Permutationally Invariant Polynomial Potential Energy Surfaces for Tropolone and H and D atom Tunneling Dynamics

We report permutationally invariant polynomial (PIP) fits to energies and gradients for 15-atom tropolone. These include standard, augmented, and fragmented PIP bases. Approximately 6600 energies and associated gradients are obtained from direct-dynamics calculations using DFT/B3LYP/6-31+G(d) supplemented by grid calculations spanning an energy range up to roughly 35 000 cm-1. Three fragmentation schemes are investigated with respect to efficiency and fit precision. In addition several fits are done with reduced weight for gradient data relative to energies. These do result in more precision for the H-transfer barrier height. Properties of the fits such as stationary points, harmonic frequencies and the barrier to H-atom transfer are reported and compared to direct calculations. A 1-d model to obtain the tunneling splitting for the ground vibrational state and qualitative predictions for excited vibrational states is employed. Several 1-d double well fits to the PES are developed and used to extrapolate H and D atom tunneling splittings to values at the CCSD(T)-F12 barrier. The level of agreement is within expectations for the method adapted and the level of the electronic structure theory employed

A Comparison of the Ocean Microbarom Recorded on the Ground and in the Stratosphere

The, ocean microbarom is an acoustic signal generated via nonlinear interaction of ocean surface waves. It can propagate for thousands of kilometers and represent a significant infrasonic noise source for ground infrasound stations across the globe. However, wind noise often compromises detections at ground stations. Furthermore, the microbarom may travel in elevated acoustic ducts that do not transmit enough energy for detections on ground stations. Here the presence of the ocean microbarom on two high-altitude balloon flights is investigated. A spectral peak consistent with the microbarom was observed on sensors in the stratosphere but not on those deployed on the ground near the flight path of the balloon. This is probably due to an elevated acoustic duct and/or a superior signal-to-noise ratio in the stratosphere. Thus, microbarom activity quantified solely with ground-based sensors may underestimate the occurrence of the phenomenon. However, high levels of interference from flight system electronics and/other other payloads may have obscured other microbarom episodes during the balloon deployments

Infrasound in the middle stratosphere measured with a free-flying acoustic array

Infrasound recorded in the middle stratosphere suggests that the acoustic wavefield above the Earth's surface differs dramatically from the wavefield near the ground. In contrast to nearby surface stations, the balloon-borne infrasound array detected signals from turbulence, nonlinear ocean wave interactions, building ventilation systems, and other sources that have not been identified yet. Infrasound power spectra also bore little resemblance to spectra recorded on the ground at the same time. Thus, sensors on the Earth's surface likely capture a fraction of the true diversity of acoustic waves in the atmosphere. Future studies building upon this experiment may quantify the acoustic energy flux from the surface to the upper atmosphere, extend the capability of the International Monitoring System to detect nuclear explosions, and lay the observational groundwork for a recently proposed mission to detect earthquakes on Venus using free-flying microphones. Key Points Stratospheric infrasound is very different from infrasound recorded on ground stations Numerous broadband and narrowband signals exist in the stratosphere, some of unknown origin Acoustic networks on free floating balloons have unique advantages and disadvantages

Detecting Lightning Infrasound Using a High-Altitude Balloon

Acoustic waves with a wide range of frequencies are generated by lightning strokes during thunderstorms, including infrasonic waves (0.1 to 20 Hz). The source mechanism for these low-frequency acoustic waves is still debated, and studies have so far been limited to ground-based instruments. Here we report the first confirmed detection of lightning-generated infrasound with acoustic instruments suspended at stratospheric altitudes using a free-flying balloon. We observe high-amplitude signals generated by lightning strokes located within 100 km of the balloon as it flew over the Tasman Sea on 17 May 2016. The signals share many characteristics with waveforms recorded previously by ground-based instruments near thunderstorms. The ability to measure lightning activity with high-altitude infrasound instruments has demonstrated the potential for using these platforms to image the full acoustic wavefield in the atmosphere. Furthermore, it validates the use of these platforms for recording and characterizing infrasonic sources located beyond the detection range of ground-based instruments

Equilibrium clumped-isotope effects in doubly substituted isotopologues of ethane

