Current Theoretical Description Research Articles

Do Molecular Geometries Change Under Vibrational Strong Coupling?

As pioneering experiments have shown, strong coupling between molecular vibrations and light modes in an optical cavity can significantly alter molecular properties and even affect chemical reactivity. However, the current theoretical description is limited and far from complete. To explore the origin of this exciting observation, we investigate how the molecular structure changes under strong light-matter coupling using an ab initio method based on the cavity Born-Oppenheimer Hartree-Fock ansatz. By optimizing H2O and H2O2 resonantly coupled to cavity modes, we study the importance of reorientation and geometric relaxation. In addition, we show that the inclusion of one or two cavity modes can change the observed results. On the basis of our findings, we derive a simple concept to estimate the effect of the cavity interaction on the molecular geometry using the molecular polarizability and the dipole moments.

Eclipse timing variation of GK Vir: evidence of a possible Jupiter-like planet in a circumbinary orbit

ABSTRACT Eclipse timing variation analysis has become a powerful method to discover planets around binary systems. We applied this technique to investigate the eclipse times of GK Vir. This system is a post-common envelope binary with an orbital period of 8.26 h. Here, we present 10 new eclipse times obtained between 2013 and 2020. We calculated the O−C diagram using a linear ephemeris and verified a clear orbital period variation (OPV) with a cyclic behaviour. We investigated if this variation could be explained by the Applegate mechanism, the apsidal motion, or the light travel time (LTT) effect. We found that the Applegate mechanism would hardly explain the OPV with its current theoretical description. We obtained using different approaches that the apsidal motion is a less likely explanation than the LTT effect. We showed that the LTT effect with one circumbinary body is the most likely cause for the OPV, which was reinforced by the orbital stability of the third body. The LTT best solution provided an orbital period of ∼24 yr for the outer body. Under the assumption of coplanarity between the external body and the inner binary, we obtained a Jupiter-like planet around the GK Vir. In this scenario, the planet has one of the longest orbital periods, with a full observational baseline, discovered so far. However, as the observational baseline of GK Vir is smaller than twice the period found in the O−C diagram, the LTT solution must be taken as preliminary.

New Experiments for Improved Theoretical Description of Nonlinear Rheology of Entangled Polymers

The present work discusses four types of new experiments that can improve the current theoretical description of nonlinear rheology of entangled polymers. First, a slowly imposed strain is found to result in nonmonotonic evolution of the state of chain entanglement during quiescent relaxation, consistent with the idea of chain disentanglement after step shear. Second, the stress relaxation upon a sizable step strain is found to be identical to that for small step strain, consistent with a molecular scenario that a strained entangled melt has an entropic barrier to resist chain retraction. Third, the ability of a step-strained polymer to undergo elastic recovery is found to be the same up to strain amplitude of unity, and a sample sheared for a period much longer than the Rouse time is shown to still undergo nearly full elastic recovery. Fourth, an entangled melt, stretched at a rate significantly lower than the Rouse relaxation rate, undergoes full elastic recovery until the point of tensile force maximum. We have discussed an alternative conceptual framework to describe these nonlinear responses of entangled polymers despite the possibility that the tube model might be further remedied to characterize the new rheometric measurements presented in this work.

Predicting surface anchoring: molecular organization across a thin film of 5CB liquid crystal on silicon

Anchoring, i.e. the orientation and ordering of organic molecules at a surface and its persistence away from the interface, is a key phenomenon for liquid crystal (LC) physics and technology, as well as for organic transistors and photovoltaic cells where it strongly affects transport. Recent exciting experiments have started to explore anchoring at the nanoscale, but seemingly conflicting views are emerging, suggesting either a surface order very slowly decreasing with distance, or the expected exponential decay for adhesion energies. The current theoretical description of anchoring is, however, essentially empirical, and methods for predicting the molecular structuring and orientation are conspicuously lacking. Here we establish such predictive tools by using atomistic molecular dynamics simulations and obtain the order and molecular organization of 12 and 24 nm-thick nematic and isotropic films of the popular LC 4-n-pentyl-4-cyano biphenyl (5CB) at its interfaces with an atomically flat, H-terminated, (001) crystalline silicon surface and vacuum. We show that the film adopts a hybrid configuration, with a change of the 5CB preferred orientation from planar uniform at the silicon to perpendicular at the free surface. This variation is nearly discontinuous for the thin film, and continuous with twist and bend for the thicker one. Our findings provide for the first time a full description of the interfacial silicon-LC structure at the nanoscale level.

