arXiv subjects

W. Ubachs

Publications and source records attributed to W. Ubachs.

At least 73 records · Page 4Linked to original sources

arXiv2014

H$_2$ Lyman and Werner band lines and their sensitivity for a variation of the proton-electron mass ratio in the gravitational potential of white dwarfs

Recently an accurate analysis of absorption spectra of molecular hydrogen, observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, in the photosphere of white dwarf stars GD133 and GD29-38 was published in a Letter [Phys. Rev. Lett. 113, 123002 (2014)], yielding a constraint on a possible dependence of the proton-electron mass ratio on a gravitational field of strength 10,000 times that at the Earth's surface. In the present paper further details of that study are presented, in particular a re-evaluation of the spectrum of the $B^1Σ_u^+ - X^1Σ_g^+ (v',v'')$ Lyman bands relevant for the prevailing temperatures (12,000 - 14,000 K) of the photospheres. An emphasis is on the calculation of so-called $K_i$-coefficients, that represent the sensitivity of each individual line to a possible change in the proton-electron mass ratio. Such calculations were performed by semi-empirical methods and by ab initio methods providing accurate and consistent values. A full listing is provided for the molecular physics data on the Lyman bands (wavelengths $λ_i$, line oscillator strengths $f_i$, radiative damping rates $Γ_i$, and sensitivity coefficients $K_i$) as required for the analyses of H$_2$-spectra in hot dwarf stars. A similar listing of the molecular physics parameters for the $C^1Π_u - X^1Σ_g^+ (v',v'')$ Werner bands is provided for future use in the analysis of white dwarf spectra.

physics.atom-ph↗

arXiv2014

The inhomogeneous ISM toward PKS1830-211 SW - A detailed view on molecular gas at a look-back time of 7.5 Gyr

Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the south-western source of the gravitational lens system PKS 1830-211, which is by far the best known target to study molecular gas in absorption at intermediate redshift. Determining line parameters and optical depths and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to evaluate physical parameters of the absorbing foreground gas at z~0.89, in particular its homogeneity, and (2) to monitor the spectroscopic time variability caused by fluctuations of the z~2.5 background continuum source. We find, that the gas is quite inhomogeneous with n(H2)~2 x 10^3 cm^-3 for most molecular species but with higher values for H2CO and lower ones for SO. Measuring the CS J=1-0 transition during a time interval of more than a decade, from 2001 to 2012, the peak absorption depth of the line remains approximately constant, while the line shape undergoes notable variations. Covering the time between 1996 and 2013, CS, HCO+, and CH3OH data indicate maximal integrated optical depths in ~2001 and 2011/2012. This is compatible with a ~10 yr periodicity, which, however, needs confirmation by substantially longer time monitoring. Comparing molecular abundances with those of different types of Galactic and nearby extragalactic clouds we find that the observed cloud complex does not correspond to one particular type but to a variety of cloud types with more diffuse and denser components as can be expected for an observed region with a transverse linear scale of several parsec and a likely larger depth along the line-of-sight. A tentative detection of Galactic absorption in the c-C3H2 1(10)-1(01) line at 18.343 GHz is also reported.

astro-ph.GA↗

arXiv2014

Effect of soil temperature on one-way optical frequency transfer through dense-wavelength-division-multiplexing fibre links

Results of optical frequency transfer over a carrier-grade dense-wavelength-division-multiplexing (DWDM) optical fibre network are presented. The relation between soil temperature changes on a buried optical fibre and frequency changes of an optical carrier through the fibre is modelled. Soil temperatures, measured at various depths by the Royal Netherlands Meteorology Institute (KNMI) are compared with observed frequency variations through this model. A comparison of a nine-day record of optical frequency measurements through the 2 x 298 km fibre link with soil temperature data shows qualitative agreement. A soil temperature model is used to predict the link stability over longer periods (days-months-years). We show that one-way optical frequency dissemination is sufficiently stable to distribute and compare e.g. rubidium frequency standards over standard DWDM optical fibre networks.

