arXiv subjects

W. Ubachs

Publications and source records attributed to W. Ubachs.

At least 55 records · Page 3Linked to original sources

arXiv2017

CARS spectroscopy of the ($v=0\to1$) band in $\rm{T_2}$

Molecular hydrogen is a benchmark system for bound state quantum calculation and tests of quantum electrodynamical effects. While spectroscopic measurements on the stable species have progressively improved over the years, high resolution studies on the radioactive isotopologues $\rm{T_2}$, $\rm{HT}$ and $\rm{DT}$ have been limited. Here we present an accurate determination of $\rm{T_2}$ $Q(J = 0 - 5)$ transition energies in the fundamental vibrational band of the ground electronic state, by means of high resolution Coherent Anti-Stokes Raman Spectroscopy. With the present experimental uncertainty of $0.02\,\rm{cm^{-1}}$, which is a fivefold improvement over previous measurements, agreement with the latest theoretical calculations is demonstrated.

physics.atom-ph↗

arXiv2017

Testing the variability of the proton-to-electron mass ratio from observations of methanol in the dark cloud core L1498

The dependence of the proton-to-electron mass ratio, mu, on the local matter density was investigated using methanol emission in the dense dark cloud core L1498. Towards two different positions in L1498, five methanol transitions were detected and an extra line was tentatively detected at a lower confidence level in one of the positions. The observed centroid frequencies were then compared with their rest frame frequencies derived from least-squares fitting to a large data set. Systematic effects, as the underlying methanol hyperfine structure and the Doppler tracking of the telescope, were investigated and their effects were included in the total error budget. The comparison between the observations and the rest frame frequencies constrains potential mu variation at the level of Dmu/mu < 6 x 10^(-8), at a 3 sigma confidence level. For the dark cloud we determine a total CH3OH (A+E) beam averaged column density of 3-4 x 10^(12) cm(-2) (within roughly a factor of two), an E- to A-type methanol column density ratio of N(A-CH3OH)/N(E-CH3OH) = 1.00 +/- 0.15, a density of n(H2) = 3 x 10^5 cm^(-3) (again within a factor of two), and a kinetic temperature of Tkin = 6 +/- 1 K. In a kinetic model including the line intensities observed for the methanol lines, the n(H2) density is higher and the temperature is lower than that derived in previous studies based on different molecular species; the intensity of the 1_0 --> 1_-1 E line strength is not well reproduced.

astro-ph.CO↗

arXiv2017

Analysis of carbon monoxied absorption at z=2.5 to constrain variation of the proton-to-electron mass ratio

Absorption by carbon monoxide in the spectrum of quasar SDSS J000015.16+004833.2 is investigated in order to derive a constraint on the temporal variation of the proton-to-electron mass ratio, mu. The spectrum was recorded using VLT/UVES, and it was partially corrected for long-range wavelength scale distortions using the supercalibration technique. Eight vibrational CO singlet-singlet bands belonging to the A-X electronic absorption system, and the perturbing d3-X(5, 0) singlet-triplet band are detected in the damped Lyman-alpha system at z =2.52. The spectra are modelled using a comprehensive fitting technique, resulting in a final value of Dmu/mu=(1.8 +/- 2.2 +/- 0.4)x10e(-5), which is consistent with no variation over a look-back time of 11.2 Gyrs.

astro-ph.CO↗

arXiv2016

Optical spectroscopy of complex open 4$d$-shell ions Sn$^{7+}$-Sn$^{10+}$

We analyze the complex level structure of ions with many-valence-electron open [Kr] 4$d^\textrm{m}$ sub-shells ($\textrm{m}$=7-4) with ab initio calculations based on configuration-interaction many-body perturbation theory (CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra of Sn$^{7+}$-Sn$^{10+}$ ions were obtained using an electron beam ion trap. Semi-empirical spectral fits carried out with the orthogonal parameters technique and Cowan code calculations lead to 90 identifications of magnetic-dipole transitions and the determination of 79 energy ground-configuration levels, questioning some earlier EUV-line assignments. Our results, the most complete data set available to date for these ground configurations, confirm the ab initio predictive power of CI+MBPT calculations for the these complex electronic systems.

