Unstructured Continuum Research Articles

Detection of Low‐Frequency Continuum Radiation

AbstractThe electromagnetic spectrum at low frequencies from ∼3 to 300 kHz is dominated by impulses from lightning discharges and anthropogenic radio transmissions used for communication. Electromagnetic waves generated in near‐Earth space exhibit generally smaller amplitudes that are attenuated when travelling through the ionosphere before they can be observed at high and midlatitudes. Electromagnetic waves with yet smaller amplitudes contribute to the overall electromagnetic energy trapped within the Earth‐ionosphere cavity. At this point, the electromagnetic waves from all possible sources blend into an unstructured continuum radiation near the instrumental noise floor, which is often considered to be a fundamental limit to scientific discovery. As a result, the sources of continuum radiation have been little studied and are essentially unknown. Here we show how low‐frequency continuum radiation is detected and discriminated against known radio sources and instrumental noise by use of rigorous criteria inferred from novel precision measurements with an array of radio receivers. In particular, it is found that coherent continuum radiation from intermittent radio transmitters exhibits electric field strengths 0.5–0.7 μV/m, which are almost 2 orders of magnitude smaller when compared to the noise floor of the radio receivers ∼25 μV/m. Another part of the continuum radiation is found at local zenith above the array when it is caused by random instrumental noise. The results exemplify the possibility to extract sources from continuum radiation to study their origin and physical properties, which can contribute to an improved understanding of the impact of space weather and solar variability on the Earth's upper atmosphere.

Unstructured continuum–kinetic solver with fluid–particle interaction for in-space propulsion contaminant dispersal

A new parallel generalized unstructured grid solver for prediction of rarefied gas dynamics is presented. The method is based on the direct numerical solution of the semi-classical Boltzmann equation using a discrete velocity method (DVM) and supports accurate prediction of non-continuum flows in near-vacuum rarefied flow regimes. For in-space propulsion applications which involve mixed continuum–rarefied environments, the flow regimes are solved separately through the use of DVM and hybrid RANS/LES solution procedures, respectively, on the same computational domain while interacting with one another. The solver also supports coupled Lagrangian particle transport and coalescence/breakup modeling to enable prediction of plume flow impingement and contaminant dispersal through mixed continuum–rarefied environments around spacecraft. Such simulation capabilities are essential for the prediction of spacecraft environments concerning plume impingement pressure, heating, and contamination on spacecraft components such as solar panels and sensitive instruments. Example applications include venting and plume flow environments from spacecraft main engines and attitude control thrusters firing in near-vacuum conditions. The framework upon which the solvers are developed is introduced along with a detailed description of the direct Boltzmann solution and collision integral evaluation procedures. The accuracy of standalone kinetic and coupled continuum–kinetic predictions is validated by comparison against experimental data for shock structure using various different intermolecular interaction models. Results are presented to demonstrate the merits of the capability for prediction of high-speed gas–particle interaction through complex moving shock systems, and thruster plume flow impingement with liquid droplet tracking in near-vacuum conditions.

An algorithm for the EEG frequency architecture of consciousness and brain body coupling

THE FREQUENCY ARCHITECTURE OF THE CONSCIOUS HUMAN BRAIN The EEG is traditionally categorized into a handful of different frequency bands (δ, θ, α, β, γ; c.f. Schomer and Lopes da Silva, 2011). This implies that EEG frequencies do not represent an unstructured continuum. But what could be the reasons for that? One obvious reason is that frequency bands such as e.g., θ and α exhibit a clear task and event related behavior (Klimesch, 1999, 2012; Buzsaki, 2006). But here the emphasis is on a formal aspect, which is to avoid unwanted “spurious” phase synchronization. If the numerical ratio between two frequencies (f1, f2; f1 < f2) is harmonic (f2 = I ∗ f1; I = integer), the excitatory phases of the two frequencies can meet and synchronize according to a strict and regular pattern. This is of great advantage when phase coupling between frequencies is an important aspect of neuronal communication. If the ratio differs from a harmonic, spurious (unwanted) phase synchronization will appear in an uncontrolled way. Pletzer et al. (2010) have shown mathematically that the golden mean (g = 1.618 . . ..) is the best possible ratio to avoid spurious phase synchronization (see also Roopun et al., 2008). These aspects of phase synchronization can be summarized by two assumptions. (a) The center frequency of each EEG band is harmonically related to those of neighboring bands. A good estimate for δ, θ, α, β and γ is 2.5, 5, 10, 20, and 40Hz. (b) The width of a band is defined on the basis of the “golden mean role” (Klimesch, 2012; for an illustration, see Figure 1A left panel) to guarantee minimal interference between bands. EEG center frequencies which have these properties are termed frequency domains in the following. When considering the numerical relation between δ, θ, α, γ as described by assumption (a) it can easily be recognized that each center frequency is twice as high than its (lower) neighbor. An exponential function with base 2 defines that property.

