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Ultrafast photorelaxation of uracil can be hindered by its natural RNA environment. Multiscale quantum dynamical simulations show that the wave packet can be trapped in the photoexcited electronic state, which could potentially lead to photodamage.

Directional breaking of the C-H/C-D molecular bond is manipulated in acetylene (C2H2) and deuterated acetylene (C2D2) by waveform controlled few-cycle mid-infrared laser pulses with a central wavelength around 1.6 μm at an intensity of about 8 × 1013 W/cm2. The directionality of the deprotonation of acetylene is controlled by changing the carrier-envelope phase (CEP). The CEP-control can be attributed to the laser-induced superposition of vibrational modes, which is sensitive to the sub-cycle evolution of the laser waveform. Our experiments and simulations indicate that near-resonant, intense mid-infrared pulses permit a higher degree of control of the directionality of the reaction compared to those obtained in near-infrared fields, in particular for the deuterated species.

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.

The reactive collision of chloride anions and methyl iodide molecules forming iodide anions and methyl chloride is a typical example of a concerted bimolecular nucleophilic substitution (SN2) reaction. We present wave packet dynamics calculations to investigate quantum effects in the collinear gas phase reaction. A new type of reduced coordinate system is introduced to allow for an efficient solution of the time-dependent Schrödinger equation on an ab initio potential energy surface. The reduced coordinates were designed to study the direct rebound mechanism under the Walden inversion. Especially the suppressed direct rebound mechanism at low collision energies, the quantum effects of the initial state preparation and the influence of the CH3 inversion mode are addressed. The internal energy distributions of the molecular product are evaluated from the wave packet calculations and compared to experimental results obtained with crossed-beam velocity map ion imaging. The observed reactivity is discussed in light of a dynamical barrier, a concept that is illustrated by the wave packet dynamics.

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to control the movement of nuclei with tailored light, within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics are monitored using coincident 3D momentum imaging spectroscopy, and described with a quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wavepacket by the intense off-resonant laser field., Comment: 5 pages, 4 figures

Electrons and their dynamics are involved in bond breaking and formation; thus, the idea to steer chemical reactions by localization of electronic wavepackets seems natural. The formation of a localized electronic wavepacket requires the superposition of two or more appropriate electronic states through, e.g., an external electric field. The guiding of such an electronic wavepacket is only possible within the coherence time of the system. In theoretical studies, we elucidate the role of electron wavepacket motion for the control of molecular processes. We analyze three examples of electron wavepacket-driven processes with direct connection to already performed or ongoing experiments. From these examples, we extract the system requirements defining the time window for intramolecular electronic coherence and efficient control. With this knowledge, we derived an appropriate molecular configuration in a photoreaction of a polyatomic molecule where a control by guiding electronic wavepackets is possible. For such a photoreaction, we designed a new control scheme with the carrier envelope phase as a control parameter that works at high efficiency.

Coherent control of near-fields in the proximity of nanoparticles is a promising tool for shaping the temporal and spatial properties of light at the nano-scale. Numerical simulations on a series of progressively more complex metallic nanoparticle systems were carried out to explore in detail the mechanisms of control. We find that an important element for the controllability of intensity localization is the spatial variation of the phase in the near-field that is due either to the interference between the plane incident wave and the near-field of the nanosystem or to the interference of fields generated by different parts of the nanosystem. For a collection of random nanoparticles we show that it is possible to select the appearance of highly localized hot spots.

We outline the details of our new method to calculate angular-dependent ionization probabilities based on electronic structure theory for diatomic and larger systems. To demonstrate its abilities, we compare our calculations to measured ionization probabilities of the four molecules D2, N2, O2 and CO in the strong-field regime. The calculated angular distributions yield better agreement with the experimental data than those obtained from the widely used MO-ADK theory. For CO the measured angular distributions of ionic fragments indicate contributions to the ionization from both the HOMO and the HOMO-1 orbital, an effect that is addressed by the theory.

Laser pulses with stable electric field waveforms establish the opportunity to achieve coherent control on attosecond timescales. A first successful example of electron localization and its control could be demonstrated for the molecules D 2 + and HD + . These molecules constitute simple model systems with only one electron and left the question open, whether electron localization in multi-electron systems can be achieved and controlled. In this context, we recently reported results where a high degree of light-waveform control over the directional emission of C + and O + fragments from the dissociative ionization of CO was observed [1] . Here, we compare and analyze the mechanisms leading to electron localization in the two different molecular systems D 2 + and CO + . We use our recently introduced method for multi-electron systems that allows us to describe the formation of an electronic wavepacket and to follow its evolution during the dissociation process.