physics.atom-ph↗

arXiv2016

Constraint on a varying proton-to-electron mass ratio from H2 and HD absorption at z = 2.34

Molecular hydrogen absorption in the damped Lyman-alpha system at z = 2.34 towards quasar Q1232+082 is analyzed in order to derive a constraint on a possible temporal variation of the proton-to-electron mass ratio, mu, over cosmological timescales. Some 106 H2 and HD transitions, covering the range 3290-3726 Å, are analyzed with a comprehensive fitting technique, allowing for the inclusion of overlapping lines associated with hydrogen molecules, the atomic hydrogen lines in the Lyman-alpha forest as well as metal lines. The absorption model, based on the most recent and accurate rest wavelength for H2 and HD transitions, delivers a value of dmu/mu = (19 +/- 9 +/- 5)x 10^(-6). An attempt to correct the spectrum for possible long-range wavelength distortions is made and the uncertainty on the distortion correction is included in the total systematic uncertainty. The present result is an order of magnitude more stringent than a previous measurement from the analysis of this absorption system, based on a line-by-line comparison of only 12 prominent and isolated H2 absorption lines. This is consistent with other measurements of dmu/mu from 11 other absorption systems in showing a null variation of the proton-to-electron mass ratio over a look-back time of 11 Gyrs.

astro-ph.CO↗

arXiv2016

Precision measurements and test of molecular theory in highly-excited vibrational states of H$_2$ $(v=11)$

Accurate $EF{}^1Σ^+_g-X{}^1Σ^+_g$ transition energies in molecular hydrogen were determined for transitions originating from levels with highly-excited vibrational quantum number, $v=11$, in the ground electronic state. Doppler-free two-photon spectroscopy was applied on vibrationally excited H$_2^*$, produced via the photodissociation of H$_2$S, yielding transition frequencies with accuracies of $45$ MHz or $0.0015$ cm$^{-1}$. An important improvement is the enhanced detection efficiency by resonant excitation to autoionizing $7pπ$ electronic Rydberg states, resulting in narrow transitions due to reduced ac-Stark effects. Using known $EF$ level energies, the level energies of $X(v=11, J=1,3-5)$ states are derived with accuracies of typically 0.002 cm$^{-1}$. These experimental values are in excellent agreement with, and are more accurate than the results obtained from the most advanced ab initio molecular theory calculations including relativistic and QED contributions.

physics.atom-ph↗

arXiv2016

Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048

We present the discovery of a molecular cloud at zabs=2.5255 along the line of sight to the quasar J0000+0048. We perform a detailed analysis of the absorption lines from ionic, neutral atomic and molecular species in different excitation levels, as well as the broad-band dust extinction. We find that the absorber classifies as a Damped Lyman-alpha system (DLA) with logN(HI)(cm^-2)=20.8+/-0.1. The DLA has super-Solar metallicity with a depletion pattern typical of cold gas and an overall molecular fraction ~50%. This is the highest f-value observed to date in a high-z intervening system. Most of the molecular hydrogen arises from a clearly identified narrow (b~0.7 km/s), cold component in which CO molecules are also found, with logN(CO)~15. We study the chemical and physical conditions in the cold gas. We find that the line of sight probes the gas deep after the HI-to-H2 transition in a ~4-5 pc-size cloud with volumic density nH~80 cm^-3 and temperature of only 50 K. Our model suggests that the presence of small dust grains (down to about 0.001 μm) and high cosmic ray ionisation rate (zeta_H a few times 10^-15 s^-1) are needed to explain the observed atomic and molecular abundances. The presence of small grains is also in agreement with the observed steep extinction curve that also features a 2175 A bump. The properties of this cloud are very similar to what is seen in diffuse molecular regions of the nearby Perseus complex. The high excitation temperature of CO rotational levels towards J0000+0048 betrays however the higher temperature of the cosmic microwave background. Using the derived physical conditions, we correct for a small contribution (0.3 K) of collisional excitation and obtain TCMB(z = 2.53)~9.6 K, in perfect agreement with the predicted adiabatic cooling of the Universe. [abridged]