physics.optics↗

arXiv2014

$B\,^1Σ^{+}_{u}$ and $EF\,^{1}Σ^{+}_{g}$ level energies of D$_{2}$

Accurate absolute level energies of the $B\,^1Σ^{+}_{u}$, $v=0-8, N$ and $EF\,^{1}Σ^{+}_{g}$, $v=0-21, N$ rovibrational quantum states of molecular deuterium are derived by combining results from a Doppler-free two-photon laser excitation study on several lines in the $EF\,{}^{1}Σ_{g}^{+}-X\,{}^{1}Σ_{g}^{+}$ (0,0) band, with results from a Fourier-transform spectroscopic emission study on a low-pressure hydrogen discharge. Level energy uncertainties as low as 0.0005 cm$^{-1}$ are obtained for some low-lying $E\,^{1}Σ^{+}_{g}$ inner-well rovibrational levels, while uncertainties for higher-lying rovibrational levels and those of the $F\,^{1}Σ^{+}_{g}$ outer-well states are nominally 0.005 cm$^{-1}$. Level energies of $B\,^1Σ^{+}_{u}$ rovibrational levels, for $v \leq 8$ and $N \leq 10$ are determined at an accuracy of 0.001 cm$^{-1}$. Computed wavelengths of D$_2$ Lyman transitions in the $B\,^1Σ^{+}_{u}-X\,^{1}Σ^{+}_{g}$ ($v,0$) bands are also tabulated for future applications.

physics.atom-ph↗

arXiv2014

Ramsey-type microwave spectroscopy on CO ($a^3Π$)

Using a Ramsey-type setup, the lambda-doublet transition in the $J=1,\, Ω=1$ level of the $a^3Π$ state of CO was measured to be 394 064 870(10) Hz. In our molecular beam apparatus, a beam of metastable CO is prepared in a single quantum level by expanding CO into vacuum and exciting the molecules using a narrow-band UV laser system. After passing two microwave zones that are separated by 50 cm, the molecules are state-selectively deflected and detected 1 meter downstream on a position sensitive detector. In order to keep the molecules in a single $m_J^B$ level, a magnetic bias field is applied. We find the field-free transition frequency by taking the average of the $m_J^B = +1 \rightarrow m_J^B = +1$ and $m_J^B = -1 \rightarrow m_J^B = -1$ transitions, which have an almost equal but opposite Zeeman shift. The accuracy of this proof-of-principle experiment is a factor of 100 more accurate than the previous best value obtained for this transition.

physics.chem-ph↗

arXiv2014

Precision spectroscopy of the $X\,^{1}Σ_{g}^{+}, v=0\rightarrow 1$ ($J=0-2$) rovibrational splittings in H$_{2}$, HD and D$_{2}$

Accurate experimental values for the vibrational ground tone or fundamental vibrational energy splitting of H$_2$, HD, and D$_2$ are presented. Absolute accuracies of $2\times10^{-4}$ cm$^{-1}$ are obtained from Doppler-free laser spectroscopy applied in a collisionless environment. The vibrational splitting frequencies are derived from the combination difference between separate electronic excitations from the $X^{1}Σ_{g}^{+}, v=0, J$ and $v=1, J$ vibrational states to a common $EF^{1}Σ^{+}_{g}, v=0, J$ state. The present work on rotational quantum states $J=1,2$ extends the results reported by Dickenson et al. on $J=0$ [Phys. Rev. Lett. 110 (2013) 193601]. The experimental procedures leading to this high accuracy are discussed in detail. A comparison is made with full \emph{ab initio} calculations encompassing Born-Oppenheimer energies, adiabatic and non-adiabatic corrections, as well as relativistic corrections and QED-contributions. The present agreement between the experimental results and the calculations provides a stringent test on the application of quantum electrodynamics in molecules. Furthermore, the combined experimental-theoretical uncertainty can be interpreted to provide bounds to new interactions beyond the Standard Model of Physics or fifth forces between hadrons.

