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The charge transport mechanism of a wire can be revealed by how its electrical resistance varies with length. We have measured the resistance and current-voltage characteristics of conjugated molecular wires ranging in length from 1 to 7 nanometers, connected between metal electrodes. We observe the theoretically predicted change in direct-current transport from tunneling to hopping as a function of systematically controlled wire length. We also demonstrate that site-specific disruption of conjugation in the wires greatly increases resistance in the hopping regime but has only a small effect in the tunneling regime. These nanoscale transport measurements elucidate the role of molecular length and bond architecture on molecular conductivity and open opportunities for greater understanding of electrical transport in conjugated polymer films.

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion$CpMn(CO)_5$while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.

We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

Demonstration of an optomechanical system that works as a quantum interface between light and micro-mechanical motion. Nanomechanical oscillators coupled to optic cavities The possibility of controlling the quantum states of micro- and nanomechanical oscillators has been of great interest in recent years. Although various mechanical resonators have been cooled to their quantum ground state, there are few reports of experiments in which this quantum regime is further explored and used, for example, to exchange quantum information. Previously, quantum coupling between mechanical degrees of freedom and microwave radiation has been shown. Now, Verhagen et al . demonstrate an optomechanical system, cooled by radiation pressure, that works as a quantum interface between a mechanical oscillator and optical photons, offering the advantage that standard optical fibres can be used to extract the quantum information. Optical laser fields have been widely used to achieve quantum control over the motional and internal degrees of freedom of atoms and ions 1 , 2 , molecules and atomic gases. A route to controlling the quantum states of macroscopic mechanical oscillators in a similar fashion is to exploit the parametric coupling between optical and mechanical degrees of freedom through radiation pressure in suitably engineered optical cavities 3 , 4 , 5 , 6 . If the optomechanical coupling is ‘quantum coherent’—that is, if the coherent coupling rate exceeds both the optical and the mechanical decoherence rate—quantum states are transferred from the optical field to the mechanical oscillator and vice versa. This transfer allows control of the mechanical oscillator state using the wide range of available quantum optical techniques. So far, however, quantum-coherent coupling of micromechanical oscillators has only been achieved using microwave fields at millikelvin temperatures 7 , 8 . Optical experiments have not attained this regime owing to the large mechanical decoherence rates 9 and the difficulty of overcoming optical dissipation 10 . Here we achieve quantum-coherent coupling between optical photons and a micromechanical oscillator. Simultaneously, coupling to the cold photon bath cools the mechanical oscillator to an average occupancy of 1.7 ± 0.1 motional quanta. Excitation with weak classical light pulses reveals the exchange of energy between the optical light field and the micromechanical oscillator in the time domain at the level of less than one quantum on average. This optomechanical system establishes an efficient quantum interface between mechanical oscillators and optical photons, which can provide decoherence-free transport of quantum states through optical fibres. Our results offer a route towards the use of mechanical oscillators as quantum transducers or in microwave-to-optical quantum links 11 , 12 , 13 , 14 , 15 .

Chemical mapping of a single molecule by optical means down to subnanometre resolution is achieved by spectrally matching the resonance of a nanocavity plasmon to the vibronic transitions of the molecules being studied, using tip-enhanced Raman scattering. Inner workings of a single molecule Raman spectroscopy is widely used to identify molecules by detecting their signature molecular vibrations. The technology has been refined to be effective at the single-molecule level by making use of strong localized plasmonic fields that can enhance spectral signals. This study goes further, with the demonstration of a technique related to 'tip-enhanced Raman scattering' (TERS) that allows precise tuning of the plasmon resonance and Raman spectral imaging with a spatial resolution below 1 nm, resolving even the inner structure of a single molecule and its configuration on the surface. The technique opens a new path to photochemistry at the single-molecule level, offering the potential to design, control and engineer the functionality of molecules on demand. Visualizing individual molecules with chemical recognition is a longstanding target in catalysis, molecular nanotechnology and biotechnology. Molecular vibrations provide a valuable ‘fingerprint’ for such identification. Vibrational spectroscopy based on tip-enhanced Raman scattering allows us to access the spectral signals of molecular species very efficiently via the strong localized plasmonic fields produced at the tip apex 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . However, the best spatial resolution of the tip-enhanced Raman scattering imaging is still limited to 3−15 nanometres 5 , 12 , 13 , 14 , 15 , 16 , which is not adequate for resolving a single molecule chemically. Here we demonstrate Raman spectral imaging with spatial resolution below one nanometre, resolving the inner structure and surface configuration of a single molecule. This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunnelling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunnelling gap is essential for ultrahigh-resolution imaging through the generation of an efficient double-resonance enhancement for both Raman excitation and Raman emission. Our technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of a single molecule.

