By measurement of the frequency of a vibrational overtone transition in the molecular hydrogen ion HD , we demonstrate the first optical spectroscopy of trapped molecular ions with submegahertz accuracy. We use a diode laser, locked to a stable frequency comb, to perform resonance-enhanced multiphoton dissociation spectroscopy on sympathetically cooled HD ions at 50 mK. The achieved 2-ppb relative accuracy is a factor of 150 higher than previous results for HD , and the measured transition frequency agrees well with recent high-accuracy ab initio calculations, which include highorder quantum electrodynamic effects. We also show that our method bears potential for achieving considerably higher accuracy and may, if combined with slightly improved theoretical calculations, lead to a new and improved determination of the electron-proton mass ratio.

In the field of cold quantum matter, control of the motional degrees of freedom of both neutral and charged gas-phase molecules has been achieved for a wide range of species 1-11 . However, cooling of the internal degrees of freedom remains challenging. Recently, transfer to the internal ground state by sophisticated optical techniques has been demonstrated for neutral alkali dimers created in single quantum states from ultracold atoms 12-15 . Here we demonstrate cooling of the rotational degree of freedom of heteronuclear diatomic molecules with a thermal distribution of internal states, using a simple, robust and general optical-pumping scheme with two low-power continuous-wave lasers. With trapped and translationally cooled hydrogen deuteride (HD + ) molecular ions as a model system, we achieve 78(4)% rovibrational ground-state population. The rotationally, vibrationally and translationally cold molecular ion ensemble is suitable for a number of applications, such as generation of long-lived coherences or frequency metrology of fundamental constants 16,17 .The study of cold molecular systems promises new insights and advances in many fields of physics and physical chemistry. As in atomic physics, the key to tapping the full potential of molecules is the ability to accurately control the external and internal degrees of freedom of the particles. The complex internal structure of molecules has however so far precluded direct application of many techniques developed for trapping and cooling of atoms, demanding modified or completely new approaches. Now, a large toolbox for trapping and cooling the motional degrees of freedom of both neutral and charged molecules is available 18 . Although general schemes for cooling the internal degrees of freedom of molecules have been proposed 19,20 , the most general method available at present is cryogenic buffer-gas cooling, which is efficient only for molecules in the vibrational ground state and limits the translational temperature to a few hundred millikelvin 7 . Coherent transfer to the rovibrational ground state 11-14 is most suitable when most of the molecules are initially in the same quantum state, as is the case for molecules produced by associating cold atoms.For heteronuclear molecular ensembles for which the population is distributed among many rotational levels in the v = 0 vibrational manifold, optical pumping has been proposed as an approach to rotational cooling 21,22 . We demonstrate here that a scheme using two laser fields driving a fundamental and an overtone vibrational electric dipole transition 21 yields a large ground-state population and briefly discuss the applicability of the scheme to various diatomic molecular species. Figure 1 shows the energy levels (without hyperfine structure) and electric dipole transitions of the HD + molecule relevant for the experiment. The initial internal-state distribution is given by a Boltzmann distribution reflecting thermal equilibrium with the T ∼ = 300 K blackbody radiation field emitted by the experimental Inst...

Light energy is an important factor for plant growth. In regions where the natural light source (solar radiation) is not sufficient for growth optimization, additional light sources are being used. Traditional light sources such as high pressure sodium lamps and other metal halide lamps are not very efficient and generate high radiant heat. Therefore, new sustainable solutions should be developed for energy efficient greenhouse lighting. Recent developments in the field of light source technologies have opened up new perspectives for sustainable and highly efficient light sources in the form of LEDs (light-emitting diodes) for greenhouse lighting. This review focuses on the potential of LEDs to replace traditional light sources in the greenhouse. In a comparative economic analysis of traditional vs. LED lighting, we show that the introduction of LEDs allows reduction of the production cost of vegetables in the long-run (several years), due to the LEDs' high energy efficiency, low maintenance cost and longevity. In order to evaluate LEDs as a true alternative to current lighting sources, species specific plant response to different wavelengths is discussed in a comparative study. However, more detailed scientific studies are necessary to understand the effect of different spectra (using LEDs) on plants physiology. Technical innovations are required to design and realize an energy efficient light source with a spectrum tailored for optimal plant growth in specific plant species.

Demonstrating improved confinement of energetic ions is one of the key goals of the Wendelstein 7-X (W7-X) stellarator. In the past campaigns, measuring confined fast ions has proven to be challenging. Future deuterium campaigns would open up the option of using fusion-produced neutrons to indirectly observe confined fast ions. There are two neutron populations: 2.45 MeV neutrons from thermonuclear and beam-target fusion, and 14.1 MeV neutrons from DT reactions between tritium fusion products and bulk deuterium. The 14.1 MeV neutron signal can be measured using a scintillating fiber neutron detector, whereas the overall neutron rate is monitored by common radiation safety detectors, for instance fission chambers. The fusion rates are dependent on the slowing-down distribution of the deuterium and tritium ions, which in turn depend on the magnetic configuration via fast ion orbits. In this work, we investigate the effect of magnetic configuration on neutron production rates in W7-X. The neutral beam injection, beam and triton slowing-down distributions, and the fusion reactivity are simulated with the ASCOT suite of codes. The results indicate that the magnetic configuration has only a small effect on the production of 2.45 MeV neutrons from DD fusion and, particularly, on the 14.1 MeV neutron production rates. Despite triton losses of up to 50 %, the amount of 14.1 MeV neutrons produced might be sufficient for a time-resolved detection using a scintillating fiber detector, although only in high-performance discharges.

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3  ×  1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.

The properties of cold ion plasmas with large numbers ͑Ͼ200͒ of laser-and sympathetically cooled species are modeled in detail using molecular dynamics simulations. We describe how to extract temperatures and ion numbers from CCD images. The identification of the ion species by excitation of their oscillation modes is discussed. The sympathetic cooling efficiency, effects of the rf micromotion and of collision heating by a neutral background gas are analyzed, in part experimentally.

We have cooled ensembles of the molecular hydrogen ions H2+, H3+, and all their deuterated variants to temperatures of a few mK in a radio frequency trap, by sympathetic cooling with laser-cooled beryllium ions. The molecular ions are embedded in the central regions of Coulomb crystals. Mass spectroscopy and molecular dynamics simulations were used to accurately characterize the properties of the ultracold multispecies crystals. We demonstrate species-selective purification of multispecies ensembles. These molecules are of fundamental importance as the simplest of all molecules, and have the potential to be used for precision tests of molecular structure theory, tests of Lorentz invariance, and measurements of electron to nuclear mass ratios and their time variation.

We report a high-resolution spectroscopic study of molecular ions at millikelvin temperatures. We measured several rovibrational infrared transitions in HD + molecular ions, stored in a radio-frequency trap and sympathetically cooled to Ϸ20 mK by laser-cooled Be + ions. We observed hyperfine splitting of the lines, in good agreement with theoretical predictions. The transitions were detected by monitoring the decrease in ion number after selective photodissociation of HD + ions in the upper vibrational state. The method described here is expected to be generally applicable.