Direct Measurement of Charge Transfer Probability during Photodissociation of Few-keV OD+ Beam.

We have measured the photodissociation of few-keV OD+ molecular ions into either D+ + O or O+ + D final products. The three-dimensional momentum imaging measurements of the light and massive fragments in coincidence were enabled by using an upgraded two-detector setup. In this work, we show that absorption of a single 790 or 395 nm photon excites the OD+ from its electronic ground state to the B Σ-3 state, which dissociates to the O+(4S) + D dissociation limit. To reach the other nearly degenerate dissociation limit, D+ + O(3P), a unimolecular charge transfer, B Σ-3 to X Σ-3, transition is required following the same photoexcitation. The measured branching ratio of these dissociation channels is a direct measure of the charge transfer transition probability. This measured probability as a function of energy above the dissociation limit agrees well with our calculations.

Efficiency of charge transfer in changing the dissociation dynamics of OD+ transients formed after the photo-fragmentation of D2O.

We present an investigation of the relaxation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV. We focus on the very rare D+ + O+ + D reaction channel in which the sequential fragmentation mechanisms were found to dominate the dynamics. Aided by theory, the state-selective formation and breakup of the transient OD+(a1Δ, b1Σ+) is traced, and the most likely dissociation path-OD+: a1Δ or b1Σ+ → A 3Π â†’ X 3Σ- → B 3Σ--involving a combination of spin-orbit and non-adiabatic charge transfer transitions is determined. The multi-step transition probability of this complex transition sequence in the intermediate fragment ion is directly evaluated as a function of the energy of the transient OD+ above its lowest dissociation limit from the measured ratio of the D+ + O+ + D and competing D+ + D+ + O sequential fragmentation channels, which are measured simultaneously. Our coupled-channel time-dependent dynamics calculations reproduce the general trends of these multi-state relative transition rates toward the three-body fragmentation channels.

Atomic autoionization in the photo-dissociation of super-excited deuterated water molecules fragmenting into D+ + O+ + D.

We present the relaxation dynamics of deuterated water molecules via autoionization, initiated by the absorption of a 61 eV photon, producing the very rare D+ + O+ + D breakup channel. We employ the COLd target recoil ion momentum spectroscopy method to measure the 3D momenta of the ionic fragments and emitted electrons from the dissociating molecule in coincidence. We interpret the results using the potential energy surfaces extracted from multi-reference configuration interaction calculations. The measured particle energy distributions can be related to a super-excited monocationic state located above the double ionization threshold of D2O. The autoionized electron energy shows a sharp distribution centered around 0.5 eV, which is a signature of the atomic oxygen autoionization occurring in the direct and sequential dissociation processes of D2O+* at a large internuclear distance. In this way, an O+ radical fragment and a low-energy electron are created, both of which can trigger secondary reactions in their environment.

Strong-field control of H3 + production from methanol dications: Selecting between local and extended formation mechanisms.

Using the CD3OH isotopologue of methanol, the ratio of D2H+ to D3 + formation is manipulated by changing the characteristics of the intense femtosecond laser pulse. Detection of D2H+ indicates a formation process involving two hydrogen atoms from the methyl side of the molecule and a proton from the hydroxyl side, while detection of D3 + indicates local formation involving only the methyl group. Both mechanisms are thought to involve a neutral D2 moiety. An adaptive control strategy that employs image-based feedback to guide the learning algorithm results in an enhancement of the D2H+/D3 + ratio by a factor of approximately two. The optimized pulses have secondary structures 110-210 fs after the main pulse and result in photofragments that have different kinetic energy release distributions than those produced from near transform limited pulses. Systematic changes to the linear chirp and higher order dispersion terms of the laser pulse are compared to the results obtained with the optimized pulse shapes.

Dependence on the Initial Configuration of Strong Field-Driven Isomerization of C2H2 Cations and Anions.

We have investigated the femtosecond laser-induced fragmentation of C2H2 q ion beam targets in various initial configurations, including acetylene (linear HCCH), vinylidene (H2CC), and cis/ trans. The initial configuration is shown to have a tremendous impact on the branching ratio of acetylene-like (CH q1 + CH q2) and vinylidene-like (C q1' + CH2 q2') dissociation of a specific C2H2 q molecular ion. In particular, whereas C2H2+ generated from C2H2, a linear HCCH target, exhibits comparable levels of acetylene-like and vinylidene-like fragmentation, vinylidene or cis/ trans configuration ion beams preferably undergo vinylidene-like fragmentation, with an acetylene branching ratio ranging from 13.9% to zero.

