Neutral Transition Research Articles

Influence of temporal and spectral profiles of lasers on weld quality of titanium

The effect of interaction of lasers having different spectral and temporal profiles with titanium on the weld surface quality is investigated by high-speed imaging and optical emission spectroscopy at atmospheric pressure in argon environment. High-speed imaging of the plume reveals that plume is produced during the laser pulse. Decrease in plume size and expansion rate is observed for those lasers whose spectral profiles overlap with more number of neutral atomic titanium transitions (Ti I) owing to increasing rate of reabsorption of the laser pulse primarily by the neutral titanium atoms present in the plume. Darkness around the laser spot on the titanium surface observed due to deposition of the nanoparticles increases with decrease in size and expansion rate of the plume which indicates that reabsorption of the laser pulse is the main cause of dark colouration of the surface. The bright appearance of a part of the plume produced by one laser at which another laser whose spectral profile cover many Ti I transitions is passed confirms the reabsorption process. Clean appearance of the surface around the laser spot along with enhancement of the plume size and expansion rate is observed for lasers with customized spectral profiles which do not overlap with any Ti I transitions. Modulation of the temporal profile can moderately improve the plume expansion rate and reduce the darkness around the laser spot provided that the laser spectral profile does not overlap with many Ti I transitions, particularly, with those whose energy of lower levels is less than 3 eV.

Laser-induced fluorescence measurement of very slow neutral flows in a dusty plasma experiment

Laser-Induced Fluorescence (LIF) provides the temperature and flow velocity of a target species by direct measurement of its velocity distribution via Doppler shift. A LIF diagnostic has been developed at the Caltech water-ice dusty plasma experiment that uses an ultra-narrow tunable diode laser to pump the λvac = 696.735 nm argon neutral transition. A photomultiplier detects fluorescence emission at λvac = 772.633 nm. Signal to noise ratios in excess of 100 are achieved along with a high degree of reproducibility between measurements. A Labview program fully automates data collection throughout the three-dimensional plasma volume by controlling four stepper motors and recording measured data. The argon neutral temperature is measured to be slightly above room temperature. Challenges such as the lack of absolute calibration of diode lasers and wavelength drift due to slight changes in ambient room conditions are overcome to measure bulk neutral flow speeds on the order of 1-2 m/s with resolution on the order of 2/3 of a meter per second. High-speed video shows that introducing an argon flow to a cloud of ice grains causes the cloud of ice grains to move and change shape. Ice grain motion is analyzed and found to be in agreement with neutral LIF flow measurements. Surprisingly, when the flow ceases, the ice grain cloud reverts to its original location and shape.

The Properties of the Interstellar Medium of Galaxies across Time as Traced by the Neutral Atomic Carbon [C i

Abstract We report Atacama Large Millimeter Array observations of the neutral atomic carbon transitions [C i] and multiple CO lines in a sample of ∼30 main-sequence galaxies at , including novel information on [C i] and CO for 7 of such normal objects. We complement our observations with a collection of >200 galaxies with coverage of similar transitions, spanning the z = 0–4 redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [C i]/IR and [C i]/high-J upper (J upper = 7) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main-sequence galaxies. We further report constant / ratios across the galaxy populations and redshifts, suggesting that gas temperatures T exc traced by [C i] do not strongly vary. We find only a mild correlation with T dust and that, generally, T exc ≲ T dust. We fit the line ratios with classical photodissociation region models, retrieving consistently larger densities and intensities of the UV radiation fields in submillimeter galaxies than in main-sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submillimeter emission. While we confirm the success in reproducing the CO lines, we find systematically larger [C i] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [C i] emission on galactic scales. We release our data compilation to the community.

