Symmetry breaking in the body-fixed electron emission pattern due to electron-retroaction in the photodissociation of H2+ and D2+ close to threshold

We present an experimental investigation of symmetry breaking of H2 and D2 molecules after single photoionization due to the Coulomb field of the emitted slow electron interacting with the parent cation during dissociation. The experiments were carried out by measuring the three-dimensional momentum vectors of the photoelectron and recoiling ion in coincidence using a reaction microscope. For photon energies close to threshold, the low-energy photoelectron influences the dissociation process, which results in an asymmetric molecular frame photoelectron angular distribution. This can be explained by the retroaction of the Coulomb field of the photoelectron on its parent ion and has been recently experimentally demonstrated by M. Waitz et al. [Phys. Rev. Lett. 116, 043001 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.043001], confirming theoretical predictions by V. V. Serov and A. S. Kheifets [Phys. Rev. A 89, 031402(R) (2014)PLRAAN1050-294710.1103/PhysRevA.89.031402]. High-momentum resolution and a new series of photon energies just above the dissociation threshold enable the observation of a strong influence of the electron energy and nuclear kinetic energy on the electron localization process for energies below ∼100 meV, which so far has neither been observed nor discussed by theory. Exploring the limitations of the retroaction mechanism at our lowest photon energy, we are able to single out a sensitive testbed and present data of non-Born-Oppenheimer dynamics of the simplest molecular system for future benchmark computational treatments

Symmetry breaking in the body-fixed electron emission pattern due to electron-retroaction in the photodissociation of H2+ and D2+ close to threshold

We present an experimental investigation of symmetry breaking of H2 and D2 molecules after single photoionization due to the Coulomb field of the emitted slow electron interacting with the parent cation during dissociation. The experiments were carried out by measuring the three-dimensional momentum vectors of the photoelectron and recoiling ion in coincidence using a reaction microscope. For photon energies close to threshold, the low-energy photoelectron influences the dissociation process, which results in an asymmetric molecular frame photoelectron angular distribution. This can be explained by the retroaction of the Coulomb field of the photoelectron on its parent ion and has been recently experimentally demonstrated by M. Waitz et al. [Phys. Rev. Lett. 116, 043001 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.043001], confirming theoretical predictions by V. V. Serov and A. S. Kheifets [Phys. Rev. A 89, 031402(R) (2014)PLRAAN1050-294710.1103/PhysRevA.89.031402]. High-momentum resolution and a new series of photon energies just above the dissociation threshold enable the observation of a strong influence of the electron energy and nuclear kinetic energy on the electron localization process for energies below ∼100 meV, which so far has neither been observed nor discussed by theory. Exploring the limitations of the retroaction mechanism at our lowest photon energy, we are able to single out a sensitive testbed and present data of non-Born-Oppenheimer dynamics of the simplest molecular system for future benchmark computational treatments

Tracing intermolecular Coulombic decay of carbon-dioxide dimers and oxygen dimers after valence photoionization

We have conducted an experimental study on the photo double ionization (PDI) of carbon-dioxide dimers at photon energies of 37 and 55 eV and oxygen dimers at photon energies of 38, 41.5, and 46 eV, while focusing on the dissociation dynamics upon single-photon absorption. The investigation was performed by applying the cold-target recoil-ion momentum spectroscopy method in order to collect and record the three-dimensional momenta of the ionic fragments and emitted electrons from the dissociating dimer in coincidence. The kinetic-energy release upon fragmentation and the electron angular distributions in the laboratory and body-fixed frames, as well as the relative electron-electron emission angle, show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon-dioxide dimers upon photoionization below and above the double-ionization threshold of CO2 monomers. The PDI of oxygen dimers is less conclusive and shows contributions from ICD and knock-off ionization mechanisms. As for atomic dimers, the present results reveal that ICD in CO2 dimers after valence PDI can also serve as a source for low-energy electrons, known to be very relevant in biological systems, cells, and tissues

Tracing inter-Coulombic decay of molecular dimers

We have conducted an experimental study on the photo double ionization (PDI) of carbon dioxide dimers and oxygen dimers, while focusing on the dissociation dynamics upon single photon absorption. The results in terms of the kinetic energy and angular distributions of the charged particles show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon dioxide dimers. In the oxygen dimer, the results show that ICD is accompanied by knock-off ionization mechanisms

Mechanisms and dynamics of the NH+2+ H+and NH++ H++ H fragmentation channels upon single-photon double ionization of NH3

We present state-selective measurements on the NH2+ + H+ and NH+ + H+ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH3, where the two photoelectrons and two cations are measured in coincidence using 3D momentum imaging. Three dication electronic states are identified to contribute to the NH2+ + H+ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH2+ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH2+ fragment with roughly 1 eV of internal energy. The NH+ + H+ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the NH2+ + H+ channel. We find evidence of autoionization contributing to each of the double ionization channels. The distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on both the photoionization and photodissociation mechanisms for the different dication states

Photoelectron and fragmentation dynamics of the H++ H+ dissociative channel in NH3 following direct single-photon double ionization

We report measurements on the H++H+ fragmentation channel following direct single-photon double ionization of neutral NH3 at 61.5 eV, where the two photoelectrons and two protons are measured in coincidence using three-dimensional (3D) momentum imaging. We identify four dication electronic states that contribute to H++H+ dissociation, based on our multireference configuration-interaction calculations of the dication potential energy surfaces. The extracted branching ratios between these four dication electronic states are presented. Of the four dication electronic states, three dissociate in a concerted process, while the fourth undergoes a sequential fragmentation mechanism. We find evidence that the neutral NH fragment or intermediate NH+ ion is markedly rovibrationally excited. We also identify differences in the relative emission angle between the two photoelectrons as a function of their energy sharing for the four different dication states, which bare some similarities to previous observations made on atomic targets