Lotte Holmegaard

Laser-Induced Alignment and Orientation of Quantum-State-Selected Large Molecules

A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced one-dimensional alignment (cos;(2)theta_(2D)=0.97) and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.

Photoelectron angular distributions from strong-field ionization of oriented molecules

An experimental study shows how a polar molecule can be oriented in three dimensions by using a combination of laser and electrostatic fields. The approach should help to obtain molecular-frame information about strong-field ionization processes in molecules for which the orientation cannot be determined after ionization.

X-Ray Diffraction from Isolated and Strongly Aligned Gas-Phase Molecules with a Free-Electron Laser

We report experimental results on x-ray diffraction of quantum-state-selected and strongly aligned ensembles of the prototypical asymmetric rotor molecule 2,5-diiodobenzonitrile using the Linac Coherent Light Source. The experiments demonstrate first steps toward a new approach to diffractive imaging of distinct structures of individual, isolated gas-phase molecules. We confirm several key ingredients of single molecule diffraction experiments: the abilities to detect and count individual scattered x-ray photons in single shot diffraction data, to deliver state-selected, e.g., structural-isomer-selected, ensembles of molecules to the x-ray interaction volume, and to strongly align the scattering molecules. Our approach, using ultrashort x-ray pulses, is suitable to study ultrafast dynamics of isolated molecules.

Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields

Supersonic beams of polar molecules are deflected using inhomogeneous electric fields. The quantum-state selectivity of the deflection is used to spatially separate molecules according to their quantum state. A detailed analysis of the deflection and the obtained quantum-state selection is presented. The rotational temperatures of the molecular beams are determined from the spatial beam profiles and are all approximately 1 K. Unprecedented degrees of laser-induced alignment (=0.972) and orientation of iodobenzene molecules are demonstrated when the state-selected samples are used. Such state-selected and oriented molecules provide unique possibilities for many novel experiments in chemistry and physics.

Manipulating the torsion of molecules by strong laser pulses.

We demonstrate that strong laser pulses can induce torsional motion in a molecule consisting of a pair of phenyl rings. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis, connecting the two phenyl rings, allowing a perpendicularly polarized, intense femtosecond pulse to initiate torsional motion accompanied by an overall rotation about the fixed axis. We monitor the induced motion by femtosecond time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for and generalizes the experimental findings.

Laser-induced 3D alignment and orientation of quantum state-selected molecules

A strong inhomogeneous static electric field is used to spatially disperse a rotationally cold supersonic beam of 2,6-difluoroiodobenzene molecules according to their rotational quantum state. The molecules in the lowest-lying rotational states are selected and used as targets for 3-dimensional alignment and orientation. The alignment is induced in the adiabatic regime with an elliptically polarized, intense laser pulse and the orientation is induced by the combined action of the laser pulse and a weak static electric field. We show that the degree of 3-dimensional alignment and orientation is strongly enhanced when rotational state-selected molecules, rather than molecules in the original molecular beam, are used as targets.

Pure Samples of Individual Conformers: The Separation of Stereoisomers of Complex Molecules Using Electric Fields

