Stefanie Gräfe

Attosecond-recollision-controlled selective fragmentation of polyatomic molecules.

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.

Ultrafast electron diffraction imaging of bond breaking in di-ionized acetylene

Acetylenes scission visualized by selfie Can molecules take pictures of themselves? That is more or less the principle underlying laser-induced electron diffraction (LIED): A laser field strips an electron from a molecule and then sends it back to report on the structure of the remaining ion. Wolter et al. applied this technique to acetylene to track the cleavage of its C–H bond after double ionization (see the Perspective by Ruan). They imaged the full structure of the molecule and also distinguished more rapid dissociative dynamics when it was oriented parallel rather than perpendicular to the LIED field. Science, this issue p. 308; see also p. 283 An electron transiently stripped from a molecule is used to image that molecules dissociation. Visualizing chemical reactions as they occur requires atomic spatial and femtosecond temporal resolution. Here, we report imaging of the molecular structure of acetylene (C2H2) 9 femtoseconds after ionization. Using mid-infrared laser–induced electron diffraction (LIED), we obtained snapshots as a proton departs the [C2H2]2+ ion. By introducing an additional laser field, we also demonstrate control over the ultrafast dissociation process and resolve different bond dynamics for molecules oriented parallel versus perpendicular to the LIED field. These measurements are in excellent agreement with a quantum chemical description of field-dressed molecular dynamics.

Self-healing mechanism of metallopolymers investigated by QM/MM simulations and Raman spectroscopy

The thermally induced self-healing mechanisms in metallopolymers based on bisterpyridine complexes of iron(II) sulfate and cadmium(II) bromide, respectively, were studied by means of combined quantum mechanical/molecular mechanical (QM/MM) simulations and Raman spectroscopy. Two possible healing schemes, one based on a decomplexation of the cross-linking complexes and a second one relying on the dissociation of ionic clusters, have been addressed. Temperature-dependent Raman spectroscopy displayed bathochromic shifts of the Raman intensity pattern upon heating. QM/MM simulations on the polymer models assign these alterations to a partial decomplexation of the metal terpyridine complexes, i.e. signals originating from free terpyridine ligands increase upon heating. Thus, a healing mechanisms based on partial decomplexation of the cross-linking complexes is suggested. The possibility that the dissociation of ionic clusters, which are assumed to be present in this class of self-healing polymers, is also responsible for the self-healing process was investigated as well. However, such calculations on model clusters revealed relatively strong binding of the clusters, which renders reversible cluster breaking and reformation upon temperature cycling in the range up to 100 °C unlikely.

Instantaneous dynamics and quantum control fields: Principle and numerical applications

The relation between laser pulses serving the purpose of controlling elementary molecular processes and the instantaneous dynamics of the perturbed system is investigated. The application of the conditions assuring a controlled change of the expectation value of an observable directly links the internal motion to the external perturbation. Several numerical applications document that the derived control fields are efficient and can be interpreted clearly on physical grounds.

Local control of the quantum dynamics in multiple potential wells

The driven wave-packet dynamics in potentials exhibiting several potential wells is investigated. Therefore, local-control strategies are employed where the control field is constructed from the systems dynamics at any instant of time. It is shown that particles can be moved successively between various potential minima. Furthermore, results presented indicate that the intuitive local-control scheme allows for the initiation of a clockwise or counterclockwise rotational motion of a model molecular motor.

Low-energy peak structure in strong-field ionization by midinfrared laser pulses: Two-dimensional focusing by the atomic potential

We analyze the formation of the low-energy structure (LES) in above-threshold ionization spectra first observed by Quan et al. [1] and Blaga et al. [2] using both quasi-classical and quantum approaches. We show this structure to be largely classical in origin resulting from a two-dimensional focusing in the energy-angular momentum plane of the strong-field dynamics in the presence of the atomic potential. The peak at low energy is strongly correlated with high angular momenta of the photoelectron. Quantum simulations confirm this scenario. Resulting parameter dependences agree with experimental findings [1, 2] and, in part, with other simulations [3–5].

Time-resolved photoelectron spectroscopy of coupled electron-nuclear motion

We investigate pump-probe electron detachment spectroscopy in a model system which is ideally suited to study coupled electronic and nuclear wave-packet dynamics. Time-resolved photoelectron spectra are calculated within the adiabatic approximation and a discretization of the detachment continuum. These spectra are compared to those which derive from a non-Born-Oppenheimer description and a numerically exact treatment of the detachment process. In this way it is possible to identify the influence of non-adiabatic effects on the spectra in a systematic way and also to test commonly applied approximations.

Path-selective investigation of intense laser-pulse-induced fragmentation dynamics in triply charged 1,3-butadiene

We experimentally studied proton ejection in the three-body fragmentation of triply charged 1,3-butadiene molecules prepared by intense ultrashort laser pulses using coincidence momentum imaging. The break-up dynamics along the four possible paths that a final set of three fragments can be reached is investigated for the three different fragmentation channels that are analysed. It is found that for each channel the two dominant paths are (i) proton ejection from the triply charged ion and (ii) a sequential path, where the proton is ejected from the doubly charged ion during the second fragmentation step. Based on the measured three-body momentum correlations and accompanying numerical simulations, we discuss whether the fragmentation dynamics, where the proton is ejected from the triply charged ion, proceeds concertedly or sequentially. We also investigate the dependence of the fragmentation dynamics on the intensity and polarization state of the laser pulse.

Quantum control of electron localization in molecules driven by trains of half-cycle pulses

We present numerical simulations demonstrating efficient control of electron localization dynamics in small molecular systems by a train of half-cycle pulses using the H2+ molecule as a prototypical model system. Simulations are performed using a direct numerical integration of the Schrodinger equation on a grid for a two-degree of freedom system (one electronic, one nuclear coordinate) as well as for an expansion in terms of Born–Oppenheimer basis states. By varying the parameters of the driving field, we systematically explore the underlying control mechanism of this periodically approximately impulsively driven molecular system and demonstrate the robustness of the proposed control scheme.

Time- and frequency-resolved coherent anti-Stokes Raman scattering spectroscopy with sub-25 fs laser pulses

In general, many different diagrams can contribute to the signal measured in broadband four-wave mixing experiments. Care must therefore be taken when designing an experiment to be sensitive to only the desired diagram by taking advantage of phase matching, pulse timing, sequence, and the wavelengths employed. We use sub-25 fs pulses to create and monitor vibrational wavepackets in gaseous iodine, bromine, and iodine bromide through time- and frequency-resolved femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy. We experimentally illustrate this using iodine, where the broad bandwidths of our pulses, and Boltzmann population in the lower three vibrational levels conspire to make a single diagram dominant in one spectral region of the signal spectrum. In another spectral region, however, the signal is the sum of two almost equally contributing diagrams, making it difficult to directly extract information about the molecular dynamics. We derive simple analytical expressions for the time- and frequency-resolved CARS signal to study the interplay of different diagrams. Expressions are given for all five diagrams which can contribute to the CARS signal in our case.