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 rotationally state-selected molecules, rather than molecules in the original molecular beam, are used as targets.Comment: 8 pages, 7 figures; v2: minor update

Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

The yield of strong-field ionization, by a linearly polarized probe pulse, is studied experimentally and theoretically, as a function of the relative orientation between the laser field and the molecule. Experimentally, carbonyl sulfide, benzonitrile and naphthalene molecules are aligned in one or three dimensions before being singly ionized by a 30 fs laser pulse centered at 800 nm. Theoretically, we address the behaviour of these three molecules. We consider the degree of alignment and orientation and model the angular dependence of the total ionization yield by molecular tunneling theory accounting for the Stark shift of the energy level of the ionizing orbital. For naphthalene and benzonitrile the orientational dependence of the ionization yield agrees well with the calculated results, in particular the observation that ionization is maximized when the probe laser is polarized along the most polarizable axis. For OCS the observation of maximum ionization yield when the probe is perpendicular to the internuclear axis contrasts the theoretical results.Comment: 14 pages, 4 figure

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.Comment: minor changes, references update

Ionization of 1D and 3D oriented asymmetric top molecules by intense circularly polarized femtosecond laser pulses

We present a combined experimental and theoretical study on strong-field ionization of a three-dimensionally oriented asymmetric top molecule, benzonitrile (C$_7$H$_5$N), by circularly polarized, nonresonant femtosecond laser pulses. Prior to the interaction with the strong field, the molecules are quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and oriented adiabatically using an elliptically polarized laser pulse in combination with a static electric field. A characteristic splitting in the molecular frame photoelectron momentum distribution reveals the position of the nodal planes of the molecular orbitals from which ionization occurs. The experimental results are supported by a theoretical tunneling model that includes and quantifies the splitting in the momentum distribution. The focus of the present article is to understand strong-field ionization from 3D-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals. In the preceding article [Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons.Comment: 12 pages, 9 figure

Temperature Effect on Radiative Lifetimes: The Case of Singlet Oxygen in Liquid Solvents

A change in solvent can have an appreciable effect on the rate constant for the O₂(a¹Δ_g) → O₂(X³Σ_g^–) radiative transition at ∼1275 nm. The data thus obtained have played an important role in understanding mechanisms by which environment-dependent perturbations can influence forbidden electronic transitions. We now report that the rate constant for O₂(a¹Δ_g) radiative deactivation, <i>k</i>_r, also responds to changes in temperature. This result can have practical ramifications in experiments that use O₂(a¹Δ_g) phosphorescence to quantify yields of photosensitized O₂(a¹Δ_g) production. From a fundamental perspective, this result is significant, partly because there is little precedence for temperature-dependent changes in radiative rate constants. The data also require a re-evaluation of the current model by which oxygen is perturbed by solvent. Specifically, the evidence indicates that it is not appropriate to evaluate the interaction as a 1:1 complex between a given solvent molecule M and oxygen. Rather, one must consider an ensemble of solvent molecules surrounding oxygen

LASER-INDUCED ALIGNMENT AND ORIENTATION OF QUANTUM-STATE-SELECTED LARGE MOLECULES

H. Stapelfeldt and T. Seideman, \textit{Rev.~Mod.~Phys.L. Holmegaard et al., \textit{Phys.~Rev.~Lett.Author Institution: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195; Berlin, Germany; Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, DenmarkFor many experiments in chemistry and physics, i.\,e., reactive scattering, X-ray or electron diffraction experiments, a high level of control over the spatial orientation of molecules would be very beneficial. It is well known that molecules can be aligned and oriented in space using strong dc electric fields or laser pulses.~, \textbf{75}, (2003), 543} Here, we demonstrate that the degree of laser-induced alignment and orientation can be considerably improved, if quantum state selected samples are used.~, \textbf{102}, (2009), 023001} A strong inhomogeneous electric field is used in a Stern-Gerlach-type experiment to disperse iodobenzene molecules in a supersonic jet according to their rotational quantum state. Molecules in the lowest rotational quantum states are deflected most and can be used as targets for further experiments. This method is widely applicable to all, small and large, polar molecules and should eventually enable experiments on pure samples of strongly aligned or oriented ground-state molecules offering new prospects in molecular sciences

Rational Design of an Efficient, Genetically Encodable, Protein-Encased Singlet Oxygen Photosensitizer

Singlet oxygen, O₂(a¹Δ_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 O₂(a¹Δ_g) production in or on a cell. As such, there is great need for a system that (a) selectively produces O₂(a¹Δ_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 O₂(a¹Δ_g) production. As a consequence, our protein-encased FMN system produces O₂(a¹Δ_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 H₂O/D₂O solvent isotope effects on the O₂(a¹Δ_g) formation kinetics and yields. As such, our results facilitate future systematic developments in this field

SPATIALLY SEPARATING STRUCTURAL ISOMERS OF NEUTRAL MOLECULES

L. Holmegaard et al., \textit{Phys.~Rev.~Lett.F. Filsinger et al., \textit{Phys.~Rev.~Lett.Author Institution: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195; Berlin, Germany; Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, DenmarkLarge (bio)molecules exhibit multiple conformers (structural isomers), even under the cold conditions present in a supersonic jet. For various applications, i.\,e., scattering experiments or time resolved studies, it would be highly desirable to prepare molecular packets of individual conformers. It is well known that polar molecules can be manipulated using strong electric fields. Recently, we have demonstrated that electrostatic deflection of a molecular beam can be used for quantum-state selection of large molecules.~, \textbf{102}, (2009),023001 } Here, we demonstrate how this quantum state selectivity can be exploited to spatially separate the individual conformers of large molecules based on their distinct mass-to-dipole moment (m/$\mu$) ratios. In a proof-of-principle experiment, we have spatially isolated both, cis and trans, conformers of 3-aminophenol. We will compare this approach to conformer selection using alternating gradient (dynamic) focusing in an m/$\mu$-selector.~ \textbf{100}, (2008),133003