National Institute of Standards and Technology

About: National Institute of Standards and Technology is a government organization based out in Gaithersburg, Maryland, United States. It is known for research contribution in the topics: Laser & Scattering. The organization has 26667 authors who have published 60661 publications receiving 2215547 citations. The organization is also known as: National Bureau of Standards & NIST.

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

Abstract: The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules1,2. 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 temperature3,4. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models5,6,7. 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 ζ 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 observations5,6,7 of H3+ can be resolved.

Giant Anharmonicity and Nonlinear Electron-Phonon Coupling in MgB 2 : A Combined First-Principles Calculation and Neutron Scattering Study

Abstract: First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB{sub 2} are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E{sub 2g} in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B {sigma} bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T{sub c} and boron isotope effect in MgB{sub 2} .

Impact Broadening of Microwave Spectra

Abstract: Pressure broadening in resonant and in nonresonant microwave spectra of gases, as well as the transition from resonant to nonresonant shape with increasing pressure, are treated in the impact approximation in a unified manner, based on the theories of Baranger, Kolb and Griem, and Fano. The relaxation rate in nonresonant absorption (Debye relaxation) is expressed in terms of collision cross sections; small deviations from a Debye shape are related to a second moment of the relaxation matrix, with illustrative application to C${\mathrm{H}}_{3}$Cl. A new formula is derived for the shape of resonance lines, as a function of frequency and pressure, which takes into account the special effect of collision-induced transitions between the upper and the lower levels of the line; this formula reproduces the shape of the inversion spectra of N${\mathrm{H}}_{3}$ and of N${\mathrm{D}}_{3}$.

Measurement of the temperature dependence of the Casimir-Polder force

Abstract: We report on the first measurement of a temperature dependence of the Casimir-Polder force. This measurement was obtained by positioning a nearly pure 87Rb Bose-Einstein condensate a few microns from a dielectric substrate and exciting its dipole oscillation. Changes in the collective oscillation frequency of the magnetically trapped atoms result from spatial variations in the surface-atom force. In our experiment, the dielectric substrate is heated up to 605 K, while the surrounding environment is kept near room temperature (310 K). The effect of the Casimir-Polder force is measured to be nearly 3 times larger for a 605 K substrate than for a room-temperature substrate, showing a clear temperature dependence in agreement with theory.