Time-resolved imaging of bound and dissociating nuclear wave packets in strong-field ionized iodomethane.

We report the results of a time-resolved coincident ion momentum imaging experiment probing nuclear wave packet dynamics in the strong-field ionization and dissociation of iodomethane (CH3I), a prototypical polyatomic system for photochemistry and ultrafast laser science. By measuring yields, kinetic energies, and angular distributions of CH3+ + I+ and CH3+ + I++ ion pairs as a function of the delay between two 25 fs, 790 nm pump and probe pulses, we map both, bound and dissociating nuclear wave packets in intermediate cationic states, thereby tracking different ionization and dissociation pathways. In both channels, we find oscillatory features with a 130 fs periodicity resulting from vibrational motion (C-I symmetric stretch mode) in the first electronically excited state of CH3I+. This vibrational wave packet dephases within 1 ps, in good agreement with a simple wave packet propagation model. Our results indicate that the first excited cationic state plays a key role in the dissociative ionization of CH3I and that it represents an important intermediate in the sequential double and multiple ionization at moderate intensities.

Adaptive strong-field control of vibrational population in NO2.

An adaptive closed-loop system employing coincidence time-of-flight feedback is used to determine the optimal pulse shapes for manipulating the branching ratio of NO dications following double ionization by an intense laser pulse. Selection between the long-lived NO2+ and the dissociative N+ + O+ final states requires control of the vibrational population distribution in the transient NO2+. The ability to both suppress and enhance NO2+ relative to N+ + O+ is observed, with the effectiveness of shaped pulses surpassing near Fourier transform-limited pulses by about an order of magnitude in each direction, depending on the pulse energy. The control is subsequently investigated using velocity map imaging, identifying plausible dissociation pathways leading to N+ + O+. Combining the information about the N+ + O+ dissociation with a well-defined control objective supports the conclusion that the primary control mechanism involves selectively populating long-lived NO2+ vibrational states.

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.

Carrier-envelope phase control over pathway interference in strong-field dissociation of H2+.

The dissociation of an H2+ molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10(14) W/cm2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H+ fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net zero-photon and one-photon interference predominantly contributes at H+ + H kinetic energy releases of 0.2-0.45 eV, and net two-photon and one-photon interference contributes at 1.65-1.9 eV. These measurements of the benchmark H2+ molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase.

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.

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: 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.

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.

Incorporating real time velocity map image reconstruction into closed-loop coherent control.

We report techniques developed to utilize three-dimensional momentum information as feedback in adaptive femtosecond control of molecular dynamics. Velocity map imaging is used to obtain the three-dimensional momentum map of the dissociating ions following interaction with a shaped intense ultrafast laser pulse. In order to recover robust feedback information, however, the two-dimensional momentum projection from the detector must be inverted to reconstruct the full three-dimensional momentum of the photofragments. These methods are typically slow or require manual inputs and are therefore accomplished offline after the images have been obtained. Using an algorithm based upon an "onion-peeling" (also known as "back projection") method, we are able to invert 1040 × 1054 pixel images in under 1 s. This rapid inversion allows the full photofragment momentum to be used as feedback in a closed-loop adaptive control scheme, in which a genetic algorithm tailors an ultrafast laser pulse to optimize a specific outcome. Examples of three-dimensional velocity map image based control applied to strong-field dissociation of CO and O2 are presented.

Adaptive strong-field control of chemical dynamics guided by three-dimensional momentum imaging.

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the nω+C2D4→C2D3++D interaction is manipulated. Second, isomerization of acetylene (nω+C2H2→C2H2(2+)→CH2++C+) is controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

Suppressed dissociation of H(2)(+) vibrational states by reduced dipole coupling.

The suppression of H(2)(+) strong-field dissociation has intrigued experimentalists and theorists since the early days of laser-molecular science. We unravel a vibrational suppression effect due to weak dipole-matrix element coupling strengths of certain vibrational states, dependent on the laser frequency-a form of Cooper minima. This effect is demonstrated by our full-dimensional calculations on H(2)(+) dissociation and persists for a broad range of laser conditions including both weak and strong-field dissociation. Using a crossed-beams coincidence, three-dimensional momentum-imaging technique, the vibrational suppression effect is clearly observed for H(2)(+) and HD(+) at 790 and 395 nm, in good agreement with our theory.