Understanding learning control of molecular fragmentation

Post-Ionization Dynamics of the Polar Molecule OCS in Asymmetric Laser Fields

We have investigated the dissociation mechanisms of the prototypical heavy polar molecule OCS into the two break-up channels of the dication, OCS²⁺ → O⁺ + CS⁺ and OC⁺ + S⁺, in phase-locked two-color intense laser fields. The branching ratio of the breaking of the C–O and C–S bonds followed a pronounced 2π-oscillation with a modulation depth of 11%, depending on the relative phase of the two-color laser fields. The fragment ejection direction of both break-up channels reflects the anisotropy of the tunneling ionization rate, following a 2π-periodicity, as well. The two dissociation pathways in the C–S bond breaking channel show different phase dependencies of the fragment ejection direction, which are assigned to post-ionization dynamics. These observations, resulting from the excitation with asymmetric two-color intense laser fields, supported by state-of-the-art theoretical simulations, reveal the importance of post-ionization population dynamics in addition to tunneling ionization in the molecular fragmentation processes, even for heavy polar molecules.

Association Reaction of Gaseous C2H4 in Femtosecond Laser Filaments Studied by Time-of-Flight Mass Spectrometry

Association reactions by femtosecond laser filamentation in gaseous C2H4 were studied by time-of-flight mass spectrometry of neutral reaction products. Direct sampling from the reaction cell to a mass spectrometer via a differential pumping stage allowed the identification of various hydrocarbon molecules C n H m with n = 3-7 and m = 4-7, which includes species not observed in the previous studies. It was found that products containing three and four carbon atoms dominate the mass spectrum with smaller yields for higher-mass species, suggesting that carbon chain growth proceeds through the reaction with C2H4 in the reaction cell. The product distribution showed a clear dependence on the laser pulse energy for filamentation.

Selective bond breaking of CO2 in phase-locked two-color intense laser fields: laser field intensity dependence

One of the two equivalent C–O bonds of CO₂ can be selectively broken by phase-locked two-color intense laser fields.

Ultrafast photofragment ion spectroscopy of the Wolff rearrangement in 5-diazo Meldrum's acid

We investigate the gas-phase photochemistry of 5-diazo Meldrum's acid (DMA), a photoactive compound used in lithography, by femtosecond photofragment ion spectroscopy. Transient-absorption studies in solution had revealed an ultrafast intramolecular Wolff rearrangement to a ketene after UV excitation, followed by reactions which also involve the solvent. Due to the absence of solvent molecules in this gas-phase study, we are able to focus purely on the photochemistry of the Wolff rearrangement and subsequent reaction steps. The observation of the time-resolved photofragment ion signals allows us to discriminate the dynamics of ketene and carbene products. By identification of the different possible molecular origins for a certain fragment ion signal, the time scale of the Wolff rearrangement and the lifetime of the ketene product are inferred. We further identified experimental signatures of a second Wolff rearrangement emanating from the carbene product, as had been conjectured indirectly for this molecule from pyrolysis studies.

Manipulating quantum pathways on the fly

The expectation value of a quantum system observable can be written as a sum over interfering pathway amplitudes. In this Letter, we demonstrate for the fist time adaptive manipulation of quantum pathways using the Hamiltonian encoding-observable decoding (HE-OD) technique. The principles of HE-OD are illustrated for population transfer in atomic rubidium using shaped femtosecond laser pulses. The ability to manipulate multiple pathway amplitudes is of fundamental importance in all quantum control applications.

Comparative study of the dissociative ionization of 1,1,1-trichloroethane using nanosecond and femtosecond laser pulses

Changes in the laser induced molecular dissociation of 1,1,1-trichloroethane (TCE) were studied using a range of intensities and standard laser wavelengths with nanosecond and femtosecond pulse durations. TCE contains C-H, C-C and C-Cl bonds and selective bond breakage of one or more of these bonds is of scientific interest. Using laser ionization time of flight mass spectrometry, it was found that considerable variation of fragment ion peak heights as well as changes in relative peak ratios is possible by varying the laser intensity (by attenuation), wavelength and pulse duration using standard laser sources. The nanosecond laser dissociation seems to occur via C-Cl bond breakage, with significant fragmentation and only a few large mass ion peaks observed. In contrast, femtosecond laser dissociative ionization results in many large mass ion peaks. Evidence is found for various competing dissociation and ionization pathways. Variation of the nanosecond laser intensity does not change the fragmentation pattern, while at high femtosecond intensities large changes are observed in relative ion peak sizes. The total ionization yield and fragmentation ratios are presented for a range of wavelengths and intensities, and compared to the changes observed due to a linear chirp variation.

Quantum control experiment reveals solvation-induced decoherence

Coherent control holds the promise of becoming a powerful spectroscopic tool for the study of complex molecular systems. Achieving control requires coherence in the quantum system under study. In the condensed phase, coherence is typically lost rapidly because of fluctuating interactions between the solvated molecule and its surrounding environment. We investigate the degree of attainable control on a dye molecule when the fluctuations of its environment are systematically varied. A single successful learning curve for optimizing stimulated emission from the dye in solution is reapplied for a range of solvents with varying viscosity, revealing a striking trend that is correlated directly with the dephasing time. Our results provide clear evidence that the environment limits the leverage of control on the molecular system. This insight can be used to enhance the yield of control experiments greatly.

Dissociative wave packets and dynamic resonances

The authors examine the role of dynamic resonances in laser driven molecular fragmentation. The yields of molecular fragments can undergo dramatic changes as an impulsively excited dissociative wave packet passes through a dynamic resonance. The authors compare three different kinds of dynamic resonances in a series of molecular families and highlight the possibility of characterizing the dissociative wave function as it crosses the resonance location.