astro-ph.GA↗

arXiv2016

High-precision spectroscopy of the HD+ molecule at the 1-p.p.b. level

Recently we reported a high precision optical frequency measurement of the (v,L):(0,2)->(8,3) vibrational overtone transition in trapped deuterated molecular hydrogen (HD+) ions at 10 mK temperature. Achieving a resolution of 0.85 parts-per-billion (p.p.b.) we found the experimental value ($ν_0= 383,407,177.38(41)$ MHz) to be in agreement with the value from molecular theory ($ν_\text{th}=383,407,177.150(15)$ MHz) within 0.6(1.1) p.p.b. [Biesheuvel et al., Nat. Commun. 7, 10385 (2016)]. This enabled an improved test of molecular theory (including QED), new constraints on the size of possible effects due to 'new physics', and the first determination of the proton-electron mass ratio from a molecule. Here, we provide the details of the experimental procedure, spectral analysis, and the assessment of systematic frequency shifts. Our analysis focuses in particular on deviations of the HD+ velocity distribution from thermal (Gaussian) distributions under the influence of collisions with fast ions produced during (laser-induced) chemical reactions, as such deviations turn out to significantly shift the hyperfine-less vibrational frequency as inferred from the saturated and Doppler-broadened spectrum, which contains partly unresolved hyperfine structure.

physics.atom-ph↗

arXiv2016

Constraint on a cosmological variation in the proton-to-electron mass ratio from electronic CO absorption

Carbon monoxide (CO) absorption in the sub-damped Lyman-$α$ absorber at redshift $z_{abs} \simeq 2.69$, toward the background quasar SDSS J123714.60+064759.5 (J1237+0647), was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio, $μ$, over a cosmological time-scale. The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 kms$^{-1}$ per pixel at $\sim 5000$ Å. Thirteen CO vibrational bands in this absorber are detected: the A$^{1}Π$ - X$^{1}Σ^{+}$ ($ν'$,0) for $ν' = 0 - 8$, B$^{1}Σ^{+}$ - X$^{1}Σ^{+}$ (0,0), C$^{1}Σ^{+}$ - X$^{1}Σ^{+}$ (0,0), and E$^{1}Π$ - X$^{1}Σ^{+}$ (0,0) singlet-singlet bands and the d$^{3}Δ$ - X$^{1}Σ^{+}$ (5,0) singlet-triplet band. An updated database including the most precise molecular inputs needed for a $μ$-variation analysis is presented for rotational levels $J = 0 - 5$, consisting of transition wavelengths, oscillator strengths, natural lifetime damping parameters, and sensitivity coefficients to a variation of the proton-to-electron mass ratio. A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of $Δμ/μ= (0.7 \pm 1.6_{stat} \pm 0.5_{syst}) \times 10^{-5}$ was derived from CO only. A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of $Δμ/μ= (-5.6 \pm 5.6_{stat} \pm 3.1_{syst}) \times 10^{-6}$, which is consistent with no variation over a look-back time of $\sim 11.4$ Gyrs.

astro-ph.CO↗

arXiv2016

Analysis of the fine structure of Sn$^{11+...14+}$ ions by optical spectroscopy in an electron beam ion trap

We experimentally re-evaluate the fine structure of Sn$^{11+...14+}$ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole $M1$ transitions, in a wavelength range covering 260\,nm to 780\,nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of \emph{ab initio} calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited.

physics.atom-ph↗

arXiv2016

Probing QED and fundamental constants through laser spectroscopy of vibrational transitions in HD+

The simplest molecules in nature, molecular hydrogen ions in the form of H2+ and HD+, provide an important benchmark system for tests of quantum electrodynamics in complex forms of matter. Here, we report on such a test based on a frequency measurement of a vibrational overtone transition in HD+ by laser spectroscopy. We find that the theoretical and experimental frequencies are equal to within 0.6(1.1) parts per billion, which represents the most stringent test of molecular theory so far. Our measurement not only confirms the validity of high-order quantum electrodynamics in molecules, but also enables the long predicted determination of the proton-to-electron mass ratio from a molecular system, as well as improved constraints on hypothetical fifth forces and compactified higher dimensions at the molecular scale. With the perspective of comparisons between theory and experiment at the 0.01 part-per-billion level, our work demonstrates the potential of molecular hydrogen ions as a probe of fundamental physical constants and laws.

