Temperature-dependent bulk viscosity of nitrogen gas determined from spontaneous Rayleigh-Brillouin scattering

Values for the bulk viscosity of molecular nitrogen gas (N2) were derived from spontaneous Rayleigh-Brillouin (RB) scattering at ultraviolet wavelengths (366.8 nm) and at a 90 degree scattering angle. Analysis of the scattering profiles yield values showing a linear increasing trend in the temperature interval from 255 K to 340 K. The present values, pertaining to hypersound acoustics at frequencies in the GHz domain, are found to be in agreement with results from acoustic attenuation experiments in N2 performed at MHz frequencies

Cavity Ring-down UV spectroscopy of the C2Σ+^2\Sigma^+-X2Π^2\Pi electronic transition of CH

Rotationally resolved spectra of the C$^2\Sigma^+$-X$^2\Pi$ electronic system of the CH radical were measured using cavity ring-down spectroscopy in supersonically expanding, planar hydrocarbon plasma. The experimental conditions allowed the study of highly excited rotational levels starting from vibrationally excited states. Here we present some 200+ new or more accurately recorded transitions in the 0-0, 1-1 and 2-2 vibronic bands in the ultraviolet between 30900-32400 cm$^{-1}$ (324-309 nm). The resulting data, compared to earlier measurements, allows for the determination of more precise molecular constants for each vibrational state and therefore more precise equilibrium values. From this an equilibrium bond length of 1.115798(17) \r{A} for the C$^2{\Sigma}^+$ state is determined. A comprehensive list with observed transitions for each band has been compiled from all available experimental studies and constraints are placed on the predissociation lifetimes

Rayleigh-Brillouin scattering of carbon dioxide

The spectral lineshape of spontaneous Rayleigh-Brillouin scattering in CO2 is studied in a range of pressures. The spectrum is influenced by the bulk viscosity, which is a relaxation phenomenon involving the internal degrees of freedom of the molecule. The associated relaxation rates can be compared to the frequency shift of the scattered light, which demands precise measurements of the spectral lineshape. We find the value of the bulk viscosity around 5.7 X 10^{-6} kg/(ms) for the range of pressures p= 2-4 bar and for conditions of room temperature

Sensitivity of rotational transitions in CH and CD to a possible variation of fundamental constants

The sensitivity of rotational transitions in CH and CD to a possible variation of fundamental constants has been investigated. Largely enhanced sensitivity coefficients are found for specific transitions which are due to accidental degeneracies between the different fine-structure manifolds. These degeneracies occur when the spin-orbit coupling constant is close to four times the rotational constant. CH and particularly CD match this condition closely. Unfortunately, an analysis of the transition strengths shows that the same condition that leads to an enhanced sensitivity suppresses the transition strength, making these transitions too weak to be of relevance for testing the variation of fundamental constants over cosmological time scales. We propose a test in CH based on the comparison between the rotational transitions between the e and f components of the Omega'=1/2,J=1/2 and Omega'=3/2,J=3/2 levels at 532 and 536 GHz and other rotational or Lambda-doublet transitions in CH involving the same absorbing ground levels. Such a test, to be performed by radioastronomy of highly redshifted objects, is robust against systematic effects

Search For A Variation Of The Proton-electron Mass Ratio From Methanol Observations

A limit on a possible cosmological variation of the proton-to-electron mass ratio $\mu$ is derived from observation of methanol lines in the PKS1830-211 lensed galaxy at redshift $z \sim 0.89$ with the Effelsberg 100 m single-dish radio telescope (at frequencies 6.5 - 32 GHz), the IRAM 30 m telescope (at frequencies 80 - 160 GHz), and band-6 of the novel ALMA telescope array (at 260 GHz). Ten different absorption lines of CH$_3$OH are detected covering a wide range of sensitivity coefficients $K_{\mu}$. Systematic effects of chemical segregation, excitation temperature, frequency dependence, and time variability of the back- ground source are quantified. A robust constraint of $\Delta\mu/\mu = (1.0 \pm 0.8_{stat} \pm 1.0_{syst}) \times 10^{-7}$ is derived from this large sample of lines belonging to a single molecular species. Analysis of additional observations at the E-VLA radio telescope is under way

Extraction of the translational Eucken factor from light scattering by molecular gas

Although the thermal conductivity of molecular gases can be measured straightforwardly and accurately, it is difficult to experimentally determine its separate contributions from the translational and internal motions of gas molecules. Yet this information is critical in rarefied gas dynamics as the rarefaction effects corresponding to these motions are different. In this paper, we propose a novel methodology to extract the translational thermal conductivity (or equivalently, the translational Eucken factor) of molecular gases from the Rayleigh-Brillouin scattering (RBS) experimental data. From the numerical simulation of the \cite{LeiJFM2015} model we find that, in the kinetic regime, in addition to bulk viscosity, the RBS spectrum is sensitive to the translational Eucken factor, even when the total thermal conductivity is fixed. Thus it is not only possible to extract the bulk viscosity, but also the translational Eucken factor of molecular gases from RBS light scattering spectra measurements. Such experiments bear the additional advantage that gas-surface interactions do not affect the measurements. For the first time, bulk viscosities (due to the rotational relaxation of gas molecules only) and translational Eucken factors of N2, CO2 and SF6 are simultaneously extracted from RBS experiments

High-order harmonic generation yielding tunable extreme-ultraviolet radiation of high spectral purity

Production of extreme-ultraviolet radiation by high-order harmonic generation is demonstrated to yield unprecedented spectral purity of λ/Δλ=2.5×105 at wavelengths covering the entire range 40–100 nm. Tunability and sub-cm-1 bandwidth of the harmonics are demonstrated in recordings of the He (1s4p) and Ar (3p53d′) resonance lines at 52.2 and 86.6 nm. Frequency shift of the harmonics due to chirp-induced phenomena are investigated and found to be small, resulting in a frequency accuracy of about 5×10-7 in the domain of extreme-ultraviolet radiation