Elucidation of Control Mechanisms Discovered during Adaptive Manipulation of [Ru(dpb)3](PF6)2 Emission in the Solution Phase

To design methodologies that will allow researchers to directly correlate the results of adaptive control experiments with physiochemical control pathways in arbitrary complex molecular systems it is imperative that prototype systems are developed and that exigent control pathways are understood. We have been interested in the results of adaptive control experiments in our laboratory involving the maximization of a ratio of two experimental observables: (1) the thermalized emission from the solution-phase coordination complex [Ru(dpb)3](PF6)2 and (2) the second harmonic signal (a purely intensity-dependent phenomenon) of the shaped laser fields. Using a rational pulse shaping strategy, we have made a measurement of the ratio spectrum (in essence the two-photon absorption cross section) for the molecule [Ru(dpb)3](PF6)2 in a room temperature solution of acetonitrile. This spectrum is highly varied across the accessible two-photon power spectrum of our broad-band laser pulses and demonstrates the existence of a control pathway wherein a shaped laser field can manipulate excited-state population (with respect to SHG) by conforming to the second-order spectral response of the molecule in solution. We show that our adaptive control algorithm is capable of taking advantage of these control pathways using simulated adaptive control experiments. Finally, we measure second-harmonic spectra of shaped laser fields discovered during an adaptive control experiment and show that these agree with simulation. These results suggest that our adaptive control experiment can be understood in the context of the elucidated spectral control pathway.

Isomeric identification by laser control mass spectrometry

The influence shaped femtosecond laser pulses have on molecular photofragmentation and ionization, coupled with the intrinsic sensitivity of mass spectrometry, results in a powerful tool for fast, accurate, reproducible and quantitative isomeric identification. Complex phase functions are introduced to enhance differences during the laser-molecule interactions, which depend on geometric structure, resulting in different fragmentation fingerprints. A full account is given on the setup and results leading to a technique that can be used to distinguish between compounds normally indistinguishable by conventional electron ionization mass spectrometry. We demonstrate geometric and structural isomer identification of cis-/trans-3-heptene, cis-/trans-4-methyl-2-pentene, o-/p-cresol and o-/p-xylene. For the positional isomers of xylene we present a complete dataset consisting of 1024 different phases to explore phase complexity. A selection of two phases from that data can then be used to achieve quantitative identification in mixtures of xylene isomers. Finally, we evaluate receiver operational curves obtained from our experimental data to demonstrate the reliability that can be achieved by femtosecond laser control mass spectrometry.

Quantum control mechanism analysis through field based Hamiltonian encoding

Optimal control of quantum dynamics in the laboratory is proving to be increasingly successful. The control fields can be complex, and the mechanisms by which they operate have often remained obscure. Hamiltonian encoding (HE) has been proposed as a method for understanding mechanisms in quantum dynamics. In this context mechanism is defined in terms of the dominant quantum pathways leading to the final state of the controlled system. HE operates by encoding a special modulation into the Hamiltonian and decoding its signature in the dynamics to determine the dominant pathway amplitudes. Earlier work encoded the modulation directly into the Hamiltonian operators. This present work introduces the alternative scheme of field based HE, where the modulation is encoded into the control field and not directly into the Hamiltonian operators. This distinct form of modulation yields a new perspective on mechanism and is computationally faster than the earlier approach. Field based encoding is also an important step towards a laboratory based algorithm for HE as it is the only form of encoding that may be experimentally executed. HE is also extended to cover systems with noise and uncertainty and finally, a hierarchical algorithm is introduced to reveal mechanism in a stepwise fashion of ever increasing detail as desired. This new hierarchical algorithm is an improvement over earlier approaches to HE where the entire mechanism was determined in one stroke. The improvement comes from the use of less complex modulation schemes, which leads to fewer evaluations of Schrodinger's equation. A number of simulations are presented on simple systems to illustrate the new field based encoding technique for mechanism assessment.

General Method for the Dimension Reduction of Adaptive Control Experiments

Adaptive femtosecond control experiments are expanding the possibilities for using laser pulses as photophysical and photochemical reagents. However, because of the large number of variables necessary to perform these experiments (usually 100-200), it has proven difficult to elucidate the underlying control mechanisms from the optimized pulse shapes. If adaptive control is to become a widespread tool for examining chemical dynamics, methods must be developed that reveal latent control mechanisms. This manuscript presents a generally applicable method for dimension reduction of adaptive control experiments based on partial least squares regression analysis (PLS) of the normalized covariance matrix of the total data set. When applied to experimental results obtained in our laboratory, it shows that only seven fundamental dimensions from an original 208-dimension search space are needed to account for approximately 90% of the variance in the observed fitness of 11,700 laser-pulse shapes explored during the optimization experiment. Furthermore, the seven dimensions have a remarkable regularity in their functional form. It is anticipated that this work will facilitate theoretical treatments directly linking the optimal fields to control mechanisms, allow quantitative comparisons of independent control results, and suggest new experimental methods for rapid adaptive searches.

Interpreting closed-loop learning control of molecular fragmentation in terms of wave-packet dynamics and enhanced molecular ionization

We interpret a molecular fragmentation experiment using shaped, ultrafast laser pulses in terms of enhanced molecular ionization during dissociation. A closed-loop learning control experiment was performed to maximize the CF3+CH3+ production ratio in the dissociative ionization of CH3COCF3. Using ab inito molecular structure calculations and quasistatic molecular ionization calculations along with data from pump-probe experiments, we identify the primary control mechanism which is quite general and should be applicable to a broad class of molecules.