Magnetically controllable photon blockade under a weak quantum-dot–cavity coupling condition

Quantum control of photons has become an area of great interest in the development of quantum technology. Here we explore how an external magnetic field can be exploited to control the statistical properties of photons in a bimodal cavity quantum electrodynamics (CQED) system. Our CQED system is composed of a bimodal micropillar Fabry-P\'erot cavity containing a single $V$-level quantum dot (QD) involving fine-structure-splitting neutral exciton transitions under the effect of the applied magnetic field in the presence of an incident pump laser field driving one cavity mode. With the introduction of the external magnetic field, not only is the conversion of polarization characteristics of photons achieved, but the statistics properties of photons are well engineered and the switching between photon bunching and antibunching is realized in the weak-coupling regime of CQED. The superiority of our system is manifested in the following aspects. (i) Using experimental parameters, we can manipulate the statistical properties of photons by appropriately tuning the strength of the magnetic field under the weak-coupling CQED condition, accompanied by either enhanced antibunching or a transition from antibunching to strong bunching of the light and vice versa. So we can arrive at selective photon statistics. (ii) The strong photon antibunching effect, under nonresonant scenarios but without the need to match the resonant or near-resonant conditions for the cavity and QD, can appear. Furthermore, we find that the multifrequency photon blockade can be generated in the present system, which is different from previous studies about photon blockade only for specific optical detuning. (iii) The high photon occupations or transmissions, simultaneously accompanied by strong photon antibunching in the system, are beneficial to the correlation measurement in practical experiments. This process works with a high quality of photon antibunching and bunching over a wide range of parameters and has potential applications in on-chip quantum information processing. It is hoped that the proposed scheme provides a possibility for the generation of tunable single-photon sources or controllable photon quantum gates.

Deceptively cold dust in the massive starburst galaxy GN20 at z ∼ 4

We present new observations, carried out with IRAM NOEMA, of the atomic neutral carbon transitions [C I](3P1–3P0) at 492 GHz and [C I](3P2–3P1) at 809 GHz of GN20, a well-studied star-bursting galaxy at z = 4.05. The high luminosity line ratio [C I](3P2–3P1) /[C I](3P1–3P0) implies an excitation temperature of 48+14−9 K, which is significantly higher than the apparent dust temperature of Td = 33 ± 2 K (β = 1.9) derived under the common assumption of an optically thin far-infrared dust emission, but fully consistent with Td = 52 ± 5 K of a general opacity model where the optical depth (τ) reaches unity at a wavelength of λ0 = 170 ± 23 μm. Moreover, the general opacity solution returns a factor of ∼2× lower dust mass and, hence, a lower molecular gas mass for a fixed gas-to-dust ratio, than with the optically thin dust model. The derived properties of GN20 thus provide an appealing solution to the puzzling discovery of starbursts appearing colder than main-sequence galaxies above z > 2.5, in addition to a lower dust-to-stellar mass ratio that approaches the physical value predicted for starburst galaxies.

Quantum dynamics analysis of transition-state spectrum for the SH + H2S → H2S + SH reaction.

We present the results of quantum wave packet calculations analyzing the experimental transition-state spectrum for the SH + H2S hydrogen transfer reaction based on photodetachment of the H3S2- anion. We used a reduced-dimensionality model in which four normal-mode coordinates were considered for the dynamics of the neutral transition state. The four-dimensional potential energy surfaces for the anionic and neutral states were constructed using four different levels of theory, namely, MP2, B3LYP, CAM-B3LYP, and LC-BLYP, with the aug-cc-pVDZ basis set. The spectrum calculated using the scaled MP2 potential energy surface was in reasonable agreement with the experimental spectrum. The present theoretical study confirms that the vibrational progression observed experimentally is associated with the antisymmetric motion of the transferred hydrogen atom. We also found that the S-S stretching motion plays an important role in the transition-state dynamics.

Excited State Dynamics of a Self‐Doped Conjugated Polyelectrolyte

AbstractThe growing number of applications of doped organic semiconductors drives the development of highly conductive and stable materials. Lack of understanding about the formation and properties of mobile charges limits the ability to improve material design. Thus the largely unexplored photophysics of doped systems are addressed here to gain insights about the characteristics of doping‐induced polarons and their interactions with their surroundings. The study of the ultrafast optical processes in a self‐doped conjugated polyelectrolyte reveals that polarons not only affect their environment via Coulomb effects but also strongly couple electronically to nearby neutral sites. This is unambiguously demonstrated by the simultaneous depletion of both the neutral and polaronic transitions, as well as by correlated excited state dynamics, when either transition is targeted during ultrafast experiments. The results contrast with the conventional picture of localized intragap polaron states but agree with revised models for the optical transitions in doped organic materials, which predict a common ground level for polarons and neighboring neutral sites. Such delocalization of polarons into the frontier transport levels of their surroundings could enhance the electronic connectivity between doped and undoped sites, contributing to the formation of conductive charges.

