14 research outputs found

    Photoelectron and fragmentation dynamics of the H+^{+} + H+^{+} dissociative channel in NH3_3 following direct single-photon double ionization

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    We report measurements on the H+^{+} + H+^{+} fragmentation channel following direct single-photon double ionization of neutral NH3_{3} at 61.5 eV, where the two photoelectrons and two protons are measured in coincidence using 3-D 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. Of these 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 ro-vibrationally 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.Comment: 15 pages, 13 figures, 3 table

    Mechanisms and dynamics of the NH2+_2^{+} + H+^{+} and NH+^{+} + H+^{+} + H fragmentation channels upon single-photon double ionization of NH3_3

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    We present state-selective measurements on the NH2+_2^{+} + H+^{+} and NH+^{+} + H+^{+} + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH3_{3}, where the two photoelectrons and two cations are measured in coincidence using 3-D momentum imaging. Three dication electronic states are identified to contribute to the NH2+_2^{+} + H+^{+} dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH2+_2^+ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH2+_2^+ 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+_2^{+} + 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.Comment: 18 pages, 21 figures, 3 table

    Strong Field Ionization of Water II: Electronic and Nuclear Dynamics En Route to Double Ionization

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    We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D+^{+}/D+^{+}/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D2_2O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data

    Filming enhanced ionization in an ultrafast triatomic slingshot

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    Abstract Filming atomic motion within molecules is an active pursuit of molecular physics and quantum chemistry. A promising method is laser-induced Coulomb Explosion Imaging (CEI) where a laser pulse rapidly ionizes many electrons from a molecule, causing the remaining ions to undergo Coulomb repulsion. The ion momenta are used to reconstruct the molecular geometry which is tracked over time (i.e., filmed) by ionizing at an adjustable delay with respect to the start of interatomic motion. Results are distorted, however, by ultrafast motion during the ionizing pulse. We studied this effect in water and filmed the rapid “slingshot” motion that enhances ionization and distorts CEI results. Our investigation uncovered both the geometry and mechanism of the enhancement which may inform CEI experiments in many other polyatomic molecules

    Step-by-step state-selective tracking of fragmentation dynamics of water dications by momentum imaging.

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    The double photoionization of a molecule by one photon ejects two electrons and typically creates an unstable dication. Observing the subsequent fragmentation products in coincidence can reveal a surprisingly detailed picture of the dynamics. Determining the time evolution and quantum mechanical states involved leads to deeper understanding of molecular dynamics. Here in a combined experimental and theoretical study, we unambiguously separate the sequential breakup via D+ + OD+ intermediates, from other processes leading to the same D+ + D+ + O final products of double ionization of water by a single photon. Moreover, we experimentally identify, separate, and follow step by step, two pathways involving the b 1Σ+ and a 1Δ electronic states of the intermediate OD+ ion. Our classical trajectory calculations on the relevant potential energy surfaces reproduce well the measured data and, combined with the experiment, enable the determination of the internal energy and angular momentum distribution of the OD+ intermediate
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