The combination of photoelectron spectroscopy and ultrafast light sources is on track to set new standards for detailed interrogation of dynamics and reactivity of molecules [1][2][3][4][5][6][7]. A crucial prerequisite for further progress is the ability to not only detect the electron kinetic energy, as done in traditional photoelectron spectroscopy, but also the photoelectron angular distributions (PADs) in the molecular frame [1,4,5,[7][8][9][10]. Until recently the only method relied on determining the orientation of the molecular frame after ionization [1,[11][12][13].This requires that ionization leads to fragmentation thereby limiting both the species and the specific processes that can be studied. An attractive alternative is to fix the molecular frame prior to ionization. The only demonstrations hitherto involved aligned small linear unpolar molecules [4,5,8]. A decisive milestone is extension to the general class of polar molecules. Here carbonylsulfide (OCS) and benzonitrile (C 7 H 5 N) molecules, fixed in space by combined laser and electrostatic fields, are ionized with intense, circularly polarized, 30 femtosecond laser pulses. For 1-dimensionally oriented OCS the molecular frame PADs exhibit pronounced anisotropies, perpendicular to the fixed permanent dipole moment, that are absent in PADs from randomly oriented molecules.For 3-dimensionally oriented C 7 H 5 N additional striking structures appear due to suppression of electron emission in nodal planes of the fixed electronic orbitals. Our theoretical analysis, relying on tunneling ionization theory [14,15], shows that the PADs reflect nodal planes, permanent dipole moments and polarizabilities of both the neutral molecule and its cation. The calculated results are exponentially sensitive to changes in these molecular properties thereby pointing to exciting opportunities for time-resolved probing of valence electrons dynamics by intense circularly polarized pulses. Molecular frame PADs from oriented molecules will prove important in other contexts notably in emerging free-electron-laser studies where localized inner shell electrons are knocked off by x-ray pulses.Experimentally a target of adiabatically aligned and oriented molecules is created by the combined action of a 10 nanosecond laser pulse and a weak static electric field [16,17].Here alignment refers to confinement of molecule-fixed axes along laboratory fixed axes, and orientation refers to the molecular dipole moment pointing in a particular direction 2 [18]. Before reaching the interaction point with the laser pulses and the static field the molecules are selected in the lowest-lying rotational quantum states by an electrostatic deflector [19]. Hereby alignment and orientation is optimized, which is crucial for observation of the molecular frame PAD effects discussed next. The degree of alignment and orientation is initially measured by Coulomb exploding the molecules using an intense femtosecond (fs) probe laser pulse ( Supplementary Information, SI).For the PAD experiments a circul...

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.

Supersonic beams of polar molecules are deflected using inhomogeneous electric fields. The quantumstate-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 ( cos 2 θ2D = 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.

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.

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.