Lattice calculation of the pion transition form factor with Nf=2+1 Wilson quarks

We present a lattice QCD calculation of the double-virtual neutral pion transition form factor, with the goal to cover the kinematic range relevant to hadronic light-by-light scattering in the muon $g-2$. Several improvements have been made compared to our previous work. First, we take into account the effects of the strange quark by using the $N_f=2+1$ CLS gauge ensembles. Secondly, we have implemented the on-shell $\mathcal{O}(a)$-improvement of the vector current to reduce the discretization effects associated with Wilson quarks. Finally, in order to have access to a wider range of photon virtualities, we have computed the transition form factor in a moving frame as well as in the pion rest-frame. After extrapolating the form factor to the continuum and to physical quark masses, we compare our results with phenomenology. We extract the normalization of the form factor with a precision of 3.5\% and confirm within our uncertainty previous somewhat conflicting estimates for a low-energy constant that appears in chiral perturbation theory for the decay $\pi^0 \to \gamma\gamma$ at NLO. With additional input from experiment and theory, we reproduce recent estimates for the decay width $\Gamma(\pi^0 \to \gamma\gamma)$. We also study the asymptotic large-$Q^2$ behavior of the transition form factor in the double-virtual case. Finally, we provide as our main result a more precise model-independent lattice estimate of the pion-pole contribution to hadronic light-by-light scattering in the muon $g-2$: $a_{\mu}^{\mathrm{HLbL}; \pi^0} = (59.7 \pm 3.6) \times 10^{-11}$. Using in addition the normalization of the form factor obtained by the PrimEx experiment, we get the lattice and data-driven estimate $a_{\mu}^{\mathrm{HLbL}; \pi^0} = (62.3 \pm 2.3) \times 10^{-11}$.

Implication of RD(*) anomalies on semileptonic decays of Σb and Ωb baryons

The flavor changing decays of heavy bottom quarks to the corresponding lighter quarks ($u$, $c$ and $s$) in various $B$-meson decays via charged current and neutral current semileptonic transitions have emerged as promising candidates to explore the physics beyond the standard model. Experimentally the lepton flavor universality violation in $b \to (c,u)\,l\,\nu$ and $b \to s\,l^+ l^-$ transitions have been reported to a higher precision. The measurements of the lepton flavor violating ratios such as $R_{D^{(*)}}$, $R_{J/\Psi}$ and $R_{K^{(*)}}$ are observed to deviate from the standard model expectations at the level of $1.4\sigma$, $2.5\sigma$, $1.5\sigma$, $2.4\sigma$ and $2.2\sigma$ respectively. Motivated by these anomalies, we investigate the lepton flavor universality violation in $\Sigma_b \to \Sigma_c l \nu$ and $\Omega_b \to \Omega_c l \nu$ decays. We follow a model independent effective field theory formalism and study the implications of $R_{D^{(*)}}$ anomalies on $\Sigma_b \to \Sigma_c \tau \nu$ and $\Omega_b \to \Omega_c \tau \nu$ decay modes. We give predictions of various physical observables such as the ratio of branching ratios, total differential decay rate, forward-backward asymmetry, lepton side polarization fraction and convexity parameter within the standard model and within various new physics scenarios.

