Patrick Nuernberger

Femtosecond quantum control of molecular dynamics in the condensed phase

We review the progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses. Due to its high particle density the condensed phase has both high relevance and appeal for chemical synthesis. Thus, in recent years different methods have been developed to manipulate the dynamics of condensed-phase systems by changing one or multiple laser pulse parameters. Single-parameter control is often achieved by variation of the excitation pulses wavelength, its linear chirp or its temporal subpulse separation in case of pulse sequences. Multiparameter control schemes are more flexible and provide a much larger parameter space for an optimal solution. This is realized in adaptive femtosecond quantum control, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm. Several experiments are presented that illustrate the different control concepts and highlight their broad applicability. These fascinating achievements show the continuous progress on the way towards the control of complex quantum reactions in the condensed phase.

Coherent two-dimensional ultraviolet spectroscopy in fully noncollinear geometry

We introduce fully noncollinear coherent two-dimensional (2D) spectroscopy in the UV domain with an all-reflective and miniaturized setup design. Phase stability is achieved via pairwise beam manipulation, and the concept can be transferred to all wavelength regimes. Here we present results from an implementation that has been optimized for wavelengths between 250 and 375 nm. Interferometric measurements prove phase stability over several hours. We obtained 2D spectra of the nonpolar UV chromophore p-terphenyl in ethanol, excited with 50 fs pulses at 287 nm.

Multidimensional Electronic Spectroscopy of Photochemical Reactions

Coherent multidimensional electronic spectroscopy can be employed to unravel various channels in molecular chemical reactions. This approach is thus not limited to analysis of energy transfer or charge transfer (i.e. processes from photophysics), but can also be employed in situations where the investigated system undergoes permanent structural changes (i.e. in photochemistry). Photochemical model reactions are discussed by using the example of merocyanine/spiropyran-based molecular switches, which show a rich variety of reaction channels, in particular ring opening and ring closing, cis-trans isomerization, coherent vibrational wave-packet motion, radical ion formation, and population relaxation. Using pump-probe, pump-repump-probe, coherent two-dimensional and three-dimensional, triggered-exchange 2D, and quantum-control spectroscopy, we gain intuitive pictures on which product emerges from which reactant and which reactive molecular modes are associated.

Multidimensional spectroscopy of photoreactivity

Significance Many chemical reactions typically involve ultrafast reaction steps and are highly complex. Thus, one requires special methods to observe the associated atomic movements in real time. Though conventional femtosecond spectroscopy techniques are in principle capable of providing the necessary temporal resolution, they often suffer from the fact that they cannot isolate the overlapping spectral signatures of reactants, intermediates, and products. Here we demonstrate that this issue is unraveled using 2D and 3D electronic spectroscopy that directly visualizes the photochemical connectivity between photoreactive molecular species. Hence, this approach not only provides an intuitive and direct picture for which reactants can be turned into which products, but also exposes the reactive molecular modes connecting them with unprecedented perspicuity. Coherent multidimensional electronic spectroscopy is commonly used to investigate photophysical phenomena such as light harvesting in photosynthesis in which the system returns back to its ground state after energy transfer. By contrast, we introduce multidimensional spectroscopy to study ultrafast photochemical processes in which the investigated molecule changes permanently. Exemplarily, the emergence in 2D and 3D spectra of a cross-peak between reactant and product reveals the cis–trans photoisomerization of merocyanine isomers. These compounds have applications in organic photovoltaics and optical data storage. Cross-peak oscillations originate from a vibrational wave packet in the electronically excited state of the photoproduct. This concept isolates the isomerization dynamics along different vibrational coordinates assigned by quantum-chemical calculations, and is applicable to determine chemical dynamics in complex photoreactive networks.

Femtosecond quantum control of molecular bond formation

Ultrafast lasers are versatile tools used in many scientific areas, from welding to eye surgery. They are also used to coherently manipulate light–matter interactions such as chemical reactions, but so far control experiments have concentrated on cleavage or rearrangement of existing molecular bonds. Here we demonstrate the synthesis of several molecular species starting from small reactant molecules in laser-induced catalytic surface reactions, and even the increase of the relative reaction efficiency by feedback-optimized laser pulses. We show that the control mechanism is nontrivial and sensitive to the relative proportion of the reactants. The control experiments open up a pathway towards photocatalysis and are relevant for research in physics, chemistry, and biology where light-induced bond formation is important.