We combine path-integral Monte Carlo methods with a new intramolecular potential energy surface to quantify the equilibrium enrichment of doubly substituted ethane isotopologues due to clumped-isotope effects. Ethane represents the simplest molecule to simultaneously exhibit 13C–13C, 13C–D, and D–D clumped-isotope effects, and the analysis of corresponding signatures may provide useful geochemical and biogeochemical proxies of formation temperatures or reaction pathways. Utilizing path-integral statistical mechanics, we predict equilibrium fractionation factors that fully incorporate nuclear quantum effects, such as anharmonicity and rotational-vibrational coupling which are typically neglected by the widely used Urey model. The magnitude of the calculated fractionation factors for the doubly substituted ethane isotopologues indicates that isotopic clumping can be observed if rare-isotope substitutions are separated by up to three chemical bonds, but the diminishing strength of these effects suggests that enrichment at further separations will be negligible. The Urey model systematically underestimates enrichment due to 13C–D and D–D clumped-isotope effects in ethane, leading to small relative errors in the apparent equilibrium temperature, ranging from 5 K at 273.15 K to 30 K at 873.15 K. We additionally note that the rotameric dependence of isotopologue enrichment must be carefully considered when using the Urey model, whereas the path-integral calculations automatically account for such effects due to configurational sampling. These findings are of direct relevance to future clumped-isotope studies of ethane, as well as studies of 13C–13C, 13C–D, and D–D clumped-isotope effects in other hydrocarbons

Renormalization flow of Yang-Mills propagators

We study Landau-gauge Yang-Mills theory by means of a nonperturbative vertex expansion of the quantum effective action. Using an exact renormalization group equation, we compute the fully dressed gluon and ghost propagators to lowest nontrivial order in the vertex expansion. In the mid-momentum regime, $p^2\sim\mathcal{O}(1)\text{GeV}^2$, we probe the propagator flow with various {\em ans\"atze} for the three- and four-point correlations. We analyze the potential of these truncation schemes to generate a nonperturbative scale. We find universal infrared behavior of the propagators, if the gluon dressing function has developed a mass-like structure at mid-momentum. The resulting power laws in the infrared support the Kugo-Ojima confinement scenario.Comment: 28 pages, 5 figures. V2: Typos corrected and reference adde

Topographically Scattered Infrasound Waves Observed on Microbarometer Arrays in the Lower Stratosphere

When an acoustic wave strikes a topographic feature, some of its energy is scattered. Sensors on the ground cannot capture these scattered signals when they propagate at high angles. We report observations of upwardly-scattered acoustic waves prior to refraction back to the ground, intercepting them with a set of balloon-borne infrasound microbarometers in the lower stratosphere over northern Sweden. We show that these scattered waves generate a coda whose presence can be related to topography beneath balloons and low-altitude acoustic ducts. The inclination of the coda signals changes systematically with time, as expected from waves arriving from scatterers successively closer to receivers. The codas are present when a temperature inversion channels infrasound from a set of ground chemical explosions along the ground, but are absent following the inversion's dissipation. Since scattering partitions energy away from the main arrival, these observations imply a mechanism of amplitude loss that had previously been inaccessible to measurement. As such, these data and results allow for a better comprehension of interactions between atmospheric infrasound propagation and the solid earth

Scaling behavior of the overlap quark propagator in Landau gauge

The properties of the momentum space quark propagator in Landau gauge are examined for the overlap quark action in quenched lattice QCD. Numerical calculations are done on three lattices with different lattice spacings and similar physical volumes to explore the approach of the quark propagator toward the continuum limit. We have calculated the nonperturbative momentum-dependent wave function renormalization function Z(p) and the nonperturbative mass function M(p) for a variety of bare quark masses and perform an extrapolation to the chiral limit. We find the behavior of Z(p) and M(p) are in reasonable agreement between the two finer lattices in the chiral limit, however the data suggest that an even finer lattice is desirable. The large momentum behavior is examined to determine the quark condensate.Comment: 9 pages, 5 figures, Revtex 4. Streamlined presentation, additional data. Final versio

A dynamical gluon mass solution in a coupled system of the Schwinger-Dyson equations

We study numerically the Schwinger-Dyson equations for the coupled system of gluon and ghost propagators in the Landau gauge and in the case of pure gauge QCD. We show that a dynamical mass for the gluon propagator arises as a solution while the ghost propagator develops an enhanced behavior in the infrared regime of QCD. Simple analytical expressions are proposed for the propagators, and the mass dependency on the $\Lambda_{QCD}$ scale and its perturbative scaling are studied. We discuss the implications of our results for the infrared behavior of the coupling constant, which, according to fits for the propagators infrared behavior, seems to indicate that $\alpha_s (q^2) \to 0$ as $q^2 \to 0$.Comment: 17 pages, 7 figures - Revised version to be consistent with erratum to appear in JHE