The technological needs imposed by the exponential miniaturization trend of conventional electronic devices has drawn attention towards the development of smaller and faster devices like ultrafast molecular switches. In recent years molecular switches emerge again in the focus of active and innovative research with state-of-the-art optical tools recording their dynamics in real time. Still many questions about the underlying microscopic mechanism are left open, including potential factors that effect the switching process in either way, improve or worsen it. Due to the complexity of such molecules it is difficult to obtain a global answer from experiment alone. On the other side molecular switches are generally too large for a complete quantum chemical and quantum dynamical calculation. In our group we therefore developed an ab initio based modular model to handle the laser induced quantum dynamics in molecular switches like fulgides. It enables us to study the effect of internal molecular coupling and of the molecular response to external fields. We can investigate the related wave packet dynamics, the switching efficiency and the controllability. Our results focus on the laser induced ring opening in fulgides, which equals one direction of the switching process. Presented are the influence of a conical intersection seam and of time-dependent potentials, mimicking the mean interaction with the environment. Furthermore the relation of controllability and the wave packet's momentum is studied and the influence of potential barriers on the switching dynamics is shown.

In this theoretical study vibrational ladder climbing in transition metal carbonyl complexes, as a possible means to initialize chemical ground state reactions, and the resulting vibrational population distribution using chirped mid-infrared femtosecond laser pulses is investigated. Our model system is MnBr(CO)(5), a strong IR-absorber within an experimentally easily accessible wavelength region. Special emphasis is put on the perturbation due to additional vibrational modes, especially on one, which allows dissociation at low energies. The related potential energy surface for the three representative modes is calculated, whereon quantum dynamics calculations, including the laser-molecule interaction, are performed. No significant coupling could be detected, neither in the bound, nor in the dissociative region. Contrarily, we found a dynamical barrier even for energies high above the dissociation limit. Different vibrational population distributions after the laser excitation of the CO stretching mode could be generated in dependence of the chirp parameters. Based on these findings we simulated the laser excitation corresponding to an experiment by M. Joffre et al., Proc. Natl. Acad. Ssi. U. S. A., 2004, 101(36), 13216-13220, where coherent vibrational ladder climbing in carboxyhemoglobin was demonstrated and we could offer an explanation for an open question, concerning the interpretation of the spectroscopic data.

Molecular switches are discussed as promising nanoscale devices for various applications including data storage and information processing. For this purpose a high controllability of the complete switching process is desirable. We propose to use shaped femtosecond laser pulses to initiate and control a fast and efficient switching process mediated by conical intersections. We focus on photoreactions of bistable molecules in which the ring opening of 1,3-cyclohexadiene constitutes the key activity. The investigations of control schemes for this type of reactions are carried out in a model based on ab initio and quantum dynamical calculations of the ring-opening reaction of cyclohexadiene. By defining an intermediate target, we can achieve product control with optimized laser pulses in this model.

Coherent control theory has been applied to steer molecular vibrational dynamics so far disregarding the rotational degree of freedom. In this paper the complete rovibrational molecular Hamiltonian is considered, especially its effect on the controllability. It is shown that suitably shaped pulses can overcome the difficulties arising from the reorientational motion of the molecule achieving again a high degree of laser control.

To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [Cr(CO)6], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm−1 to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule.

Ultrafast excited-state dynamics in indigo-based photochromic compounds was studied with the transient absorption spectroscopy and ab initio calculations. We demonstrated an approach of adjusting excited state relaxation routes and photoisomerization rates for applications where fast photoswitching is needed.

Being the simplest molecule on the planet, H-2, as well as its isotopes, have been the prototype systems for strong-field molecular physics for decades. Photoionization and dissociation of H-2 have been extensively investigated. After single ionization, the electron left in the molecular ion and its microscopic localization around the two dissociating nuclei can be effectively manipulated using intense femtosecond laser fields with broken symmetry. In this paper, we review the recent progress made on the sub-cycle directional control of the dissociative ionization of hydrogen molecules by tailoring the waveform of femtosecond laser fields, including few-cycle pulses and 2-color fields polarized along the same direction (one-dimension) or different directions (two-dimension).

Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform.

