Neutral dissociation of the I, I′, and I″ vibronic progressions of O2

Disentangling Multiphoton Ionization and Dissociation Channels in Molecular Oxygen Using Photoelectron-Photoion Coincidence Imaging

Multiphoton excitation of molecular oxygen in the 392-408 nm region is studied using a tunable femtosecond laser coupled with a double velocity map imaging photoelectron-photoion coincidence spectrometer. The laser intensity is held at ≤∼1 TW/cm² to ensure excitation in the perturbative regime, where the possibility of resonance enhanced multiphoton ionization (REMPI) can be investigated. O₂⁺ production is found to be resonance enhanced around 400 nm via three-photon excitation to the e'³Δ_u(v = 0) state, similar to results from REMPI studies using nanosecond dye lasers. O⁺ production reaches 7% of the total ion yield around 405 nm due to two processes: autoionization following five-photon excitation of O₂, producing O₂⁺(X(v)) in a wide range of vibrational states followed by two- or three-photon dissociation, or six-photon excitation to a superexcited O₂** state followed by neutral dissociation and subsequent ionization of the electronically excited O atom. Coincidence detection is shown to be crucial in identifying these competing pathways.

Symmetry of molecular Rydberg states revealed by XUV transient absorption spectroscopy

Transient absorption spectroscopy is utilized extensively for measurements of bound- and quasibound-state dynamics of atoms and molecules. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses has the potential to attain unprecedented time resolution. Here we apply this technique to aligned-in-space molecules. The XUV pulses are much shorter than the time during which the molecules remain aligned, typically \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<$$\end{document}<100 fs. However, transient absorption is not an instantaneous probe, because long-lived coherences re-emit for picoseconds to nanoseconds. Due to dephasing of the rotational wavepacket, it is not clear if these coherences will be evident in the absorption spectrum, and whether the properties of the initial excitations will be preserved. We studied Rydberg states of N\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2 and O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2 from 12 to 23 eV. We were able to determine the polarization direction of the electronic transitions, and hence identify the symmetry of the final states.

Transient absorption spectroscopy is used to identify the structural characteristics of the atoms and molecules. Here the authors used extreme ultraviolet transient absorption spectroscopy to identify the Rydberg state symmetry of aligned molecules.

Resonant photoionization of O2 up to the fourth ionization threshold

We present a detailed theoretical study of valence-shell photoionization of the oxygen molecule by using the recently proposed XCHEM method. This method makes use of a hybrid Gaussian and B-spline basis in the framework of a close-coupling approach to describe electron correlation in the molecular electronic continuum at a level comparable to that provided by multi-reference configuration interaction methods in bound state calculations. The computed total and partial photoionization cross sections are presented and discussed, with emphasis on the series of autoionizing resonances that appear between the first and the fourth ionization thresholds, i.e., photon energies between 12 and 18 eV. More than fifty autoionizing states are identified, including series not previously reported in the literature, and their energy positions and widths are provided. The present results illustrate the potential of the XCHEM approach to accurately describe molecular autoionization, which is mostly due to electron correlation. This is relevant in view of current experimental efforts aimed at providing real-time (attosecond) imaging of autoionization dynamics in molecules.

Breakdown of the Spectator Concept in Low-Electron-Energy Resonant Decay Processes

We suggest that low-energy electrons, released by resonant decay processes, experience substantial scattering on the electron density of excited electrons, which remain a spectator during the decay. As a result, the angular emission distribution is altered significantly. This effect is expected to be a common feature of low-energy secondary electron emission. In this Letter, we exemplify our idea by examining the spectator resonant interatomic Coulombic decay of Ne dimers. Our theoretical predictions are confirmed by a corresponding coincidence experiment.

Effect of a triplet to singlet state interaction on photofragmentation dynamics: highly excited states of HBr probed by VMI and REMPI as a case study

Analysis of mass resolved spectra as well as velocity map images derived from resonance enhanced multiphoton ionization (REMPI) of HBr via resonance excitations to mixed Rydberg (6pπ ³Σ^-(v' = 0)) and valence (ion-pair) (V ¹Σ⁺(v' = m + 17)) states allows characterization of the effect of a triplet-to-singlet state interaction on further photoexcitation and photoionization processes. The analysis makes use of rotational spectra line shifts, line intensity alterations, kinetic energy release spectra as well as angular distributions. Energy-level-dependent state mixing of the resonance excited states is quantified and photoexcitation processes, leading to H⁺ formation, are characterized in terms of the states and fragmentation processes involved, depending on the state mixing.

Rydberg, valence, and ion-pair quintet states of O2

We report an ab initio study of the quintet states of molecular oxygen. The calculations are carried out employing the multireference single and double excitation configuration interaction package. Potential energy curves of the six quintet valence states dissociating into ground state atoms and of the four quintet states dissociating to ion-pair atoms were computed. A number of bound quintet Rydberg series converging to the a(4)Πu and b(4)Σg(-) states of the O2(+) cation have been identified.

Ion-pair dissociation dynamics of O2 in the range 17.2-17.5 eV studied by XUV laser and velocity map imaging method

The ion-pair dissociation dynamics of O2, O2 + hv → O(+)((4)S) + O(-)((2)P(1/2, 3/2)), in the photon energy range 17.20-17.50 eV has been studied using extreme ultraviolet laser and velocity map imaging method. The ion-pair yield spectrum and the fine structure resolved photofragment O(-)((2)P(j)) velocity map images have been recorded. The branching ratios between the two spin-orbit components O(-)((2)P(3/2)) and O(-)((2)P(1/2)) and the corresponding anisotropy parameters describing their angular distributions have been determined. It is found that the fragments O(-)((2)P(1/2)) are all from parallel transitions, while the fragments O(-)((2)P(3/2)) are from both parallel and perpendicular transitions. The main products for most of the excitation photon energies are O(-)((2)P(1/2)). The dissociation dynamics has been discussed based on the ab initio potential energy curves of the ion-pairs. The major peaks in the ion-pair yield spectrum have been assigned based on the angular distribution of the photofragments. The experimental results suggest that the so-called strong and weak series of Rydberg states converging to O2(+)b(4)Σg(-) should have symmetries of (3)Σu(-) and (3)Πu, respectively. In addition to the Rydberg states converging to O2(+)b(4)Σg(-), the Rydberg states converging to O2(+)A(2)Πu should also play a role in the ion-pair dissociation of O2.