High resolution X-ray emission spectroscopy of water and its assignment based on two structural motifs

We evaluate proposed interpretations of previous X-ray emission spectroscopy (XES) data on liquid water. The split peak in the lone-pair orbital region has been interpreted in terms of either two different structural motifs, tetrahedral and distorted, or as due to core-hole-induced dissociative dynamics; here we add new data on a 1:1 H 2O/D 2O isotopic mixture and additional spectrum simulations including the core-hole-induced dynamics. The XES spectrum of HDO is quite nicely reproduced as the sum of spectra of H 2O and D 2O, which we interpret as that core-hole-induced dynamics contribute only to the peak shape and do not affect the intensity ratio between tetrahedrally coordinated and distorted. We find the simulation-based interpretation of the two lone-pair peaks as being of completely different symmetries, molecular 1b 1 and dissociated 3a 1, difficult to reconcile with the experimental intensities in the 1b 2 and 3a 1 spectral regions. We report extensive theoretical simulations of spectra probing both the distance and velocity quantum distributions of the internal OH stretch; sharp features not associated with the lone-pair, that are seen when the OH stretch is treated as a classical oscillator, become smeared out when the zero-point Franck–Condon profile and momentum distribution in the v = 0 level of the OH stretch are taken into account. This demonstrates that neglecting zero-point motion in simulating XES spectra of water generates artificially sharp structures. XES spectra of 1 M and 4 M hydrochloric acid (HCl) and sodium hydroxide (NaOH) are reported. These spectra indicate that dissociated species most likely can be excluded as the origin of the double 1b 1 peak structure. We thus argue that the experimental observation of two distinct peaks in the lone-pair region is less likely to be explained by an unstructured continuum model of the liquid, but is easily explained within a two-component fluctuating model.

Single and double ionization of strontium in the vicinity of four-photon excitation of the 5p2 1S0 doubly excited state

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 1011 W cm−2 and within the 710–740 nm wavelength range. The Sr+ spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p2 1S0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr2+ ion. A weakly evident but reproducible inflection point (‘knee’ structure) appears in the intensity dependence of the Sr2+ yield at the location of the 5p2 1S0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p1/2,3/2, 6s1/2 and 5d3/2,5/2 excited Sr+ states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr+ spectrum. The population accumulation in the 5p1/2,3/2 ionic states unambiguously proves the absorption of two photons above the first 5s1/2 atomic threshold while that of the 5d3/2,5/2 and 6s1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

Short-range repulsion in the D 10 +( 1Σ +)-state potential of the CdRG (RG = Ar, Kr) molecules determined from a direct continuum ← bound excitation detected at the D 10 + ← X 10 +( 1Σ +) transition

The supersonic beam method combined with techniques of laser spectroscopy has been applied to determine the repulsive wall of the D 10 +( 1Σ +) excited-state potential of CdAr and CdKr molecules. The molecules were produced in a continuous supersonic-expansion beam and excited with a dye-laser beam directly from the X 10 +( 1Σ +) to the excited state. Analysis of the laser-induced fluorescence of the unstructured continuum ← bound profiles, recorded for the first time in excitation at the D 10 + ← X 10 + transition, yielded information on the short range D 10 +-state potential of the molecules.

Population trapping and laser-induced continuum structure in helium: Experiment and theory

We report the observation of laser-induced structure (LICS) in the flat photoionization continuum of helium. The structure is a strong and spectrally sharp resonance, showing both enhanced and diminished ionization (a Beutler-Fano profile). Our observations show, in an otherwise unstructured continuum, pronounced ionization suppression (as much as 70{percent}), which is due to population trapping associated with LICS. We also show the effect of dynamic Stark shifts upon the LICS profile. A discussed theoretical model, incorporating significant dynamic Stark shifts, gives a quantitatively accurate description of the line profile for a wide range of pulse intensities. {copyright} {ital 1998} {ital The American Physical Society}

Fluorescence excitation and depletion spectroscopy of the BAr complex: Electronic states correlating with the excited valence B(2s2p2 2D) asymptote

Laser fluorescence excitation and depletion spectra of the BAr van der Waals complex, in the spectral region around the B atomic 2s2p2 2D←2s22p 2P transition at 208.9 nm, are reported. The fluorescence excitation spectrum displays a series of structured bands, whose excited levels are weakly bound (⩽60 cm−1), and an unstructured continuum to the blue. The onset of the latter allows determination of the ground state dissociation energy D0″=102.4±0.3 cm−1. The fluorescence depletion spectrum reveals, in addition, bands at lower wave numbers to a strongly bound, predissociating state, assigned as C2Δ. The bands observed by fluorescence excitation are assigned to the D2Π, E2Σ+–X2Π transitions. The derived spectroscopic data are used to construct potential energy curves for the C2Δ and D2Π states. The predissociation of the C2Δ state is due to spin–orbit coupling to a repulsive 4Π state correlating with the B(2s2p2 4P)+Ar asymptote, as shown in the accompanying paper [K. Sohlberg and D. R. Yarkony, J. Chem. Phys. 106, 6607 (1997)]. A weak nonradiative decay process is also observed for the D2Π state.