Femtosecond pulse shaping using a spatial light modulator and degenerate four-wave-mixing (DFWM) spectroscopy have been combined to control the vibrational wave packet motion in the electronic ground and excited states of the prototype molecule K2 and, thus, study the influence of phase and amplitude modulated pulses on the molecular FWM response. It is shown that the vibrational motion of the potassium dimer is sensitive to the temporal interpulse separation and the adjusted phase differences. A simple theoretical model for such arbitrary shaped pulse excitation in a FWM process has been developed, which proves to be in excellent agreement with the observed FWM signal. In addition, the spectrally resolved DFWM signal works as a molecular FROG providing information about the activated molecular vibrations, the DFWM signal itself and the shapes of the pulses used in the excitation sequence. Furthermore, the experiment was ideal to test the robustness of the evolutionary algorithm previously used to find new control processes.

For the ultrafast photoinduced ring opening of cyclohexadiene the S1 state plays a central role, providing the possibility to rapidly decay to the ground state. In this paper we follow the path of a wavepacket propagating in the reactive coordinate space of the S1 surface. We present a detailed analysis of the corresponding electronic and nuclear motions. With the help of a projection method and a normal-mode analysis, the vibrational modes driving the reaction might be detected experimentally.

An extended theory of optimal control strategies for the design of feasible laser pulses is presented. The theoretical framework includes the control of population-transfer and expectation values. Selected studies for two-dimensional model systems providing aspects of uni- and bimolecular reactions are presented. Subsequent analysis of the obtained laser fields is performed to gain insight into the underlying reaction mechanisms. The effect of non-uniform spatial laser profiles and the orientation of the molecules versus the laser beam on controlling quantum dynamics is investigated with the main interest focussed on the achievable quantum yield and information on variable pathways stored in the laser field.

The conditions for spectroscopic studies of fast decaying excited states with femtosecond laser pulses are examined both experimentally and by quantum dynamical calculations. The sodium-ammonia complex (NaNH 3 ) acts as a model system which can be treated in quantum chemical calculations and is of chemical relevance for metal-molecular interaction in polar liquids. The state selective dynamics is studied in a two colour pump-probe experiment. The fundamental wavelength of a Ti:sapphire generated femtosecond laser pulse excites the NaNH 3 complex and the second harmonic is used for ionization. The calculations performed on ab initio potential energy and transition dipole moment surfaces show that for laser intensities with a Rabi cycle less than π the pump-probe process is a pure two-photon ionization process. Under these conditions the observed ion signal reflects directly the population of the excited state and the observed wavelength dependence resembles the vibrational spectrum of the NaNH 3 complex.

Interpretation of the vibrational absorption spectrum for Na3(X) → Na3(B) based on full ab-initio quantum mechanical description is presented. Potential energy surfaces (PES) for the ground and excited state and dipole transitions have been calculated employing ab-initio accurate correlated wavefunctions. All three vibrational modes of the molecule are included in the theoretical treatment. The absorption spectrum is calculated in two ways, namely a time-independent approach employing the sinc-function discrete variable representation (DVR) method in which eigenvalues as well as eigenfunctions are evaluated directly, and a time-dependent approach performing a three-dimensional wave packet propagation in the B state, after instantaneous absorption from the X state. On the basis of our calculations we discuss two possible assignments of the absorption lines up to 330 cm−1 above the origin. The calculated spectrum agrees satisfactorily well with the measured spectrum and most of its features can be explained, including not only the dominant progressions of angular and radial pseudorotations but also the rather weak signatures of the symmetric stretch. Nevertheless some open questions remain concerning comparison between theoretical and experimental findings.

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.

We highlight the effect of the nuclear motion on the control scheme and in particular on the oscillating dipole.

We present a new dynamic continuum ansatz to describe the frictional force exerted on moving wave packets in a solvent cage. The solvent interferes on the femtosecond time scale, but decides the reaction outcome.

We demonstrate how the fast electron dynamics in molecules and hence the reaction of the system can be efficiently manipulated by controlling the temporal phase of an ultrashort laser pulse with attosecond precision.

UV-induced photolesions are studied in the visible and IR. While structural distortions of the DNA-backbone at the moment of light absorption are prerequisite for CPD-formation, strain from the backbone guides Dewar-formation.