Neutral carbon and highly excited CO in a massive star-forming main sequence galaxy at z = 2.2

We used the Plateau De Bure Interferometer to observe multiple CO and neutral carbon transitions in a z = 2.2 main sequence disk galaxy, BX610. Our observation of CO(7-6), CO(4-3), and both far-infrared (FIR) [CI] lines complements previous observations of Hα and low-J CO, and reveals a galaxy that is vigorously forming stars with UV fields (Log(GG0−1) ≲ 3.25); although less vigorously than local ultra-luminous infrared galaxies or most starbursting submillimeter galaxies in the early universe. Our observations allow new independent estimates of the cold gas mass which indicate Mgas ∼ 2 × 1011 M⊙, and suggest a modestly larger αCO value of ∼8.2. The corresponding gas depletion timescale is ∼1.5 Gyr. In addition to gas of modest density (Log(n cm3) ≲ 3) heated by star formation, BX610 shows evidence for a significant second gas component responsible for the strong high-J CO emission. This second component might either be a high-density molecular gas component heated by star formation in a typical photodissociation region, or could be molecular gas excited by low-velocity C shocks. The CO(7-6)-to-FIR luminosity ratio we observe is significantly higher than typical star-forming galaxies and suggests that CO(7-6) is not a reliable star-formation tracer in this galaxy.

Charge neutral crystal field transitions: A measure of electron–phonon interaction

We report a study on the charge neutral crystal field (d–d) transitions and their possible correlation with coupling between continuum states above the optical band gap in pure and irradiated PrFeO3. For this purpose, room temperature optical absorption spectroscopy and Raman spectroscopy measurements were carried out. It was observed that width (Wd–d) and relative intensity (ID) of d–d transition scaled with the extent of disorder created by laser irradiation. The consistent broadening of the d–d transition with irradiation indicates enhanced coupling between crystal field excitations and continuum states above the gap. These results are consistent with observed asymmetry in Raman modes and the Gaussian-like nature of excitonic spectral line shapes of optical absorption spectra with laser irradiation. Thus, we demonstrated a new way to probe the electron–exciton interactions in terms of crystal field transitions and excitonic line shapes in highly correlated systems. Additionally, the present work provides crucial information regarding lattice disordering in polycrystalline PrFeO3 with irradiation and may have significant implications for caution in using this material in extremely high radiation environments at room temperature. The results are further supported with a first-principle calculation carried out using density functional theory.

Search for Lepton Flavor Non-universality with B→ (K, K*) τ+τ- Decays

The decay modes of B meson consisting tau leptons as final state particle are seemed to be the key feature for searching new physics. As the experimental measurements for b → sτ+τ− processes are still not confirmed, the theoretical study of these decays is playing vital role in recent times. The lepton flavor universality violation gains much curiosity after the observation of RK and $$ {R}_{K^{\ast }} $$ anomalies in semileptonic $$ B\to \left(K,{K}^{\ast}\right)\mu \overline{\mu} $$ channel which provides a hint towards μ − e non-universality in flavor sector. In this paper, we have devoted our concern to the decay mode B → (K, K∗)τ+τ− to predict the branching ratio. We have also searched for the τ − l (where l = e, μ) lepton flavor non-universality for neutral current transitions. We have estimated our results in the standard model as well as in Z′ model.

Polarimetry of photon echo on charged and neutral excitons in semiconductor quantum wells

Coherent optical spectroscopy such as four-wave mixing and photon echo generation deliver rich information on the energy levels involved in optical transitions through the analysis of polarization of the coherent response. In semiconductors, it can be applied to distinguish between different exciton complexes, which is a highly non-trivial problem in optical spectroscopy. We develop a simple approach based on photon echo polarimetry, in which polar plots of the photon echo amplitude are measured as function of the angle φ between the linear polarizations of the two exciting pulses. The rosette-like polar plots reveal a distinct difference between the neutral and charged exciton (trion) optical transitions in semiconductor nanostructures. We demonstrate this experimentally by photon echo polarimetry of a CdTe/(Cd, Mg)Te quantum well. The echoes of the trion and donor-bound exciton are linearly polarized at the angle 2φ with respect to the first pulse polarization and their amplitudes are weakly dependent on φ. While on the exciton the photon echo is co-polarized with the second exciting pulse and its amplitude scales as cosφ.

