Session J4: Photoionization or Photofragmentation of Homonuclear Molecules

A Direct Measurement of the Dissociative Lifetimes for Superexcited States of Molecular Oxygen

Using an attosecond extreme ultraviolet (XUV) pump-probe set up, we performed a {\it direct} measurement of the dissociation lifetime of superexcited states corresponding to the $nl\sigma_g(c^4\Sigma_u^-)$ Rydberg series of O$_2$. Superexcited states are highly-excited, neutral molecular states which lie far above the first ionization potential. These states are found to play a major role in the chemistry of the upper atmosphere but are difficult to model due to their non-Born-Oppenheimer behavior. Using a direct time-domain scheme, we measured a dissociation lifetime of $\tau_d=105\pm8$ fs, a factor 1.5 longer than dissociation lifetimes reported for the molecular ion-core. These results indicate the influence of the Rydberg electron on the ultrafast fragmentation dynamics of the ion-core and can provide insight into interaction between electronic and nuclear degrees of freedom in the non-Born-Oppenheimer regime.   

Ultrafast Dynamics of Ozone Exposed to Ionizing Radiation

By irradiating ozone molecules with few-femtosecond soft x-ray pulses and probing the fragmentation pathways, we find that any excess energy is rapidly and efficiently transferred into internal excitation of the triatomic molecule. We explore the Coulomb explosion of O$_{3}$ when irradiated with soft x-rays above the double ionization threshold, at photon energies in the XUV (43 eV) range. The super-excited states are then probed using a femtosecond infrared (IR) pulse in combination with 3D coincidence momentum imaging (COLTRIMS). By comparing the O$_{3}^{+}$ and O$_{2}^{+}$ ion fragmentation yields and their kinetic energies as a function of time delay between the XUV pump and IR probe, we find that the triatomic ozone molecule shows an ability to absorb any excess energy internally rather than emerging as kinetic energy released by the Coulomb exploding fragments. This internal conversion of energy in a triatomic molecule before explosion is very different from the case of diatomic molecular oxygen.   

Two are better than one: Combining ATI and KER spectra

Molecular breakup in a strong laser field is a vital topic, because measurement of the resulting fragments is a key tool for learning about their dynamics. Studies of kinetic energy release (KER) and above-threshold-ionization (ATI) spectra as a function of, e.g., molecular alignment and carrier-envelope-phase have revealed important information about both nuclear and electronic behavior. We explore the potential of gaining even more insight by investigating the breakup probability as a function of all fragment energies at once. As ATI and KER spectra are projections of the joint energy spectrum, this joint spectrum gives a more detailed look into fragmentation dynamics. Validating our strong-field-approximation-based qualitative picture for H$_2^+$ allows us to generalize our studies to larger molecules. In particular, we show that when the fragmentation probability of even a complex molecule is resolved onto the energies of all fragments, the probability peaks on surfaces separated by the photon energy where the distribution on a given multi-photon surface reflects structure and dynamics of the molecule.   

R-dependent strong field ionization from a neutral ground state diatomic molecule

Strong-field ionization of molecules is significantly more complicated than for atoms, due to the rotational and vibrational degrees of freedom and the closer energy spacing of different orbitals. However, a complete understanding of strong field ionization may lead to new techniques for understanding the electronic structure of molecules. Especially interesting would be a way to probe how the orbital structure changes as a function of internuclear separation. To this end, we have begun a series of experiments on I$_{2}$ molecules in which we create a coherent vibrational wavepacket centered on v=33 of the ground state of the neutral molecule. This allows us to probe ionization as a function of internuclear separation in the neutral ground state, for the first time.   

Effect of moving nuclei in multiphoton ionization of the H$_2^{\,+}$ ion

We propose an accurate {\it ab initio} numerical method to depict the dynamics of the nuclear fragments and the entangled motion of the nuclei and the electron in the laser-driven H$_2^{\,+}$ ion. Building on recent work~[1], we solve the time-dependent Schr\"odinger equation in prolate spheroidal coordinates and extract the angle-differential cross section for the photo\-electron as well as the kinetic energy release spectra of the nuclei. Assuming that the nuclei are frozen in their lowest rotational state, the nuclear coordinate in the wave function is discretized through a finite-element discrete-variable representation (FE-DVR), which is coupled to other FE-DVRs for the electron coordinate. The present procedure in full dimensionality allows us to go beyond the popular fixed-nuclei approximation, the two-channel approximation, and reduced-dimensional models. As a specific example, we discuss the effect of the nuclear motion in the H$_2^{\,+}$ ion followed by two- and three-photon absorption, both in the parallel and perpendicular geometries.\\[4pt] [1] X.~Guan, E.~Secor, K.~Bartschat, and B.~I.~Schneider, Phys.~Rev.~A~{\bf 84} (2011) 032420.   