Laser pulses of moderate intensities with durations of either 1.5 ps or 120 fs were employed to excite the Na3 molecule to its electronic B state. Using a pump–probe technique the temporal evolution of the two‐photon ionization signal could be resolved in real time. Different vibrational modes of the excited trimer are detected selectively. While the ps laser pulses yield preferential excitation of the slow pseudorotational mode with a period of 3 ps, the use of ∼10 times shorter pulses allows the trimer’s symmetric stretch mode with a 310–320 fs period for the first 5 ps to be observed. These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the pump–probe experiments. The calculations are performed on three‐dimensional ab initio potential energy and transition dipole surfaces. Thus all three vibrational degrees of freedom of the Na3 molecule are included in the theoretical treatment. The time‐dependent wave‐packet dynamics elucidate the effect of u...

Experimental and theoretical pump−probe studies are performed for the K2 molecule. Special molecular spectroscopic properties combined with the dynamics induced by “femtosecond state preparation” facilitate the transition from pump−probe to control spectroscopy. Hereby, the intensity of the laser field serves as a control parameter in the observed multiphoton processes. In the monitored transient ion signal we can distinguish the effect of two processes, i.e., multiphoton ionization (MPI) and resonant impulsive stimulated Raman scattering (RISRS). As a consequence, the temporal evolution of the ion signal reflects the induced dynamics of either the first excited state A1Σu+ or the ground state X1Σg+ of the potassium dimer.

The steering of electron motion in molecules is accessible with waveform-controlled few-cycle laser light and may control the outcome of light-induced chemical reactions. An optical cycle of light, however, is much shorter than the duration of the fastest dissociation reactions, severely limiting the degree of control that can be achieved. To overcome this limitation, we extended the control metrology to the midinfrared studying the prototypical dissociative ionization of D(2) at 2.1 μm. Pronounced subcycle control of the directional D(+) ion emission from the fragmentation of D(2)(+) is observed, demonstrating unprecedented charge-directed reactivity. Two reaction pathways, showing directional ion emission, could be observed and controlled simultaneously for the first time. Quantum-dynamical calculations elucidate the dissociation channels, their observed phase relation, and the control mechanisms.

The transition from the multiphoton to the tunneling regime for ionization is characterized by the Keldysh parameter equation, where U p ∝ I × λ2. A value of γ smaller than 1 implies tunnel ionization. The scaling of the Keldysh parameter with the wavelength allows studying atomic and molecular systems deep in the tunneling regime, when mid-infrared wavelengths are applied. In the present study we used a home-built few-cycle IR light source [1] with a center wavelength at 2.1 µm that is based on three OPCPA stages pumped by an optically synchronized 50 ps, 11 mJ Nd:YLF laser. Difference frequency mixing is employed for generating the IR seed, resulting in a signal that is self CEP-stabilized without the need for an active feed-back loop. The quality of the self-stablization of the CEP was demonstrated by imaging the directional control of the electron emission in the photoionization of xenon [2].

Laser pulses with stable electric field waveforms open the door for coherent control on attosecond timescales. First successful experimental demonstrations were reported for electron localization in the dissociative ionization of the prototype molecules D 2 and HD [1, 2]. The steering of the one electron, remaining after ionization, originates from a CEP controlled coherent superposition of two electronic states which finally controls the localization of the electron upon the break-up of the molecule. Here, we present theoretical results and analysis of the experimental findings for the direct strong-field control of electronic motion in the multi-electron system CO+.

An alternative approach for the optimization strategy in quantum control experiments is proposed. Genetic algorithms are used frequently to improve the laser fields driving quantum processes. We present a flexible scheme based on ant-colony-optimization, which introduces a correlation of the mask function pixels and allows a decrease in the shaped pulse complexity and duration without loss of efficiency.

Laser pulses with stable electric field waveforms establish the opportunity to achieve coherent control on attosecond time scales. We present experimental and theoretical results on the steering of electronic motion in a multielectron system. A very high degree of light-waveform control over the directional emission of ${\mathrm{C}}^{+}$ and ${\mathrm{O}}^{+}$ fragments from the dissociative ionization of CO was observed. Ab initio based model calculations reveal contributions to the control related to the ionization and laser-induced population transfer between excited electronic states of ${\mathrm{CO}}^{+}$ during dissociation.