Bijvoet's method, which makes use of anomalous x-ray diffraction or dispersion, is the standard means of directly determining the absolute (stereochemical) configuration of molecules, but it requires crystalline samples and often proves challenging in structures exclusively comprising light atoms. Herein, we demonstrate a mass spectrometry approach that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionizatior–induced Coulomb explosion. This technique is applied to the prototypical chiral molecule bromochlorofluoromethane and the isotopically chiral methane derivative bromodichloromethane.

How does a chemical reaction proceed at ultralow temperatures? Can simple quantum mechanical rules such as quantum statistics, single partial-wave scattering, and quantum threshold laws provide a clear understanding of the molecular reactivity under a vanishing collision energy? Starting with an optically trapped near-quantum-degenerate gas of polar ⁴⁰K⁸⁷Rb molecules prepared in their absolute ground state, we report experimental evidence for exothermic atom-exchange chemical reactions. When these fermionic molecules were prepared in a single quantum state at a temperature of a few hundred nanokelvin, we observed p-wave-dominated quantum threshold collisions arising from tunneling through an angular momentum barrier followed by a short-range chemical reaction with a probability near unity. When these molecules were prepared in two different internal states or when molecules and atoms were brought together, the reaction rates were enhanced by a factor of 10 to 100 as a result of s-wave scattering, which does not have a centrifugal barrier. The measured rates agree with predicted universal loss rates related to the two-body van der Waals length.

Laser cooling goes molecular The development of Doppler laser cooling techniques allowed unprecedented access to ultracold temperatures of less 1 millikelvin. The motion of particles effectively ceases at such temperatures, enabling physical phenomena to be studied and controlled in extraordinary detail. Although laser cooling of atoms was demonstrated about 30 years ago, these techniques had not previously been extended to molecules. Ultracold molecules may prove even more interesting than ultracold atoms, because their greater internal complexity can potentially be exploited to investigate and manipulate a wide variety of physical phenomena, ranging from quantum information processing to chemical reactions and particle physics. Currently the only technique for producing ultracold molecules is by binding together ultracold alkali atoms to produce bi-alkali molecules. A team from Yale University now presents an experimental demonstration of laser cooling of a diatomic molecule — the polar molecule strontium monofluoride (SrF). With further refinement, the technique should enable the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bi-alkalis. Laser cooling has not yet been extended to molecules because of their complex internal structure. At present, the only technique for producing ultracold molecules is to bind ultracold alkali atoms to produce bialkali molecules. These authors experimentally demonstrate laser cooling of the polar molecule strontium monofluoride, reaching temperatures of a few millikelvin or less. The technique should allow the production of molecules at microkelvin temperatures for species that are chemically distinct from bialkalis. It has been roughly three decades since laser cooling techniques produced ultracold atoms 1 , 2 , 3 , leading to rapid advances in a wide array of fields. Laser cooling has not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications 4 . For example, heteronuclear molecules possess permanent electric dipole moments that lead to long-range, tunable, anisotropic dipole–dipole interactions. The combination of the dipole–dipole interaction and the precise control over molecular degrees of freedom possible at ultracold temperatures makes ultracold molecules attractive candidates for use in quantum simulations of condensed-matter systems 5 and in quantum computation 6 . Also, ultracold molecules could provide unique opportunities for studying chemical dynamics 7 , 8 and for tests of fundamental symmetries 9 , 10 , 11 . Here we experimentally demonstrate laser cooling of the polar molecule strontium monofluoride (SrF). Using an optical cycling scheme requiring only three lasers 12 , we have observed both Sisyphus and Doppler cooling forces that reduce the transverse temperature of a SrF molecular beam substantially, to a few millikelvin or less. At present, the only technique for producing ultracold molecules is to bind together ultracold alkali atoms through Feshbach resonance 13 or photoassociation 14 . However, proposed applications for ultracold molecules require a variety of molecular energy-level structures (for example unpaired electronic spin 5 , 9 , 11 , 15 , Omega doublets 16 and so on). Our method provides an alternative route to ultracold molecules. In particular, it bridges the gap between ultracold (submillikelvin) temperatures and the ∼1-K temperatures attainable with directly cooled molecules (for example with cryogenic buffer-gas cooling 17 or decelerated supersonic beams 18 ). Ultimately, our technique should allow the production of large samples of molecules at ultracold temperatures for species that are chemically distinct from bialkalis.

A molecule or larger body is chiral if it cannot be superimposed on its mirror image (enantiomer). Electromagnetic fields may be chiral, too, with circularly polarized light (CPL) as the paradigmatic example. A recently introduced measure of the local degree of chiral dissymmetry in electromagnetic fields suggested the existence of optical modes more selective than circularly polarized plane waves in preferentially exciting single enantiomers in certain regions of space. By probing induced fluorescence intensity, we demonstrated experimentally an 11-fold enhancement over CPL in discrimination of the enantiomers of a biperylene derivative by precisely sculpted electromagnetic fields. This result, which agrees to within 15% with theoretical predictions, establishes that optical chirality is a fundamental and tunable property of light, with possible applications ranging from plasmonic sensors to absolute asymmetric synthesis.