Vacuum ultraviolet laser pulsed field ionization-photoelectron study ofcis-dichloroethene

Molecular photodissociation dynamics revealed by Coulomb explosion imaging

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, etc). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies. 'One-colour' studies (employing strong field ionisation driven by intense near infrared or single X-ray or extreme ultraviolet laser pulses) afford routes to preparing multiply charged molecular cations and exploring how their fragmentation progresses from valence-dominated to Coulomb-dominated dynamics with increasing charge and how this evolution varies with molecular size and composition. 'Two-colour' studies use one ultrashort laser pulse to create electronically excited neutral molecules (or monocations), whose structural evolution is then probed as a function of pump-probe delay using an ultrafast ionisation pulse along with time and position-sensitive detection methods. This latter type of experiment has the potential to return new insights into not just molecular fragmentation processes but also charge transfer processes between moieties separating with much better defined stereochemical control than in contemporary ion-atom and ion-molecule charge transfer studies.

Vertical and Adiabatic Ionization Potentials of Fluorinated, Chlorinated, and Chlorofluorinated Ethylenes Using G2 and G3 Theories

Adiabatic ionization potentials (AIPs) and vertical ionization potentials (VIPs) for all fluorinated, chlorinated, and chlorofluorinated ethylenes have been determined by ab initio computations. The calculated AIPs give a mean absolute deviation of 0.014 eV at G2 and 0.015 eV at G3 theories compared to experimental values. We have estimated AIPs (in eV) for AIP ((E)-CHCl=CFCl) = 9.59, AIP ((Z)-CHCl=CFCl) = 9.60, AIP (CCl2=CFCl) = 9.42, and AIP (CHF=CCl2) = 9.65. Furthermore, our calculated AIPs values of 9.58 eV for (Z)-CFCl=CFCl and 9.56 eV for (E)-CFCl=CFCl are very different from the experimental data of 10.2 eV. VIPs are calculated by Koopmans's theorem with HF methodology and by G2 and G3 theories. Koopmans's theorem fails in giving a good description of the behavior of the VIPs for fluoroethylenes. Furthermore, significant improvement in the results is observed by the mean absolute deviation from experimental data on the computed values (0.242 eV using the 6-311+G(3df,2p) basis set and 0.248 eV using the GTlarge basis set, compared with 0.049 eV at G2 and 0.045 eV at G3 theories) when orbital relaxation and changes in electronic correlation and zero-point energies are taken into account. Our estimated VIPs values calculated by G3 theory (in eV) are VIP ((E)-CHCl=CFCl) = 9.89, VIP ((Z)-CHCl=CFCl) = 9.90, VIP ((E)-CFCl=CHF)= 10.26, VIP ((Z)-CFCl=CHF) = 10.25, VIP ((E)-CFCl=CFCl) = 9.93, VIP ((Z)-CFCl=CFCl) = 9.96, VIP (CCl2=CFCl) = 9.71, VIP (CF2=CHCl) = 10.19, VIP (CHF=CCl2) = 9.96, VIP (CH2=CFCl) = 10.32, VIP ((Z)-CHCl=CHF) = 10.16, and VIP ((E)-CHCl=CHF) = 10.16. Furthermore, the variation of the VIPs and AIPs with the increase in the number of halogen atoms in the molecules presents different patterns to chloroethylenes and fluoroethylenes.

Benchmarking state-of-the-art ab initio thermochemical predictions with accurate pulsed-field ionization photoion-photoelectron measurements

This Account presents a comparison between highly precise thermochemical data, including ionization energies, 0 K dissociative photoionization thresholds, and 0 K bond dissociation energies, of selected radicals and molecules and their cations obtained by pulsed field ionization photoion-photoelectron measurements and state-of-the-art thermochemical predictions calculated by the wavefunction-based ab initio CCSD(T)/CBS procedures with high-level corrections. The CCSD(T)/CBS method combines the coupled cluster approach including single, double, and quasi-perturbative triple excitations [CCSD(T)] and the complete basis set (CBS) extrapolation approximation. This benchmarking effort indicates that the CCSD(T)/CBS procedures together with high-level corrections are capable of yielding reliable thermochemical predictions with error limits < or =10 meV for small radicals and molecules and their cations. The error limits increase to approximately 35 meV for larger molecular species, such as phenyl and benzyl radicals.