With the CARS technique (coherent anti-Stokes Raman scattering), we found the first, second and third overtone O, Q and S lines of NaH in a Na (3p) + H2 gas mixture, when the CARS pump-laser frequency was tuned close to electronic resonances between X 1Σ+ (ν″=O, J″) and the first excited state A 1Σ+ (ν′=1, J′=J″±1) in NaH. This opens new possibilities in the investigation of the nascent molecular products of the reaction between excited sodium atoms and hydrogen molecules, since resonance CARS can be used with good spatial and temporal resolution. We discuss the possible origin of the NaH CARS lines produced in the Na(3p) + H2 mixture.

We present a theoretical approach for the simulation of the cooling of internal and external molecular degrees of freedom in a cavity. The idea is to sequentially depopulate excited ro-vibrational levels by vacuum-stimulated Raman scattering into the mode of a high-finesse cavity. As a model system we choose the OH radical. By using a combination of ab initio quantum chemical and experimental data we are able to determine the internal energy level structure and the transition frequencies and strengths needed for our cooling scheme. We take into account the perturbing spontaneous and the coherent Raman processes amplified by the cavity to get a realistic model. This gives us insight in the cooling process and lets us predict the time scales needed to prepare a molecule in its ground state.

We demonstrate the importance of ionization from Rydberg orbitals via experimental and theoretical work focusing on the strong-field dissociative single ionization of small hydrocarbons. Our findings suggest that Rydberg states should be routinely considered when studying polyatomic molecules in intense laser fields.

Our concept for a quantum computational system is based on qubits encoded in vibrational normal modes of polyatomic molecules. The quantum gates are implemented by shaped femtosecond laser pulses. We adopt this concept to the new species manganese pentacarbonyl bromide [MnBr(CO)5] and show that it is a promising candidate in the mid-infrared (IR) frequency range to connect theory and experiment. As direct reference for the ab initio calculations we evaluated experimentally the absorption bands of MnBr(CO)5 in the mid-IR as well as the related transition dipole moments. The two-dimensional potential-energy surface spanned by the two strongest IR active modes and the dipole vector surfaces are calculated with density-functional theory. The vibrational eigenstates representing the qubit system are determined. Laser pulses are optimized by multitarget optimal control theory to form a set of global quantum gates: NOT, CNOT, Pi, and Hadamard. For all of them simply structured pulses with low pulse energies around 1 microJ could be obtained. Exemplarily for the CNOT gate we investigated the possible transfer to experimental shaping, based on the mask function for pulse shaping in the frequency regime as well as decomposition into a train of subpulses.

A molecular approach to quantum computing is demonstrated. The vibrational normal modes of molecules in an ensemble are used to encode qubit systems. In each mode a certain degree of excitation is referred to as |0>, another degree of excitation in the same mode is referred to as |1/spl rarr/. For the realization of quantum gates within and between the qubits, single shaped femtosecond laser pulses in the IR-regime is applied.

The ultrafast dynamics of small Na(NH3)n clusters are studied using amplified 150 fs Ti:Sapphire laser radiation. In the two colour pump‐probe experiment it has been found, that the first electronically excited state of NaNH3 is stable for at least time t∼1 ns. In contrast, Na(NH3)+2 ions can only be observed for pump probe delay times shorter than 32 ps. The simultaneously recorded increasing Na+ ion signal indicates fragmentation of the complex. Repeating this experiment with Na(ND3)2 a dramatic change of the fragmentation time by a factor 30 to 60 is observed. This experimental finding can be understood by an internal rearrangement corresponding to the NH3 (or ND3) tunnelling motion after being excited by the pump laser pulse. First quantum chemical studies support this assumption.

Experimental and theoretical pump-probe studies were performed for the K2 molecule. Special molecular spectroscopic properties combined with the induced dynamics by the ’femtosecond state preparation’ allow the transition from pump-probe to pump-control spectroscopy. Hereby, the intensity of the laser field serves as a control parameter in the multi photon processes. We can distinguish between the effect of two main processes, i.e. multi photon ionization (MPI) and impulsive stimulated raman scattering (ISRS).

The vibrational dynamics of single molecular ions confined in a coulomb crystal are followed by 4 fs ultraviolet pulses. Theoretical calculations predicted the optimal laser parameters for a successful experiment under background free conditions. The ions are subject to internal heating, which has to be treated in the theory and handled in the experiment. Beyond the experiment the pump-probe scheme is extended by a coherent control scheme designed to suppress unwanted vibrational heating.

An elementary chemical reaction proceeds through two distinct conical intersections. Benzhydryl radicals are formed with a delay of 80 fs, cations after 125 fs. The optical signal increases more slowly due to planarization and solvation.