Quantitative atomic spectroscopy, a review of progress in the optical-UV region and future opportunities

The development of tunable dye lasers and a simple atomic and ionic beam source for all elements were critical in establishing a reliable absolute scale for atomic transition probabilities in the optical to near UV regions. The laboratory astrophysics program at the University of Wisconsin - Madison (UW) concentrates on neutral and singly-ionized species transitions that are observable in astronomical spectra of cool stars, emphasizing the rare earth n(eutron)-capture elements and the Fe-group elements that are important inputs to early Galactic nucleosynthesis studies. The UW program is one of several productive efforts on atomic transition probabilities. These programs generally use time-resolved laser-induced-fluorescence (TR-LIF) to accurately measure total decay rates and data from high resolution Fourier transform spectrometers (FTSs) to determine emission branching fractions (BFs). The UW laboratory results almost always are directly linked to astronomical chemical composition efforts. There are good opportunities to extend similar research to other wavelength regions.

IrF8 Molecular Crystal under High Pressure.

An important goal in chemistry is to prepare F-rich transition metal fluorides due to the high oxidation states and potential applications such as oxidating and fluorinating agents. Thus far, the highest F stoichiometry in the neutral transition metal fluorides is 7. Here, we identify a hitherto unknown IrF8 compound through first-principles swarm-intelligence structure search calculations under high pressure. The three identified IrF8 phases exhibit typical molecular crystal characters, showing +8 oxidation state in Ir. The spatial symmetry of the basic building block in the three IrF8 phases gradually increases with pressure (e.g., dodecahedron [Formula: see text] square antiprism [Formula: see text] quasicube). The pressure-induced faster increase of Ir 5d orbital energy level with respect to F 2p provides a strong charge transfer driving force from Ir 5d to F 2p, facilitating the formation of F-rich compounds. More interestingly, the predicted electron affinities of the three predicted IrF8 phases are comparable/larger than that of PtF6, the strongest oxidation agent in the third row transition metal hexafluorides. The built high-pressure phase diagram of Ir-F binary compounds provides useful information for experimental synthesis.

Recent results on Kaon Physics at KLOE-2

KLOE-2 extends the physics program of the forerunner KLOE experiment, especially in the field of discrete symmetries tests with neutral kaons. KLOE and KLOE-2 have collected together the largest sample of electron-positron collisions at an energy equal to the φ-meson mass, corresponding to about 2.4×1010 produced φ mesons. The latest results on neutral kaon physics at KLOE will be reviewed, together with the status and prospects of the analyses of KLOE-2 data. A new measurement of the charge asymmetry in KS semileptonic decays with 1.7 fb−1 of KLOE data, which improves the sensitivity of previous measurements of about a factor two, will be presented. Furthermore, the status of the analysis devoted to directly test T and CPT symmetries in neutral kaons transitions, as well as the search of the pure CP-violating KS → 3π0 decay using part of the recently acquired KLOE-2 dataset, will be presented.

The metallo-formate anions, M(CO2)-, M = Ni, Pd, Pt, formed by electron-induced CO2 activation.

The metallo-formate anions, M(CO2)-, M = Ni, Pd, and Pt, were formed by electron-induced CO2 activation. They were generated by laser vaporization and characterized by a combination of mass spectrometry, anion photoelectron spectroscopy, and theoretical calculations. While neutral transition metal atoms are normally unable to activate CO2, the addition of an excess electron to these systems led to the formation of chemisorbed anionic complexes. These are covalently bound, formate-like anions, in which their CO2 moieties are significantly reduced. In addition, we also found evidence for an unexpectedly attractive interaction between neutral Pd atoms and CO2.

Molecular gas and star formation in an absorption-selected galaxy: Hitting the bull’s eye atz≃ 2.46