Femtosecond XUV Transient Absorption of Strong-Field Induced Vibrational Wavepackets in Molecular Bromine

The development of table-top extreme ultraviolet (XUV) transient absorption spectroscopy has allowed for the investigation of chemical dynamics with both elemental specificity and chemical environment sensitivity on the femtosecond and even attosecond timescales in real-time. In this experiment, vibrational wavepackets in molecular bromine are simultaneously prepared by high field ionization on the neutral and ionized ground state potentials via an 800 nm (2.0 x 10$^{14 }$W/cm$^{2})$ pump. High harmonic generation from a semi-infinite gas cell source is then employed to probe the wavepacket evolutions via the core level transient absorption of the bromine 3d electrons at 65 -- 72 eV with 20 fs, 200 meV spectral resolution. By monitoring the molecular bromine neutral depletion and ion absorption transitions, recurrences are observed in the transient absorption signal amplitude, with 105 fs and 92 fs periods, respectively, which are indicative of the creation of vibrational coherences in both states. The subsequent dissociative ionization of atomic bromine is also observed through the Br $^{2}$P$_{3/2}-^{2}$D$_{5/2}$, $^{2}$P$_{1/2}-^{2}$D$_{3/2}$, $^{2}$P$_{3/2}-^{2}$D$_{3/2}$, and $^{2}$P$_{3/2}-^{2}$D$_{5/2}$ transitions. Probing of the vibrational coherences via core level excitation allows for state selective investigation of the strong-field ionization dynamics from an elemental perspective. Extension of this work to polyatomics should allow for site and state selective investigation of both vibrational and electronic coherences.   

Angle-resolved and internuclear-separation-resolved measurements of the ionization rate of the B state of I2 by strong laser ?elds

For the first time, angle and internuclear separation resolved measurements of the single ionization rate of neutral I$_{2}$ have been obtained. By launching a wavepacket in the B$^3\Pi_u^+$ (B-state) of I$_{2}$ with a 50 fs tunable pump pulse we can measure the ionization rate as a function of internuclear separation as the wavepacket evolves in the B-state. Moreover, since the ground to B-state optical transition dipole moment is parallel to the internuclear axis, the B-state sub-population of the I$_{2}$ thermal ensemble will have a high degree of alignment, allowing for angular measurements. The B-state shows the well-known effect of enhanced ionization at a critical separation R$_c$ with an enhancement factor of 22 when the ionizing field is polarized along the internuclear axis and the enhanced ionization decreases when the angle between the field and the axis increases from 0$^\circ$ to 90$^\circ$, finally disappearing at 90$^\circ$. These results on the enhanced ionization of the B-state of I$_{2}$ give the most precise determination of R$_{c}$ for any molecule and agree extremely well with the prediction from a simple model of electron localization.   

Routes to formation of highly excited neutral atoms in the break-up of strongly driven hydrogen molecule

We present a theoretical quasiclassical treatment of the formation, during Coulomb explosion, of highly excited neutral H atoms for strongly-driven hydrogen molecule. This process, where after the laser field is turned off, one electron escapes to the continuum while the other occupies a Rydberg state, was recently reported in an experimental study in Phys. Rev. Lett 102, 113002 (2009). We find that two-electron effects are important in order to correctly account for all pathways leading to highly excited neutral hydrogen formation [1]. We identify two pathways where the electron that escapes to the continuum does so either very quickly or after remaining bound for a few periods of the laser field. These two pathways of highly excited neutral H formation have distinct traces in the probability distribution of the escaping electron momentum components. \\[4pt] [1] A. Emmanouilidou, C. Lazarou, A. Staudte and U. Eichmann, Phys. Rev. A (Rapid) 85 011402 (2012).   

Double ionization of dimers in intense laser pulses

Experiments on double ionization of rare gas dimers by weak synchrotron radiation as well as by strong infrared laser pulses have been reported recently. New pathways to double ionization, in which the electrons are emitted either from the same atom or different atoms in the dimer, have been proposed on the basis of the experimental data. We apply a recently developed theoretical two-electron model to explore the correlated emission of two electrons in a rare gas dimer due to the interaction with attosecond VUV and XUV radiation and/or intense near-infrared laser pulses. In particular, we study the double ionization mechanisms mediated by electron correlation and their temporal resolution.   

Internuclear separation resolved asymmetric dissociation of I$_{2}$ in a two-color laser field

We have designed a pump-probe experiment to excite I$_{2}$ to the $B$ state and subsequently ionize the molecule with a two-color (800 and 400 nm) probe pulse. By varying the relative phase of the two colors we are able to probe the asymmetric dissociation of I$_{2}^{2+} \rightarrow$ I$^{2+} +$ I and we observe spatial asymmetries in the ion yield of this (2,0) channel. Because the durations (35 fs) of the pump and probe pulses are much shorter than the vibrational period of the $B$ state (700 fs) we can fully resolve the dynamics as a function of internuclear separation $R$. We find that the amplitude of the spatial asymmetry increases as a function of $R$ and that the relative phase of the two colors that produces the maximum asymmetry is independent of $R$. Both of these observations are consistent with ionization of I$_2$ directly into the field-dressed potential curves of I$_{2}^{2+}$, which we model with a two-electron 1-D double-well potential in an external field. Interestingly, we find a spatial asymmetry for dissociation channels with a charge difference $\Delta q = 2$, ((2,0) and (3,1)), but not for $\Delta q = 1$, ((1,0), (2,1), (3,2)). Finally, substructure in the time-of-flight data shows two distinct states leading to the (2,0) dissociation limit.