K. A. Hanold

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−.

Translational spectroscopy studies of the photodissociation dynamics of O−4

An investigation of the photodissociation dynamics of the dimer anion O−4 at 523.6, 349.0, and 261.8 nm is reported. Product translational energy and angular distributions have been obtained using photofragment translational spectroscopy in a fast ion beam. At all wavelengths photodissociation (O−4+hν→O2+O−2) is observed to proceed via a rapid parallel electronic transition, with the photofragment angular distribution strongly peaked along the laser electric vector. The lowest energy photodissociation channel produces O2(a1Δg) and ground state O−2(X2Πg), indicating that O−4 is a doublet anion. The partitioning of energy in the dissociation reveals a complicated wavelength dependence.

Energy and angular distributions in dissociative photodetachment of O 4

An anisotropic product angular distribution has been observed in the dissociative photodetachment of O−4 at 523 nm. Energy and angular distributions of coincident O2 products from the process O−4+hν→O2+O2+e− were measured using translational energy spectroscopy in a fast ion beam. The angular distribution peaks perpendicular to the electric vector of the laser beam.

Complete kinematic measurement of three-body reaction dynamics: Dissociative photodetachment of O6− at 532 nm

Measurement of the translational energy partitioning in the three-body dissociative photodetachment of O6− (O6−+hν→O2+O2+O2+e−) at 532 nm is reported. Using photoelectron and photofragment translational energy spectroscopies in coincidence, a complete kinematic measurement of the three-body dissociation of neutral O6 is obtained. Vibrationally resolved product translational energy distributions are observed. The results provide insights into the structure, binding energy, and dissociation dynamics of O6− and O6 and illustrate a new approach to the study of three-body reaction dynamics.

Fast‐ion‐beam photoelectron spectrometer

A high‐collection‐efficiency fast‐ion‐beam photoelectron spectrometer is described. In a straight time‐of‐flight mode, the spectrometer collects ∼1% of the photoelectrons and achieves an energy resolution of ΔE/E of ∼5%. For coincidence experiments requiring greater collection efficiency, a paraboloidal electrostatic mirror is used. The mirror collects ∼40% of the photoelectrons while maintaining ΔE/E≤35%. In both modes of operation, a time‐ and position‐sensitive electron detector allows conversion of the photoelectron laboratory energy to center‐of‐mass energy. The fast‐ion‐beam photoelectron spectrometer is used to prepare mass‐ and energy‐selected neutral molecules which are used in molecular dissociation studies.

Photoelectron–neutral–neutral coincidence studies of dissociative photodetachment

Photoelectron–neutral–neutral coincidence spectra have been measured for the dissociative photodetachment of O4− (O4−+hν→O2+O2+e−) at 523 and 349 nm. The neutral photofragment translational energy spectrum, the photoelectron spectrum, and the correlations of the translational energy and photoelectron energy are presented here. The correlation spectra reveal phenomena that are not discernable in either one‐dimensional measurement. Features are observed which indicate that non‐Franck–Condon processes play a role in the dissociative photodetachment of O4− at 349 nm.

Photodissociation dynamics of O3− at 523 nm

Abstract The photodissociation dynamics of O 3 − at 523 nm have been studied using fast-ion-beam translational energy spectroscopy. Translational energy and angular distributions of coincident O − + O 2 products from the process O 3 − + hv → O − + O 2 were measured. O 3 − was generated by electron-impact in a pulsed beam from two precursors — neat O 2 and a seeded beam of O 3 . The observed photodissociation dynamics are very different in the two cases, indicating a great difference in internal excitation of ozonide in the two sources.

Excited state dynamics in clusters of oxygen

Considerable insights into the dynamics of both ionic (photodissociation) and neutral (dissociative photodetachment) decomposition pathways of O4- and O6- have been gained using photoelectron and photofragment translational spectroscopy in a fast-ion beam. The O4- data at 532 nm reveal a novel process involving sequential photodetachment of an electron with a near-zero binding energy from photodissociating O4-. Studies of O6- at 532 nm reveal that addition of a third O2 to the O4- core leads to a dramatic change in the photodissociation dynamics, producing highly vibrationally excited O2- photofragments not observed in the case of O4-. At 355 nm, both O4- and O6- yield vibrationally excited O2- photofragments, as observed by autodetachment of the nascent O2- (v5)→O2+e-. At 266 nm, photofragment time-of-flight (TOF) measurements on O6- and O4- show that the dynamics of dissociative photodetachment in O6- are only slightly perturbed relative to O4-. The anisotropic product angular distribution previously observed in O4- is observed to persist in the three-body neutral decomposition O6-+hν→O2+O2+O2+e-. The origins of these diverse phenomena in O4- and O6- are discussed.

Dynamics of dissociative photodetachment in cluster anions: O4- and O2-H2O

A preliminary report is presented on experiments using fast ion-beam translational energy spectroscopy to study dissociative photodetachment and photodissociation dynamics in the small cluster ions O4- and O2-(DOT)(H2O). Translational energy and angular distributions of coincident molecular fragments were recorded from the photodestruction of O4- and O2-(DOT)(H2O) at 523, 349, and 262 nm. At each wavelength, the O4- results confirm the existence of at least two distinct channels: dissociative photodetachment (O4- + h(nu) yields O2 + O2 + e-) and photodissociation (O4- + h(nu) yields O2 + O2-). Observation of strongly anisotropic angular distributions shows that dissociation occurs on the time-scale of molecular rotation in both processes. The photodissociation of O4- at 523 nm gives a new value for the O2-O2- bond energy, DO equals 0.39 +/- 0.05 eV. In O2-(DOT)(H2O), a single dissociative photodetachment channel (O2-(DOT)(H2O) + h(nu) yields O2 + H2O + e-) is observed at all wavelengths. Angular distributions from this process are slightly anisotropic and exhibit a small wavelength dependence.