Absorption spectra of size-selected solvated metal cations: Electronic states, symmetries, and orbitals in Sr+(NH3)1,2 and Sr+(H2O)1,2

This paper presents photodissociation spectra for the solvated metal ion clusters Sr+(NH3)1,2 and Sr+(H2O)1,2 from 420 to 740 nm in the visible region of the spectrum. The spectra have a banded structure, corresponding to transitions from ground electronic states based upon the 2S configuration of the Sr+ ion to excited states based primarily upon p-orbitals of the excited Sr+. The photodissociation cross sections are large, ∼10−17–10−16 cm2. For the same solvent, spectral band positions are only weakly dependent upon the degree of solvation. We show that a dramatic reduction in intensity of the second excited state band in the Sr+(NH3)2 spectrum suggests that this state has strong atomic ion d-orbital parentage and that the molecule is centrosymmetric. Photodissociation of the H2O solvated species proceeds through three excited electronic states corresponding principally to three different orientations of the metal p orbitals with respect to solvent symmetry axis. Absorption band positions for Sr+(H2O)2 are shifted slightly from those of Sr+(H2O) and the presence of a substantial unstructured continuum appears in the doubly solvated ion. The absorption spectra for the Sr+(H2O)1,2 species are significantly blue-shifted and narrowed relative to those of Sr+(NH3)1,2, an observation that is understood through simple molecular orbital diagrams incorporating the fact that the ionization potential of H2O is 2.4 eV larger than that of NH3.

Experimental observation of angular correlation between Rydberg electrons in the Ba n'l'nl autoionizing series.

We have observed the Ba n'p 1/2 np and n'p 1/2 nd double Rydberg series (n>60, n' 0.75n the Rydberg structure that is observed for the lower n' levels disappears into an apparently unstructured continuum

The production of electronically-excited species from the photolysis of N2H4and N2D4at 193 nm

Emission in the spectral region 300–900 nm has been observed following 193 nm excimer laser photolysis of N2H4 and N2D4. Electronically-excited NH(A) and ND(A) are formed by a two photon process with rotational and vibrational distributions that can be characterized by a single temperature of ∼ 4000 K and ∼ 3800 K, respectively. The visible spectrum in the region 420–900 nm shows a broad and largely unstructured continuum emission upon which is superimposed a large number of well-resolved spectral features corresponding to NH2(Ã) (or ND2(Ã)). This emission is formed by a single photon process that is essentially synchronous with the laser pulse and it is suggested that NH*2 is formed directly by cleavage of the N-N bond in hydrazine. The visible emission is found to show two lifetimes; one of ∼ 25 μs and the other in excess of 100 μs, depending whether the measurement is performed on discrete spectral features or on the total emission. It is suggested that the longer component is associated with the continuum emission and possibly results from a Renner-Teller coupling of the excited NH2(Ã, 2 A 1) state with high vibrational levels of the ground state, NH2([Xtilde], 2 B 1). Experiments with various buffer gases show that for the high vibrational levels of the NH*2 populated, vibrational relaxation of the NH*2 takes place on a timescale that is comparable to electronic quenching.

Laser induced bound–bound and bound–continuum emission of the Sr2 A 1Σ+u–X 1Σ+g system

Laser-induced emission of the Sr2 molecule consisting of discrete molecular fluorescence series, regularly modulated continuous fluorescence and an unstructured continuum was observed in the gas phase at excitation wavelengths between 600 and 450 nm. The analysis of the molecular line spectra provides Dunham coefficients and RKR potentials for the X 1Σ+g ground state and the excited A 1Σ+u state. The main spectroscopic constants are ωe=40.32 cm−1, Re=4.45 Å and De=1060 cm−1 for the X state and ωe=85.07 cm−1, Re=3.95 Å and De=5400 cm−1 for the A state. The modulated continuum which is due to bound-continuum transitions from a specific exited level v′, j′ to the continuum of the ground state was used to determine the repulsive part of the ground state potential up to 3000 cm−1 above the dissociation limit. Calculated discrete and continuous Franck–Condon factors are in good agreement with the experiment, both depend sensitively on the rotational quantum number J. The unstructured continuum can be interpreted as a quasicontinuous superposition of many fluorescence spectra. Calculations show that laser excitations from levels v″, J″ slightly below the dissociation limit and excitation from continuous states lead to the observed continuum.

Laser-induced chemiluminescence of NaMg

An unstructured continuum emission around 670 nm was observed when Mg was added to an optically pumped heat pipe containing Na and K vapor, in good agreement with ab initio calculations of the NaMg potential energy curves. The corresponding excitation spectrum showed that the incident radiation was observed by NaK molecules (X → C transitions).