Resonance signatures in the body-frame valence photoionization of CF4.

We present a combined experimental and theoretical investigation of the electron dynamics and body-frame angular dependence of valence photo-single ionization of CF4 and subsequent dissociation into CF3+ and F. Ionization from a valence t2 orbital shows overlapping shape resonances close to threshold that couple to the same total symmetry, leading to striking changes in the photoelectron angular distributions when viewed in the body-frame.

Native Frames: Disentangling Sequential from Concerted Three-Body Fragmentation.

A key question concerning the three-body fragmentation of polyatomic molecules is the distinction of sequential and concerted mechanisms, i.e., the stepwise or simultaneous cleavage of bonds. Using laser-driven fragmentation of OCS into O^{+}+C^{+}+S^{+} and employing coincidence momentum imaging, we demonstrate a novel method that enables the clear separation of sequential and concerted breakup. The separation is accomplished by analyzing the three-body fragmentation in the native frame associated with each step and taking advantage of the rotation of the intermediate molecular fragment, CO^{2+} or CS^{2+}, before its unimolecular dissociation. This native-frame method works for any projectile (electrons, ions, or photons), provides details on each step of the sequential breakup, and enables the retrieval of the relevant spectra for sequential and concerted breakup separately. Specifically, this allows the determination of the branching ratio of all these processes in OCS^{3+} breakup. Moreover, we find that the first step of sequential breakup is tightly aligned along the laser polarization and identify the likely electronic states of the intermediate dication that undergo unimolecular dissociation in the second step. Finally, the separated concerted breakup spectra show clearly that the central carbon atom is preferentially ejected perpendicular to the laser field.

The importance of Rydberg orbitals in dissociative ionization of small hydrocarbon molecules in intense laser fields.

Much of our intuition about strong-field processes is built upon studies of diatomic molecules, which typically have electronic states that are relatively well separated in energy. In polyatomic molecules, however, the electronic states are closer together, leading to more complex interactions. A combined experimental and theoretical investigation of strong-field ionization followed by hydrogen elimination in the hydrocarbon series C2D2, C2D4 and C2D6 reveals that the photofragment angular distributions can only be understood when the field-dressed orbitals rather than the field-free orbitals are considered. Our measured angular distributions and intensity dependence show that these field-dressed orbitals can have strong Rydberg character for certain orientations of the molecule relative to the laser polarization and that they may contribute significantly to the hydrogen elimination dissociative ionization yield. These findings suggest that Rydberg contributions to field-dressed orbitals should be routinely considered when studying polyatomic molecules in intense laser fields.

Note: Determining the detection efficiency of excited neutral atoms by a microchannel plate detector.

We present a method for determining the detection efficiency of neutral atoms relative to keV ions. Excited D* atoms are produced by D2 fragmentation in a strong laser field. The fragments are detected by a micro-channel plate detector either directly as neutrals or as keV ions following field ionization and acceleration by a static electric field. Moreover, we propose a new mechanism by which neutrals are detected. We show that the ratio of the yield of neutrals and ions can be related to the relative detection efficiency of these species.

Note: Position dependence of time signals picked off a microchannel plate detector.

Using an ultrafast laser and a precision mask, we demonstrate that time signals picked off directly from a microchannel plate detector depend on the position of the hit. This causes a time spread of about 280 ps, which can affect the quality of imaging measurements using large detectors.

The prevalence of facial asymmetry in the dentofacial deformities population at the University of North Carolina.

In a retrospective survey of 1,460 patients evaluated in the Dentofacial Clinic at the University of North Carolina, 495 (34%) were found to have clinically apparent facial asymmetry. When present, asymmetry affected the upper face in only 5% (n = 23), the midface (primarily the nose) in 36% (n = 178), and the chin in 74% (n = 365). The occlusal plane was canted, indicating vertical asymmetry, in 41% (n = 201). Patients with Class II problems, whether or not due to mandibular deficiency, had a 28% prevalence of asymmetry; those with other types of problems (e.g., Class III, long face, Class I) had a 40% prevalence, which is significantly higher than those with Class II occlusions. When the chin deviated transversely, there was an 80% chance that the deviation was to the left. Only in patients with long face was there an equal distribution of left-right chin asymmetry. In the other groups, the prevalence of deviation of the chin to the left approached 90%. These findings are meaningful for clinicians because asymmetry must be identified and planned for prior to initiating treatment.