physics.atom-ph↗

arXiv2015

Spectroscopy and perturbation analysis of the CO A$^1Π-$X$^1Σ^+$ (2,0), (3,0) and (4,0) bands

The (2,0) (3,0) and (4,0) bands of the A$^1Π-$X$^1Σ^+$ system of $^{12}$C$^{16}$O have been re-investigated by high-resolution vacuum ultraviolet absorption spectroscopy. A VUV Fourier-transform spectrometer, illuminated by synchrotron radiation, was applied to record a jet-cooled spectrum, a room temperature static gas spectrum and a high temperature (900 K) quasi-static gas spectrum, resulting in absolute accuracies of 0.01$-$0.02 cm$^{-1}$ for the rotational line frequencies. Precise laser-based data were included in the analysis allowing for a highly accurate determination of band origins. Rotational levels up to $J=52$ were observed. The data were used to perform an improved analysis of the perturbations in the A$^1Π$, $v=2$, $v=3$, and $v=4$ levels by vibrational levels of the D$^1Δ$, I$^1Σ^-$, e$^3Σ^-$, d$^3Δ$, and a$'^3Σ^+$ states.

physics.atom-ph↗

arXiv2015

Constraint on a varying proton-to-electron mass ratio from molecular hydrogen absorption toward quasar SDSS J123714.60+064759.5

Molecular hydrogen transitions in the sub-damped Lyman alpha absorber at redshift z = 2.69, toward the background quasar SDSS J123714.60+064759.5, were analyzed in order to search for a possible variation of the proton-to-electron mass ratio mu over a cosmological time-scale. The system is composed of three absorbing clouds where 137 H2 and HD absorption features were detected. The observations were taken with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 32 per 2.5 km/s pixel, covering the wavelengths from 356.6 to 409.5 nm. A comprehensive fitting method was used to fit all the absorption features at once. Systematic effects of distortions to the wavelength calibrations were analyzed in detail from measurements of asteroid and `solar twin' spectra, and were corrected for. The final constraint on the relative variation in mu between the absorber and the current laboratory value is dmu/mu = (-5.4 \pm 6.3 stat \pm 4.0 syst) x 10^(-6), consistent with no variation over a look-back time of 11.4 Gyrs.

astro-ph.CO↗

arXiv2015

Test of quantum chemistry in vibrationally-hot hydrogen molecules

Precision measurements are performed on highly excited vibrational quantum states of molecular hydrogen. The $v=12, J=0-3$ rovibrational levels of H$_2$ ($X^1Σ_g^+$), lying only $2000$ cm$^{-1}$ below the first dissociation limit, were populated by photodissociation of H$_2$S and their level energies were accurately determined by two-photon Doppler-free spectroscopy. A comparison between the experimental results on $v=12$ level energies with the best \textit{ab initio} calculations shows good agreement, where the present experimental accuracy of $3.5 \times10^{-3}$ cm$^{-1}$ is more precise than theory, hence providing a gateway to further test theoretical advances in this benchmark quantum system.