physics.atom-ph↗

arXiv2014

Detection of vibronic bands of C$_3$ in a translucent cloud towards HD 169454

We report the detection of eight vibronic bands of C$_3$, seven of which have been hitherto unobserved in astrophysical objects, in the translucent cloud towards HD~169454. Four of these bands are also found towards two additional objects: HD~73882 and HD~154368. Very high signal-to-noise ratio ($\sim$1000 and higher) and high resolving power ($R=80,000$) UVES-VLT spectra (Paranal, Chile) allow for detecting novel spectral features of C$_3$, even revealing weak perturbed features in the strongest bands. The work presented here provides the most complete spectroscopic survey of the so far largest carbon chain detected in translucent interstellar clouds. High-quality laboratory spectra of C$_3$ are measured using cavity ring-down absorption spectroscopy in a supersonically expanding hydrocarbon plasma, to support the analysis of the identified bands towards HD~169454. A column density of N(C$_3$) = $(6.6 \pm 0.2) \times 10^{12}$ cm$^{-2}$ is inferred and the excitation of the molecule exhibits two temperature components; $T_{exc}= 22 \pm 1$ K for the low-$J$ states and $T_{exc}= 187 \pm 25$ K for the high-$J$ tail. The rotational excitation of C$_3$ is reasonably well explained by models involving a mechanism including inelastic collisions, formation and destruction of the molecule, and radiative pumping in the far-infrared. These models yield gas kinetic temperatures comparable to those found for $T_{exc}$. The assignment of spectral features in the UV-blue range 3793-4054 Å may be of relevance for future studies aiming at unravelling spectra to identify interstellar molecules associated with the diffuse interstellar bands (DIBs).

astro-ph.GA↗

arXiv2013

Novel techniques in VUV high-resolution spectroscopy

Novel VUV sources and techniques for VUV spectroscopy are reviewed. Laser-based VUV sources have been developed via non-linear upconversion of laser pulses in the nanosecond (ns), the picosecond (ps), and femtosecond (fs) domain, and are applied in high-resolution gas phase spectroscopic studies. While the ns and ps pulsed laser sources, at Fourier-transform limited bandwidths, are used in wavelength scanning spectroscopy, the fs laser source is used in a two-pulse time delayed mode. In addition a Fourier-transform spectrometer for high resolution gas-phase spectroscopic studies in the VUV is described, exhibiting the multiplex advantage to measure many resonances simultaneously.

physics.optics↗

arXiv2013

Bounds on fifth forces at the sub-Angstrom length scale

Constraints on a possible fifth-force interaction between hadrons are derived based on an analysis of results from laser precision frequency measurements of antiprotonic helium atoms, both $\bar{p}\,^{4}$He$^{+}$ and $\bar{p}\,^{3}$He$^{+}$ species, and from experiments on resonant formation rates of $ddμ^{+}$-ions in muon-catalyzed fusion processes. A comparison is made between accurate experimental data and first-principles theoretical descriptions of the exotic systems within a quantum electrodynamical framework. The agreement between theory and experiment sets limits on a possible additional hadron-hadron interaction written in the form of a Yukawa potential $V_5(r)=α_5 \exp(-r/λ)/r$, with $λ$ representing the characteristic length scale associated with the mass of a hypothetical force-carrying particle via $λ=\hbar/(m_5c)$. The laser spectroscopic data of antiprotonic helium set a constraint of $α_5/α_{EM} < 10^{-8}$ for $λ< 1$ Å, while the binding energy of the muonic molecular deuterium ion delivers a constraint of $α_5/α_{EM} \sim 10^{-5}$ for $λ< 0.05$ Å, where $α_{EM}$ represents the strength of the electromagnetic interaction or the fine structure constant.

hep-ph↗

arXiv2013

VUV Spectroscopic Study of the BB" 1Su+ State of H2

Spectral lines, probing rotational quantum states J'=0,1,2 of the inner well vibrations (v' =< 8) in the BB" 1Su+ state of molecular hydrogen, were recorded in high resolution using a vacuum ultraviolet (VUV) Fourier transform absorption spectrometer in the wavelength range 73-86 nm. Accurate line positions and predissociation widths are determined from a fit to the absorption spectra. Improved values for the line positions are obtained, while the predissociation widths agree well with previous investigations.

physics.chem-ph↗

arXiv2013

Bounds on fifth forces from precision measurements on molecules

Highly accurate results from frequency measurements on neutral hydrogen molecules H_2, HD and D_2 as well as the HD^+ ion can be interpreted in terms of constraints on possible fifth-force interactions. Where the hydrogen atom is a probe for yet unknown lepton-hadron interactions, and the helium atom is sensitive for lepton-lepton interactions, molecules open the domain to search for additional long-range hadron-hadron forces. First principles calculations in the framework of quantum electrodynamics have now advanced to the level that hydrogen molecules and hydrogen molecular ions have become calculable systems, making them a search-ground for fifth forces. Following a phenomenological treatment of unknown hadron-hadron interactions written in terms of a Yukawa potential of the form V_5(r)=βexp(-r/λ)/r current precision measurements on hydrogenic molecules yield a constraint β< 1 \times 10^{-7} eVÅfor long-range hadron-hadron interactions at typical force ranges commensurate with separations of a chemical bond, i.e. λ~1 Åand beyond.