Many complex molecules have multiple structural isomers; that is, multiple local minima on their potential energy surface. About twenty-five years ago, it was observed that multiple conformers of tryptophan are present even at the low temperatures of a few Kelvin in a supersonic jet. These conformers have been studied extensively since then with sophisticated spectroscopic techniques. Individual conformers can be identified from their distinct electronic or microwave spectra. Information on the conformational structures can be obtained using microwave or multipleresonance infrared spectroscopy, for example. In similar experiments it was even possible to obtain information on the barriers separating the conformers. The preparation of spatially separated conformers would provide unique possibilities for advanced further investigations. The chemical properties of the individual species and their differences could be directly studied in reactive scattering experiments. Such pure samples would also enable a new class of experiments, such as electron and X-ray diffraction or tomographic imaging experiments of complex molecules in the gas phase. Molecular-frame photoelectron angular distributions, ultrafast time-resolved photoelectron spectroscopy, and ultrafast dynamics studies would also benefit from the availability of these pure samples. For charged species, the separation of molecules with different shapes has been demonstrated by utilizing ion mobility in drift tubes. For neutral molecules, the abundance of the conformers in molecular beams can be partly influenced by selective over-the-barrier excitation in the early stage of the expansion or by changing the carrier gas. Herein, we demonstrate that electrostatic deflection, a classic molecular beam manipulation method that dates back to the 1920s, allows the spatial separation of the conformers of a neutral molecule when it is applied to intense beams of rotationally cold molecules produced by a state-ofthe-art pulsed supersonic expansion source. The idea of exploiting electrostatic deflection to separate quantum states was already conceived by Stern in 1926 for light diatomic molecules, and these ideas were recently extended to proposals for the separation of conformers of large molecules. Polar molecules experience a force in an inhomogeneous electric field. This force is due to the spatial variation in the potential energy of the molecules, and is given by F ! = meff·r ! E. The effective dipole moment meff of a molecule in a given quantum state is the negative gradient of the potential energy with respect to the electric field strength E. This force has been used to decelerate small molecules in a supersonic jet to a standstill and subsequently trap them. Similarly, large neutral molecules have been deflected, focused, and decelerated. Passing polar molecules through a strong inhomogenous electric field will spatially disperse them according to their effective dipole moment. In particular, the conformers of a specific biomolecule all have the same mass m, but differ by the relative orientations of their functional groups. Typically, these functional groups have large local dipole moments associated with them, and the vectorial sum of these local dipole moments largely determines the overall dipole moment of the molecule. Herein, we show that the resulting different overall dipole moments of the conformers can be exploited to select individual conformers using an electrostatic deflector. The cis and trans conformers of 3-aminophenol (Figure 1) are used herein as prototypical structural isomers of complex molecules. From the precisely known rotational constants and dipole moments, the energies of the rotational states of cis3-aminophenol and trans-3-aminophenol are calculated as a function of electric field strength. Figure 1 shows the resulting Stark curves for the lowest rotational states of both species. From Figure 1, it is obvious that the effective dipole moments meff of the states of cis-3-aminophenol are considerably larger than for trans-3-aminophenol, and therefore, a strong spatial separation of the conformers is expected. The results on the separation of the cis and trans conformers of 3-aminophenol are complementary to our previous experiments on the separation of the same species using an ac (alternating current) focusing device, and we will briefly discuss the merits of the individual techniques in the Summary. The experimental setup is shown in Figure 2, and a detailed description is given elsewhere. 3-aminophenol (Sigma–Aldrich, 98%), seeded in 90 bar of helium, is released from a pulsed valve into high vacuum. The molecular beam is [*] F. Filsinger, Dr. J. K pper, Prof. Dr. G. Meijer Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+49)30-8413-5892 http://www.fhi-berlin.mpg.de/mp/jochen E-mail: jochen@fhi-berlin.mpg.de

Rational design of an efficient, genetically encodable, protein-encased singlet oxygen photosensitizer.

Singlet oxygen, O(2)(a(1)Δ(g)), plays a key role in many processes of cell signaling. Limitations in mechanistic studies of such processes are generally associated with the difficulty of controlling the amount and location of O(2)(a(1)Δ(g)) production in or on a cell. As such, there is great need for a system that (a) selectively produces O(2)(a(1)Δ(g)) in appreciable and accurately quantifiable yields and (b) can be localized in a specific place at the suborganelle level. A genetically encodable, protein-encased photosensitizer is one way to achieve this goal. Through a systematic and rational approach involving mutations to a LOV2 protein that binds the chromophore flavin mononucleotide (FMN), we have developed a promising photosensitizer that overcomes many of the problems that affect related systems currently in use. Specifically, by decreasing the extent of hydrogen bonding between FMN and a specific amino acid residue in the local protein environment, we decrease the susceptibility of FMN to undesired photoinitiated electron-transfer reactions that kinetically compete with O(2)(a(1)Δ(g)) production. As a consequence, our protein-encased FMN system produces O(2)(a(1)Δ(g)) with the uniquely large quantum efficiency of 0.25 ± 0.03. We have also quantified other key photophysical parameters that characterize this sensitizer system, including unprecedented H(2)O/D(2)O solvent isotope effects on the O(2)(a(1)Δ(g)) formation kinetics and yields. As such, our results facilitate future systematic developments in this field.

Singlet-Oxygen-Mediated Cell Death Using Spatially-Localized Two-Photon Excitation of an Extracellular Sensitizer

Controlling and quantifying the photosensitized production of singlet oxygen are key aspects in mechanistic studies of oxygen-dependent photoinitiated cell death. In this regard, the commonly accepted practice of using intracellular photosensitizers is, unfortunately, plagued by problems that include the inability to accurately (1) quantify the sensitizer concentration in the irradiated domain and (2) control the local environment that influences light delivery and sensitizer photophysics. However, capitalizing on the fact that singlet oxygen produced outside a cell is also cytotoxic, many of these problems can be avoided with the use of an extracellular sensitizer. For the present study, a hydrophilic dendrimer-encased membrane-impermeable sensitizer was used to generate an extracellular population of singlet oxygen upon spatially localized two-photon irradiation. Through the use of this sensitizer and this approach, it is now possible to better control the singlet oxygen dose in microscope-based time- and space-resolved single cell experiments. Thus, we provide a solution to a limiting problem in mechanistic studies of singlet-oxygen-mediated cell death.

Spatial separation of state- and size-selected neutral clusters

We demonstrate the spatial separation of the prototypical indole(H