physics.atom-ph↗

arXiv2015

Constraints on extra dimensions from precision molecular spectroscopy

Accurate investigations of quantum level energies in molecular systems are shown to provide a test ground to constrain the size of compactified extra dimensions. This is made possible by the recent progress in precision metrology with ultrastable lasers on energy levels in neutral molecular hydrogen (H$_2$, HD and D$_2$) and the molecular hydrogen ions (H$_2^+$, HD$^+$ and D$_2^+$). Comparisons between experiment and quantum electrodynamics calculations for these molecular systems can be interpreted in terms of probing large extra dimensions, under which conditions gravity will become much stronger. Molecules are a probe of space-time geometry at typical distances where chemical bonds are effective, i.e. at length scales of an Å. Constraints on compactification radii for extra dimensions are derived within the Arkani-Hamed-Dimopoulos-Dvali framework, while constraints for curvature or brane separation are derived within the Randall-Sundrum framework. Based on the molecular spectroscopy of D$_2$ molecules and HD$^+$ ions, the compactification size for seven extra dimensions (in connection to M-theory defined in 11 dimensions) of equal size is shown to be limited to $R_7 < 0.6 μ$m. While limits on compactification sizes of extra dimensions based on other branches of physics are compared, the prospect of further tightening constraints from the molecular method is discussed.

physics.atom-ph↗

arXiv2015

A constraint on a varying proton--electron mass ratio 1.5 billion years after the Big Bang

A molecular hydrogen absorber at a lookback time of 12.4 billion years, corresponding to 10$\%$ of the age of the universe today, is analyzed to put a constraint on a varying proton--electron mass ratio, $μ$. A high resolution spectrum of the J1443$+$2724 quasar, which was observed with the Very Large Telescope, is used to create an accurate model of 89 Lyman and Werner band transitions whose relative frequencies are sensitive to $μ$, yielding a limit on the relative deviation from the current laboratory value of $Δμ/μ=(-9.5\pm5.4_{\textrm{stat}} \pm 5.3_{\textrm{sys}})\times 10^{-6}$.

astro-ph.CO↗

arXiv2015

High-precision laser spectroscopy of the CO A$^1Π$ - X$^1Σ^+$ (2,0), (3,0) and (4,0) bands

High-precision two-photon Doppler-free frequency measurements have been performed on the CO A$^1Π$ - X$^1Σ^+$ fourth-positive system (2,0), (3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for rotational quantum numbers up to $J = 5$, have been determined at an accuracy of $1.6\times10^{-3}$ cm$^{-1}$, using advanced techniques of two-color 2+1' resonance-enhanced multi-photon ionization, Sagnac interferometry, frequency-chirp analysis on the laser pulses, and correction for AC-Stark shifts. The accurate transition frequencies of the CO A$^1Π$ - X$^1Σ^+$ system are of relevance for comparison with astronomical data in the search for possible drifts of fundamental constants in the early universe. The present accuracies in laboratory wavelengths of $Δλ/λ= 2 \times 10^{-8}$ may be considered exact for the purpose of such comparisons.

physics.atom-ph↗

arXiv2014

Constraints on changes in the proton-electron mass ratio using methanol lines

We report Karl G. Jansky Very Large Array (VLA) absorption spectroscopy in four methanol (CH$_3$OH) lines in the $z = 0.88582$ gravitational lens towards PKS1830-211. Three of the four lines have very different sensitivity coefficients $K_μ$ to changes in the proton-electron mass ratio $μ$; a comparison between the line redshifts thus allows us to test for temporal evolution in $μ$. We obtain a stringent statistical constraint on changes in $μ$ by comparing the redshifted 12.179 GHz and 60.531 GHz lines, $[Δmu/μ] \leq 1.1 \times 10^{-7}$ ($2σ$) over $0 < z \leq 0.88582$, a factor of $\approx 2.5$ more sensitive than the best earlier results. However, the higher signal-to-noise ratio (by a factor of $\approx 2$) of the VLA spectrum in the 12.179 GHz transition also indicates that this line has a different shape from that of the other three CH$_3$OH lines (at $> 4σ$ significance). The sensitivity of the above result, and that of all earlier CH$_3$OH studies, is thus likely to be limited by unknown systematic errors, probably arising due to the frequency-dependent structure of PKS1830-211. A robust result is obtained by combining the three lines at similar frequencies, 48.372, 48.377 and 60.531 GHz, whose line profiles are found to be in good agreement. This yields the $2σ$ constraint $[Δμ/μ] \lesssim 4 \times 10^{-7}$, the most stringent current constraint on changes in $μ$. We thus find no evidence for changes in the proton-electron mass ratio over a lookback time of $\approx 7.5$ Gyrs.

astro-ph.CO↗