The FeH F$^{4}\Delta$–X$^{4}\Delta$ system

Context. Lines of diatomic molecules are ideal tools for studying cool stellar atmospheres and the internal structure of sunspots and starspots, given their temperature and pressure sensitivities, which are typically higher than in atomic lines. The Wing-Ford FeH F 4 Δ-X 4 Δ system represents such a diatomic molecule that is, in addition, highly sensitive to magnetic fields. The current theoretical description of those transitions that include the involved molecular constants, however, are only based on intensity measurements because polarimetric observations have not been available until now, which limits their diagnostic value. Furthermore, the theory has so far been optimized to reproduce energy levels and line strengths without taking magnetic sensitivities into account. Aims. The FeH F 4 Δ-X 4 Δ system is produced by transitions between two electronic states with the coupling of the angular momenta that is intermediate between limiting Hund's cases (a) and (b). Our goal is to investigate the diagnostic capabilities of the current theoretical description of the molecule FeH. Methods. Using the most precise available Hamiltonian, we carried out the perturbation calculation of the molecular Zeeman effect for this transition and computed the Lande factors of the energy levels and of transitions. We extracted Lande factors from a comparison of observed and calculated Stokes I and V profiles. Certain spectral lines, most frequently with high magnetic sensitivity, exhibited discrepancies between the theory and observations. We extended the theoretical model with a semi-empirical approach to obtain a diagnostic tool that is able to reproduce many of the interesting spectral lines. Results. We find that the current theory successfully reproduces the magnetic properties of a large number of lines in the FeH F 4 Δ-X 4 Δ system and that the modified Hamiltonian allows us to synthesize and successfully reproduce the most sensitive lines. Thus, our observations have provided valuable constraints for determining empirical molecular constants and Lande factors. Conclusions. The FeH F 4 Δ-X 4 Δ system is found to be a very sensitive magnetic diagnostic tool. Polarimetric data of these lines, in contrast to intensity measurements, provide us with more direct and detailed information to study the coolest parts of sunspot and starspot umbrae, as well as cool active dwarfs.

Intrinsic Properties of the Magnetically Collimated H2O Maser Jet of W43A

Water maser polarization observations in the precessing jet of W43A have revealed that it is magnetically collimated. Here we present a detailed description of the physical properties of the water maser environment in the jet. We discuss the maser saturation level and beaming angle as well as the intrinsic temperatures and densities. Additionally, we show that the polarization angle of the strongest red-shifted maser feature undergoes a fast rotation of 90 degrees across the maser. Along with the variation of linear polarization fraction, this strongly supports the current theoretical description of maser linear polarization.

Unravelling the Peculiar Nucleation Mechanisms for Non-Ideal Binary Mixtures with Atomistic Simulations

Recent experiments reveal unusual nucleation behavior for seemingly simple mixtures that cannot be described by the classical theory. Molecular simulations using a combination of aggregation-volume-bias Monte Carlo, umbrella sampling, and histogram reweighting methods were carried out to study the nucleation events involved in the water/ethanol, water/n-nonane, and n-nonane/ethanol mixtures. These simulations reproduced their different nonideal behaviors observed by the experiments. Furthermore, the finding of their strikingly distinct mechanisms, as implied from the calculated free-energy maps, challenges the current theoretical description of this phenomenon.