physics.atom-ph↗

arXiv2013

Synchrotron VUV radiation studies of the D^1Π_u State of H_2

The 3pπD^1Π_u state of the H_2 molecule was reinvestigated with different techniques at two synchrotron installations. The Fourier-Transform spectrometer in the vacuum ultraviolet wavelength range of the DESIRS beamline at the SOLEIL synchrotron was used for recording absorption spectra of the D^1Π_u state at high resolution and high absolute accuracy, limited only by the Doppler contribution at 100 K. From these measurements line positions were extracted, in particular for the narrow resonances involving ^1Π_u^- states, with an accuracy estimated at 0.06 cm^{-1} . The new data also closely match MQDT-calculations performed for the Π^- components observed via the narrow Q-lines. The Λ-doubling in the D^1Π_u state was determined up to v=17. The 10 m normal incidence scanning monochromator at the beamline U125/2 of the BESSY II synchrotron, combined with a home built target chamber and equipped with a variety of detectors was used to unravel information on ionization, dissociation and intramolecular fluorescence decay for the D^1Π_u vibrational series. The combined results yield accurate information of the characteristic Beutler-Fano profiles associated with the strongly predissociated Π_u^+ parity components of the D^1Π_u-levels. Values for the parameters describing the predissociation width as well as the Fano-q line shape parameters for the J=1 and J=2 rotational states were determined for the sequence of vibrational quantum numbers up to v=17.

physics.atm-clus↗

arXiv2013

VUV Spectroscopic Study of the D^1ΠState of Molecular Deuterium

The D^1Π_u - X^1Σ_g^+ absorption system of molecular deuterium has been re-investigated using the VUV Fourier -Transform (FT) spectrometer at the DESIRS beamline of the synchrotron SOLEIL and photon-induced fluorescence spectrometry (PIFS) using the 10 m normal incidence monochromator at the synchrotron BESSY II. Using the FT spectrometer absorption spectra in the range 72 - 82 nm were recorded in quasi static gas at 100 K and in a free flowing jet at a spectroscopic resolution of 0.50 and 0.20 cm^{-1} respectively . The narrow Q-branch transitions, probing states of Π^- symmetry, were observed up to vibrational level v = 22. The states of Π^+ symmetry, known to be broadened due to predissociation and giving rise to asymmetric Beutler-Fano resonances, were studied up to v = 18. The 10 m normal incidence beamline setup at BESSY II was used to simultaneously record absorption, dissociation, ionization and fluorescence decay channels from which information on the line intensities, predissociated widths, and Fano q-parameters were extracted. R-branch transitions were observed up to v = 23 for J = 1-3 as well as several transitions for J = 4 and 5 up to v = 22 and 18 respectively. The Q-branch transitions are found to weakly predissociate and were observed from v = 8 to the final vibrational level of the state v = 23. The spectroscopic study is supported by two theoretical frameworks. Results on the Π^- symmetry states are compared to ab initio multi-channel-quantum defect theory (MQDT) calculations, demonstrating that these calculations are accurate to within 0.5 cm^-1.

physics.atm-clus↗

arXiv2013

The D^1Π state of HD and the mass scaling relation of its predissociation widths

Absorption spectra of HD have been recorded in the wavelength range of 75 to 90 nm at 100 K using the vacuum ultraviolet Fourier transform spectrometer at the Synchrotron SOLEIL. The present wavelength resolution represents an order of magnitude improvement over that of previous studies. We present a detailed study of the D^1Π- X^1Σ^+_g system observed up to v'=18. The Q-branch transitions probing levels of Π^- symmetry are observed as narrow resonances limited by the Doppler width at 100 K. Line positions for these transitions are determined to an estimated absolute accuracy of 0.06 cm^{-1} . Predissociation line widths of Π^+ levels are extracted from the absorption spectra. A comparison with the recent results on a study of the D^1Πstate in H_2 and D_2 reveals that the predissociation widths scale as μ^-2J(J+1), with μthe reduced mass of the molecule and J the rotational angular momentum quantum number, as expected from an interaction with the B'^1Σ_u^+ continuum causing the predissociation.

