Dissociative photoionization of NO across a shape resonance in the XUV range using circularly polarized synchrotron radiation.

We report benchmark results for dissociative photoionization (DPI) spectroscopy and dynamics of the NO molecule in the region of the σ* shape resonance in the ionization leading to the NO+(c3Π) ionic state. The experimental study combines well characterized extreme ultraviolet (XUV) circularly polarized synchrotron radiation, delivered at the DESIRS beamline (SOLEIL), with ion-electron coincidence 3D momentum spectroscopy. The measured (N+, e) kinetic energy correlation diagrams reported at four discrete photon energies in the extended 23-33 eV energy range allow for resolving the different active DPI reactions and underline the importance of spectrally resolved studies using synchrotron radiation in the context of time-resolved studies where photoionization is induced by broadband XUV attosecond pulses. In the dominant DPI reaction which leads to the NO+(c3Π) ionic state, photoionization dynamics across the σ* shape resonance are probed by molecular frame photoelectron angular distributions where the parallel and perpendicular transitions are highlighted, as well as the circular dichroism CDAD(θe) in the molecular frame. The latter also constitute benchmark references for molecular polarimetry. The measured dynamical parameters are well described by multichannel Schwinger configuration interaction calculations. Similar results are obtained for the DPI spectroscopy of highly excited NO+ electronic states populated in the explored XUV photon energy range

Circular dichroism in molecular-frame photoelectron angular distributions in the dissociative photoionization of H2 and D2 molecules

ABSTRACT: The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecular-frame photoelectron angular distributions in dissociative photoionization of H2 [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of H2 and D2 molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Time-dependent ab initio calculations support and explain the observed results for H2 in terms of quantum interferences between direct photoionization and delayed autoionization from the Q1 and Q2 doubly excited states into ionic states (1sσg and 2pσu) of different inversion symmetry. Nevertheless, for D2 at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when D+ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny

Signature of a conical intersection in the dissociative photoionization of formaldehyde

9 pags., 4 figs., 2 tabs. -- This article is part of the themed collection: 2020 PCCP HOT ArticlesThe valence-shell photoionization of formaldehyde is investigated by means of combining Photo-Electron Photo-Ion COincidence (PEPICO) experiments and ab initio calculations. The formation of three ion fragments: HCO+, CO+ and H+2, via dissociative photoionization following excitation at 17 eV is discussed. The experimental results consisting of electron-ion kinetic energy correlation diagrams for the corresponding coincident events, i.e. (HCO+, e-), (CO+, e-) and (H+2, e-), as well as the fragment abundance as a function of the binding energy, are complemented by high level electronic structure calculations including potential energy curves and on-the-fly trajectories. The results are consistent with a main relaxation process via internal conversion into rovibrationally excited levels of the H2CO+ ground state, followed by statistical dissociation, preferentially into HCO+. The analysis of the experimental results reveals nevertheless the signature of a conical intersection controlling the dynamics and favoring dissociation into the molecular channel, CO+ + H2. In addition, the minor formation of the H+2 ion is suggested to occur through a roaming pathway on the cation excited state.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 842539 (ATTO-CONTROL) and has been financed in part by the Spanish MINECO (grant CTQ2015-65033-P) and the Spanish Ministry of Science and Innovation (grant PGC2018- 096444-B-I00)