Ultrafast Multisequential Photochemistry of 5-Diazo Meldrum’s Acid

We disclose the light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways in the photochemistry of 5-diazo Meldrums acid (DMA), a photoactive compound used in lithography, by femtosecond mid-infrared transient absorption spectroscopy covering several nanoseconds. After excitation of DMA dissolved in methanol to the second excited state S(2), 70% of excited molecules relax back to the S(0) ground state. In competing processes, they can undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they first relax to the DMA S(1) state, from where they can isomerize to a diazirine and via an intersystem crossing to a triplet carbene. For a reliable identification of the involved compounds, density functional theory calculations on the normal modes and Fourier transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and 2-propanol lead to slight spectral shifts as well as elongated time constants due to steric hindrance in transient spectra connected with the ester formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.

Ultrafast Photochemistry of a Manganese-Tricarbonyl CO-Releasing Molecule (CORM) in Aqueous Solution

Ultraviolet irradiation of a manganese-tricarbonyl CO-releasing molecule (CORM) in water eventually leads to the liberation of some of the carbon monoxide ligands. By ultraviolet pump/mid-infrared probe femtosecond transient absorption spectroscopy in combination with quantum chemical calculations, we could disclose for the exemplary compound [Mn(CO)3(tpm)](+) (tpm = tris(2-pyrazolyl)methane) that only one of the three carbonyl ligands is photochemically dissociated on an ultrafast time scale and that some molecules may undergo geminate recombination.

Removing cross-phase modulation from midinfrared chirped-pulse upconversion spectra

We observe that narrow spectral features in mid-infrared spectra obtained by chirped-pulse up-conversion are strongly distorted by cross-phase modulation between the mid-infrared field and the chirped pulse. We discuss the consequences of this effect on spectral resolution, and introduce a correction method that recovers masked lines. This simple correction can be applied either when the upconverted field is fully characterized, such as in multidimensional spectroscopy, or when causality can be used, such as in absorption spectroscopy, which we demonstrate experimentally.

Direct mid-infrared femtosecond pulse shaping with a calomel acousto-optic programmable dispersive filter

Direct amplitude and phase shaping of mid-infrared femtosecond pulses is realized with a calomel-based acousto-optic programmable dispersive filter transparent between 0.4 and 20 μm. The shaped pulse electric field is fully characterized with high accuracy, using chirped-pulse upconversion and time-encoded arrangement spectral phase interferometry for direct electric field reconstruction techniques. Complex mid-infrared pulse shapes at a center wavelength of 4.9 μm are generated with a spectral resolution of 14 cm(-1), which exceeds by a factor of 5 the reported experimental resolutions of calomel-based filters.

Ultrafast UV-Induced Photoisomerization of Intramolecularly H-Bonded Symmetric β-Diketones

In photoinduced molecular reaction dynamics, the effects of electronic charge redistribution can lead to multiple pathways that are determined by the nature of the initial structures involved and the environment the molecule of interest is studied in. The β-diketones are a common example of this complexity. They show keto-enol tautomerism that is almost totally shifted toward the enolic form. However, compared to the gas phase, the photochemistry proceeds completely differently by virtue of the solvent environment for these compounds, which are used in commercial sunscreen agents due to a high absorption in the ultraviolet (UV) and fast deactivation processes. We disclose these dynamics by investigating three symmetrical β-diketones in various solvents. To observe these effects on an ultrafast time scale directly in the UV spectral region where the relevant electronic transitions take place, we have developed and employed femtosecond transient absorption with detection capability in the deep UV. Our studies confirm that electronic excitation of the chelated enol form does not lead to any ultrafast photochemistry other than proton transfer followed by rotamerization. The formation of the nonchelated conformers takes place on a picosecond time scale through a dark state, whereas the recovery to the stable chelated enol form is a comparably slow process.