physics.atm-clus↗

arXiv2013

Precision spectroscopy of high rotational states in H_2 investigated by Doppler-free two-photon laser spectroscopy in the EF^1Σ_g^+ - X^1Σ_g^+ system

Recently a high precision spectroscopic investigation of the EF^1Σ_g^+ - X^1Σ_g^+ system of molecular hydrogen was reported yielding information on QED and relativistic effects in a sequence of rotational quantum states in the X^1Σ^+_g$ ground state of the H_2 molecule [E.J. Salumbides et al., Phys. Rev. Lett. 107, 043005 (2011)]. The present paper presents a more detailed description of the methods and results. Furthermore, the paper serves as a stepping stone towards a continuation of the previous study by extending the known level structure of the EF^1Σ^+_g state to highly excited rovibrational levels through Doppler-free two photon spectroscopy. Based on combination differences between vibrational levels in the ground state, and between three rotational branches (O, Q and S branches) assignments of excited EF^1Σ^+_g levels, involving high vibrational and rotational quantum numbers, can be unambiguously made. For the higher EF^1Σ^+_g levels, where no combination differences are available, calculations were performed using the multi-channel quantum defect method, for a broad class of vibrational and rotational levels up to J=19. These predictions were used for assigning high-J EF-levels and are found to be accurate within 5 cm^-1.

physics.atm-clus↗

arXiv2012

The CO A-X System for Constraining Cosmological Drift of the Proton-Electron Mass Ratio

The $\textrm{A}^1Π-\textrm{X}^1Σ^+$ band system of carbon monoxide, which has been detected in six highly redshifted galaxies ($z=1.6-2.7$), is identified as a novel probe method to search for possible variations of the proton-electron mass ratio ($μ$) on cosmological time scales. Laboratory wavelengths of the spectral lines of the A-X ($v$,0) bands for $v=0-9$ have been determined at an accuracy of $Δλ/λ=1.5 \times 10^{-7}$ through VUV Fourier-transform absorption spectroscopy, providing a comprehensive and accurate zero-redshift data set. For the (0,0) and (1,0) bands, two-photon Doppler-free laser spectroscopy has been applied at the $3 \times 10^{-8}$ accuracy level, verifying the absorption data. Sensitivity coefficients $K_μ$ for a varying $μ$ have been calculated for the CO A-X bands, so that an operational method results to search for $μ$-variation.

physics.atom-ph↗

arXiv2012

High-resolution Fourier-transform XUV photoabsorption spectroscopy of 14N15N

The first comprehensive high-resolution photoabsorption spectrum of 14N15N has been recorded using the Fourier-transform spectrometer attached to the Desirs beamline at the Soleil synchrotron. Observations are made in the extreme ultraviolet (XUV) and span 100,000-109,000 cm-1 (100-91.7 nm). The observed absorption lines have been assigned to 25 bands and reduced to a set of transition energies, f values, and linewidths. This analysis has verified the predictions of a theoretical model of N2 that simulates its photoabsorption and photodissociation cross section by solution of an isotopomer independent formulation of the coupled-channel Schroedinger equation. The mass dependence of predissociation linewidths and oscillator strengths is clearly evident and many local perturbations of transition energies, strengths, and widths within individual rotational series have been observed.

physics.atm-clus↗

arXiv2011

QED effects in molecules: test on rotational quantum states of H$_2$

Quantum electrodynamic effects have been systematically tested in the progression of rotational quantum states in the $X ^{1}Σ_{g}^{+}, v=0$ vibronic ground state of molecular hydrogen. High-precision Doppler-free spectroscopy of the $EF ^{1}Σ_{g}^{+} - X ^{1}Σ_{g}^{+}$ (0,0) band was performed with 0.005 cm$^{-1}$ accuracy on rotationally-hot H$_2$ (with rotational quantum states J up to 16). QED and relativistic contributions to rotational level energies as high as 0.13 cm$^{-1}$ are extracted, and are in perfect agreement with recent calculations of QED and high-order relativistic effects for the H$_2$ ground state.

physics.atom-ph↗