Todd G. Clements

Photoelectron-multiple-photofragment coincidence spectrometer

A new photoelectron–photofragment-coincidence spectrometer is described. Using a multiparticle time- and position-sensitive detector, this apparatus allows the study of dissociation processes of negative ions yielding three photofragments in coincidence with a photoelectron. The photoelectron spectrometer uses two detectors and works in time of flight mode, detecting 10% of the photoelectrons with an energy resolution of 5% at 1.3 eV as shown in studies of the photodetachment of O2−. A third detector is used for collection of multiple photofragments (up to 8) in coincidence. This multiparticle detector uses a crossed-delay-line anode and fast timing signals to encode the time- and position-of-arrival of multiple photofragments. The detector was demonstrated to record all three particles produced in a single three-body dissociation event, yielding an energy resolution of ≈15% ΔE/E at 0.7 eV in experiments on the three-body dissociative photodetachment of O6−.

Exploring the OH+CO→H+CO2 potential surface via dissociative photodetachment of (HOCO)−

Dissociative photodetachment (DPD) of the molecular anion HOCO− is used to probe the potential energy surface for the OH+CO→H+CO2 reaction. The HOCO− anion, formed by electron impact on an expansion of CH4+N2O+CO, is characterized for the first time in these experiments by photoelectron spectroscopy and photoelectron angular distribution measurements. Photodetachment of HOCO− is found to produce H+CO2+e− and OH+CO+e− products in addition to stable HOCO radicals. Ab initio calculations of the energetics and structure of HOCO− and HOCO are consistent with the experimental results and show that photodetachment to the ground electronic HOCO surface samples the vicinity of the HOCO well. The product translational energy distributions observed on the ground state surface are consistent with unimolecular decomposition out of the HOCO well. In addition, direct DPD to a repulsive excited state of HOCO, correlating to ground state OH+CO products is observed.

Predissociation dynamics of formyloxyl radical studied by the dissociative photodetachment of HCO2−/DCO2−+hν→H/D+CO2+e−

The dissociative photodetachment (DPD) of HCO2− and DCO2− was studied at 258 nm. State-resolved translational energy distributions were observed correlated to bending excitation in the CO2 product for the channel producing H/D+CO2, indicating very low rotational excitation in the products consistent with predissociation of a C2ν HCO2 molecule. No evidence was found for dissociation into OH+CO. All three low-lying electronic states (2A1, 2B2, and 2A2) were found to dissociate, but resolved progressions were only observed from photodetachment to the 2A1 and 2B2 states. Photoelectron-photofragment coincidence spectra for DCO2− show resolved vertical bands and indicate that multiple CO2 vibrational states are accessible from each vibrational level in the predissociating DCO2 molecule. The resolved structure is assigned to vibrational predissociation sequence bands, observable in this DPD process owing to the dissociation dynamics and the near degeneracy of the vibrational levels in the 2A1 and 2B2 states of H...

Energetics and dissociative photodetachment dynamics of superoxide–water clusters: O2−(H2O)n, n=1–6

The dissociative photodetachment of O2−(H2O)n=1–6 was studied at 388 and 258 nm using photoelectron–multiple-photofragment coincidence spectroscopy. Photoelectron spectra for the series indicate a significant change in the energetics of sequential solvation beyond the fourth water of hydration. Photoelectron–photofragment kinetic energy correlation spectra were also obtained for O2−(H2O)1–2, permitting a determination of the first and second energies of hydration for O2− to be 0.85±0.05 and 0.70±0.05 eV, respectively. The correlation spectra show that the peak photofragment kinetic energy release in the dissociative photodetachment of O2−(H2O) and O2−(H2O)2 are 0.12 and 0.25 eV, respectively, independent of the photon and photoelectron kinetic energies. The molecular frame differential cross section for the three-body dissociative photodetachment: O2−(H2O)2+hν→O2+2H2O+e− is also reported. The observed partitioning of momentum is consistent with either a sequential dissociation or dissociation from a range...

Transition state dynamics of the OH+OH→O+H2O reaction studied by dissociative photodetachment of H2O2−

Photoelectron-photofragment coincidence (PPC) spectroscopy has been used to study the dissociative photodetachment of H2O2− and D2O2−. The observed partitioning of photoelectron and photofragment translational energies provides information on the dynamics in the transition state region of the reaction between two hydroxyl radicals: OH+OH→O(3P)+H2O. The data reveal vibrationally resolved product translational energy distributions for both the entrance channel OH+OH and the exit channel O(3P)+H2O upon photodetachment. The total translational energy distribution shows a convoluted vibrational progression consistent with antisymmetric stretch excitation of H2O in the exit channel and OH stretch in the entrance channel. The photoelectron spectra are compared to two-dimensional time-dependent wave packet dynamics simulations based on an anharmonic potential in the anion and a model collinear potential energy surface for the neutral complex. The PPC spectra also yield the dissociation energies D0(H2O2−→H2O+O−)=1...

Transition state dynamics of the OH+H2O hydrogen exchange reaction studied by dissociative photodetachment of H3O2−

Dynamics in the transition state region of the bimolecular OH + H2O-->H2O + OH hydrogen exchange reaction have been studied by photoelectron-photofragment coincidence spectroscopy of the H3O2- negative ion and its deuterated analog D3O2-. The data reveal vibrationally resolved product translational energy distributions. The total translational energy distribution shows a vibrational progression indicating excitation of the antisymmetric stretch of the water product. Electronic structure calculations at the QCISD level of theory support this analysis. Examination of the translational energy release between the neutral products reveals a dependence on the product vibrational state. These data should provide a critical test of ab initio potential energy surfaces and dynamics calculations.

Dissociative photodetachment studies of O−(H2O)2, OH−(H2O)2, and the deuterated isotopomers: Energetics and three-body dissociation dynamics

Photoelectron–photofragment coincidence spectroscopy was used to study dissociative photodetachment of the doubly hydrated clusters of oxide and hydroxide, M−(H2O)2→M+2H2O+e− (M=O, OH). These experiments yield information on the energetics of the parent anion and the dissociation dynamics of the photodetached neutral species. Photoelectron spectra and photoelectron–photofragment coincidence spectra are presented and compared to data for O−(H2O) and OH−(H2O). Unlike the singly hydrated species, no evidence of vibrationally resolved product translational energy distributions is observed. The second hydration energy of O− with both H2O and D2O was also measured to be 0.80±0.08 and 0.81±0.08 eV, respectively. The three-body dissociation dynamics of the neutral clusters produced by photodetachment were studied by measuring the velocities and recoil angles of all the particles in coincidence. The observed partitioning of momentum is consistent with a two-step mechanism or dissociation from a wide range of start...

Three-body dissociation dynamics of (SO2)3 studied through dissociative photodetachment of (SO2)-3

The dissociative photodetachment dynamics of ðSO2Þ � were studied by photoelectron–photofragment coincidence spectroscopy at 258 nm. Correlation between the photoelectron and photofragment translational energies was observed as previously seen in the dimer system, implying the presence of a dimer core. The three-body dissociation dynamics of ðSO2Þ 3 after photodetachment are consistent with a dimer core solvated by a spectator SO2 molecule with a broad distribution in initial geometry. 2002 Elsevier Science B.V. All rights reserved.