An enhanced cosmic-ray flux towards zeta Persei inferred from a laboratory study of the H3+-e- recombination rate

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.     

Tunable nanowire nonlinear optical probe

One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion, structured illumination, and photoactivated localization microscopy, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.