We present the detection and detailed analysis of a diffuse molecular cloud atzabs= 2.4636 towards the quasar SDSS J 1513+0352 (zem≃ 2.68) observed with the X-shooter spectrograph at the Very Large Telescope. We measured very high column densities of atomic and molecular hydrogen with logN(H I, H2) ≃ 21.8, 21.3. This is the highest H2column density ever measured in an intervening damped Lyman-αsystem but we did not detect CO, implying logN(CO)/N(H2) < −7.8, which could be due to a low metallicity of the cloud. From the metal absorption lines, we derived the metallicity to beZ≃ 0.15Z⊙and determined the amount of dust by measuring the induced extinction of the background quasar light,AV≃ 0.4. We simultaneously detected Lyman-αemission at the same redshift with a centroid located at a most probable impact parameter of onlyρ≃ 1.4 kpc. We argue that the line of sight is therefore likely passing through the interstellar medium (ISM), as opposed to the circumgalactic medium (CGM), of a galaxy. The relation between the surface density of gas and that of star formation seems to follow the global empirical relation derived in the nearby Universe although our constraints on the star formation rate (SFR) and the galaxy extent remain too loose to be conclusive. We study the transition from atomic to molecular hydrogen using a theoretical description based on the microphysics of molecular hydrogen. We use the derived chemical properties of the cloud and physical conditions (Tk≃ 90 K andn≃ 250 cm−3) derived through the excitation of H2rotational levels and neutral carbon fine structure transitions to constrain the fundamental parameters that govern this transition. By comparing the theoretical and observed H Icolumn densities, we are able to bring an independent constraint on the incident ultra-violet (UV) flux, which we find to be in agreement with that estimated from the observed SFR.

Energy spectra and the expectation values of diatomic molecules confined by the shifted Deng-Fan potential

The approximate bound state solutions of the Schrodinger equation with the shifted Deng-Fan potential was obtained via a proper quantization rule. The energy spectra for the homogenous diatomic molecules (H2, I2); the heterogeneous diatomic molecules (CO, HCl, LiH); the neutral transition metal hydrides (ScH, TiH, VH, CrH); the transition-metal lithide (CuLi); the transition-metal carbides (TiC, NiC); the transition metal nitrite (ScN) and the transition metal fluoride (ScF) were calculated. By applying the Hellmann-Feynman theorem, the expression for the expectation values of the square of inverse of position r-2, potential energy V, kinetic energy T and the square of momentum p2 were derived and the numerical values for diatomic molecules were presented. The result is reliable and very consistent with the available ones in the literature.

Seasonal variability of copepod community structure and abundance modified by the El Niño-La Niña transition (2010) in the tropical Pacific off central Mexico

Seasonal variability of the community structure of copepods in the Eastern tropical Pacific off central Mexico was studied during three distinct hydrodynamic periods in 2010 using statistical and multivariate analyses. The survey period included the second half of the 2009-2010 El Niño (January), the neutral transition period (May-June), and the first half of the 2010-2011 La Niña (October). Seventy-eight copepod species were identified; richness ranged from 11 to 47 species per station, with seasonal averages from 25 species in May to 35 species in January. Cluster analysis indicated that there were four principal groups present across the surveyed periods, defined by January (El Niño), October (La Niña), May offshore stations, and May upwelling stations (cyclonic eddy and coastal stations). There were no significant differences in abundance, but the January (El Niño) cluster was most diverse with 32 species, May offshore and October (La Niña) clusters each had 25 species, and the May upwelling was the least diverse cluster with 18 species. Mesoscale processes were strongest during May, which was the only period with a significant inshore-offshore gradient of species richness and diversity. Canonical correspondence analysis (CCA) revealed that variability was primarily driven by subsurface (75-200 m) ammonium, and surface (0-50 m) temperature, nitrates+nitrites, salinity and phosphorus. Copepodites and adults of the primarily herbivorous Eucalanidae dominated the stations of the upwelling cluster, while copepodites and adults of the carnivorous Euchaetidae dominated the January (El Niño) station cluster. The higher Chl a levels during the less productive (reduced upwelling) El Niño period were probably due to reduced grazing activities and increased ammonium availability through increased zooplankton metabolism. The horizontal distribution of copepods in the Eastern Tropical Pacific off Mexico appears to be principally defined by mesoscale eddy processes (offshore) and upwelling (coastal). These mesoscale processes were affected by El Niño - La Niña transitions, which subsequently disrupted the inshore-offshore gradient and in the case of El Niño likely caused reductions in copepod abundance across the entire region which persisted for the entire study period, and possibly longer.