Far infrared (FIR) spectral measurements of wild-type (WT) and D96N mutant bacteriorhodopsin thin films have been carried out using terahertz time domain spectroscopy as a function of hydration, temperature, and conformational state. The results are compared to calculated spectra generated via normal mode analyses using CHARMM. We find that the FIR absorbance is slowly increasing with frequency and without strong narrow features over the range of 2-60 cm(-1) and up to a resolution of 0.17 cm(-1). The broad absorption shifts in frequency with decreasing temperature as expected with a strongly anharmonic potential and in agreement with neutron inelastic scattering results. Decreasing hydration shifts the absorption to higher frequencies, possibly resulting from decreased coupling mediated by the interior water molecules. Ground-state FIR absorbances have nearly identical frequency dependence, with the mutant having less optical density than the WT. In the M state, the FIR absorbance of the WT increases whereas there is no change for D96N. These results represent the first measurement of FIR absorbance change as a function of conformational state.
Time-resolved vibrational femtosecond spectroscopy is employed to investigate the photoinduced Wolff rearrangement reaction of diazonaphthoquinone (DNQ) dissolved in different solvents (methanol and water). DNQ is an important compound in commercial Novolak photoresists. Upon photoexcitation the ketene intermediate appears within 300 fs, indicating that the ketene is formed in a very fast concerted process involving N(2) loss and rearrangement. The strong shift of the vibrational band, assigned to the ketene by density functional theory calculations and experimental infrared spectra, toward higher wavenumbers is attributed to vibrational cooling. The relaxation time depends on the solvent (10 ps in methanol and 3 ps in water). However, the spectroscopic data show that the indirect ketene formation via a carbene intermediate might also be involved in the reaction process contributing to the ketene formation on the 10 ps time scale.
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 feedbackoptimized 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.femtochemistry | surface science E ver since their invention, lasers were considered the ideal tool for microscopic control over chemical bonds, and several seminal coherent control approaches have been developed (1-3). A very successful method to this task is femtosecond quantum control, where selectivity over photoinduced reactions is achieved by exploiting the coherence properties and ultrashort time scales of femtosecond laser radiation (4-6). Combined with learning algorithms processing experimental feedback to adaptively find optimized pulses best suited for solving the control task (7), chemical reactions can even be controlled without a priori knowledge about the reaction mechanisms. This scheme has been successfully applied to dissociative reactions in the gas phase, first on organometallic compounds (8) and later on many other systems. The method is not limited to gas phase experiments, as fluorescence optimizations of molecules in the liquid phase have shown (9-12). Recently, also more complex control tasks have been realized, like the energy flow in large biomolecules (13) or the quantum yield in a photoisomerization reaction (14-16). Femtosecond lasers have also been introduced to the field of photoassociation from atoms in cold traps, in both theory (17,18) and first experiments (19,20). However, the selective laser manipulation of bond-forming reactions starting from small reactant molecules that may furthermore exhibit competing bond-forming reaction channels has not been shown yet.In this contribution, we present the realization of femtosecond laser-assisted catalytic reactions of carbon monoxide and hydrogen or deuterium at a metal surface and further demonstrate that the relative reaction efficiency can be increased by the benefits of femtosecond laser pulses tailored especially for a desired reaction outcome. These experiments represent a first step and a reaction path toward laser-induced catalysis of molecular systems.Femtosecond laser sources have been employed by laser scientists to explore processes on metal surfaces as soon as they were available. Other types of lasers have been used earlier for this purpose, but starting from the first demonstration of intact desorption of ...
A shaped UV pump–MIR probe setup is employed for quantum control of the photoinduced Wolff rearrangement reaction of diazonaphthoquinone (DNQ) dissolved in methanol, yielding a ketene photoproduct. Time-resolved vibrational spectroscopy is a well-suited tool to monitor a photoreaction in the liquid phase as the narrow vibrational lines allow the observation of structural changes. Especially in the mid-infrared region, marker modes originating from different photoproducts can be identified unambiguously providing suitable feedback signals for open-loop or closed-loop control schemes. We report an experiment where the initiation of a complicated structural change of a molecule, involving bond cleavage and rearrangement, in the liquid phase can be controlled and mechanistic insight is obtained. Single-parameter scans show that the molecule is sensitive to intrapulse dumping during the excitation. Adaptive optimizations lead to pulse structures which can be understood consistently with this dumping mechanism.
We report on femtosecond laser-induced catalytic reactions of carbon monoxide and hydrogen on single crystal surfaces under high vacuum conditions. Several product molecules are synthesized, among them also species for whose formation at least three reactants are required. By applying closed-loop optimal control, we manipulate these reactions and selectively optimize the ratio of different bond-forming reaction channels, in contrast to previous quantum control experiments aiming at bond-cleavage. Further experiments explore the nontrivial control mechanism and its sensitivity to the relative proportion of the two reactant gases.
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