππ∗) state. These features make them good candidates to account for efficient radiationless deactivation in furan. The deactivation pathways considered in the present work are close analogues of those previously described for other five-membered heterocycles.

The cross section for double charge transfer between H{sup +} and H{sup -} at low collision energies (E{<=}90 eV) is calculated using a many-state molecular close-coupling model. The wave function is expanded in a diabatic representation of the seven lowest {sup 1{Sigma}}{sub g}{sup +} and the six lowest {sup 1{Sigma}}{sub u}{sup +} states of the hydrogen molecule. The calculated cross section shows clear oscillations as a function of the collision energy, similar to those observed experimentally. However, the magnitude of the calculated cross section is larger than found in experiments. Also, the cross section for double charge transfer in collisions between D{sup +} and H{sup -} is calculated.

The double valence photoionization spectra of methanol, ethanol, and n-propyl alcohol have been recorded using a time-of-flight photoelectron-photoelectron coincidence technique. The spectra show a well-defined onset followed by broad rounded bands. The lowest vertical double ionization energies have been determined for all molecules and are found to be 32.1, 29.6, and 28.2 eV, respectively. These energies have been applied along with single ionization energies from conventional photoelectron spectra to investigate a recently derived rule of thumb for determination of the lowest double ionization energy in molecules. Many-electron ab initio calculations have been performed on the dicationic ground states in good agreement with the experimental values. For methanol, also excited dicationic states have been calculated up to about 40 eV and used for a detailed interpretation of the experimental spectrum.

Single-photon core-valence double ionization of molecular oxygen has been studied using a magnetic bottle time-of-flight electron coincidence spectrometer. The K-1V-1 double ionization electron spectrum of O-2 is reported and is assigned with the aid of ab initio calculations. A direct comparison of the core-valence double ionization electron spectra with the conventional valence band photoelectron spectrum is made. The lowest core-valence double ionization energy is found to be 571.6 eV and is associated with a (3)Pi dicationic state.