Current Status of the Cosmic Microwave Background Radiation

My goal is to summarize our understanding of the cosmic microwave background radiation (CMBR) at this interesting moment after the detection of fluctuations in the background and before the next generation of satellite experiments. I begin by listing recent reviews and papers on the spectrum of the CMBR. I then sketch the current theoretical description of the power spectrum of fluctuations in the CMBR. Astronomical foregrounds and the nature of secondary fluctuations are treated next. Then I turn to observations, with special emphasis on the final results of the COBE–DMR experiment, on the growing evidence for ΔT/T = 2–3 × 10−5 fluctuations at degree scales, and on what they tell us about cosmological parameters.

Line strength in 1Σ + → 3Σ − emissions of some diatomic radicals

Line strengths in O- and S-form rotational branches of the NF (b 1Σ + → X 3Σ −) emission have been obtained. They show widely different behaviours from those of PH and NH, and new questions are posed out of the current theoretical description. A few suggestions result from a correlation between trends of the observed intensity curves with those of some particular terms in the fine structure expressions of the triplet.

Studies of Multichannel Photothermal Refraction for the Measurement of Absolute Absorptivity

Multichannel crossed-beam photothermal refraction experiments have been performed. Iron(II)-phenanthroline in methanol was the test solute. The response as a function of concentration exhibited a nonlinear relationship, which does not agree with the current theoretical description of the phenomena. Although photophysical and photochemical processes were not totally eliminated as possible causes of the nonlinear concentration response, arguments are presented which indicate, for the experimental conditions employed in this study, that these processes are negligible. It is proposed that the heat transfer model must be modified to include refractive index gradients in both the radial and axial coordinates of the pump laser beam.

X-ray study of the nematic phase and smectic-A1-to-smectic-Ã phase transition in heptylphenyl nitrobenzoloxybenzoate (DB7NO2).

We report a high-resolution x-ray study of the fluctuations in the nematic and smectic-${A}_{1}$ phases in heptylphenyl nitrobenzoloxybenzoate. As a function of decreasing temperature, the fluctuations evolve from an incommensurate region, through an intermediate region of coexisting (competing) incommensurate and smectic-$\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{C}$ fluctuations, to a region dominated by pretransitional smectic-$\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}$ fluctuations which exhibit a phase-lockin $\mathrm{smectic}\ensuremath{-}{A}_{1}\ensuremath{-}\mathrm{smectic}\ensuremath{-}\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}$ transition. This behavior directly confirms the current theoretical description of the smectic-$\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{A}$ and smectic-$\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{C}$ phases as alternatives to the incommensurate phase.

A Review of Arcing Phenomena in Vacuum and in the Transition to Atmospheric Pressure Arcs

This paper reviews vacuum-arc phenomena, and the effect of low-pressure gaseous ambients on electrode phenomena in the transition to atmospheric pressure arcs. The 5 main areas addressed are cathode-spot phenomena, anode-spot phenomena, the properties of the interelectrode plasma for both diffuse arcs and columnar arcs, the interaction of vacuum arcs with axial and transverse magnetic fields, and finally, the transition to atmospheric pressure arcs. The current levels range from 50 A to 50 kA. For each of these 5 main areas, features of the vacuum arc which can be reasonably established from the literature are filrst described, followed by a discussion of parameters requiring additional experimental and theoretical study. For example, the current densities, microstructure, and theoretical description of the cathode spot remain the subject of much debate. There is also a need for additional experimental observations of the anode ion flux and ion energies in order to clarify the overall anode-spot mechanism. With respect to high-current columnar arcs, here there is uncertainty concerning the pressure in the arc column and the mechanism of the grossly evaporating cathode spot. It is firmly established that these high-current columnar arcs can be avoided by applying an axial magnetic field parallel to the arcing axis, but a detailed understanding of the magnetic field/arc interaction remains to be established. The review concludes with a discussion of experimental investigations of electrode phenomena in the presence of low ambient gas pressures.