arXiv subjects

Wim Ubachs

Publications and source records attributed to Wim Ubachs.

At least 37 records · Page 2Linked to original sources

arXiv2018

Expansion Dynamics After Laser-Induced Cavitation in Liquid Tin Microdroplets

The cavitation-driven expansion dynamics of liquid tin microdroplets is investigated, set in motion by the ablative impact of a 15-ps laser pulse. We combine high-resolution stroboscopic shadowgraphy with an intuitive fluid dynamic model that includes the onset of fragmentation, and find good agreement between model and experimental data for two different droplet sizes over a wide range of laser pulse energies. The dependence of the initial expansion velocity on these experimental parameters is heuristically captured in a single power law. Further, the obtained late-time mass distributions are shown to be governed by a single parameter. These studies are performed under conditions relevant for plasma light sources for extreme-ultraviolet nanolithography.

physics.flu-dyn↗

arXiv2018

Measuring the electric dipole moment of the electron in BaF

We investigate the merits of a measurement of the permanent electric dipole moment of the electron ($e$EDM) with barium monofluoride molecules, thereby searching for phenomena of CP violation beyond those incorporated in the Standard Model of particle physics. Although the BaF molecule has a smaller enhancement factor in terms of the effective electric field than other molecules used in current studies (YbF, ThO and ThF$^+$), we show that a competitive measurement is possible by combining Stark-deceleration, laser-cooling and an intense primary cold source of BaF molecules. With the long coherent interaction times obtainable in a cold beam of BaF, a sensitivity of $5\times10^{-30}$ e$\cdot$cm for an $e$EDM is feasible. We describe the rationale, the challenges and the experimental methods envisioned to achieve this target.

physics.atom-ph↗

arXiv2018

Sn ion energy distributions of ns- and ps-laser produced plasmas

Ion energy distributions arising from laser-produced plasmas of Sn are measured over a wide laser parameter space. Planar-solid as well as liquid-droplet targets are exposed to infrared laser pulses with energy densities between 1J/cm$^2$ and 4kJ/cm$^2$ and durations spanning 0.5ps to 6ns. The measured ion energy distributions are compared to two self-similar solutions of a hydrodynamic approach assuming isothermal expansion of the plasma plume into vacuum. For planar and droplet targets exposed to ps-long pulses we find a good agreement between the experimental results and the self-similar solution of a semi-infinite simple planar plasma configuration with an exponential density profile. The ion energy distributions resulting from solid Sn exposed to ns-pulses agrees with solutions of a limited-mass model that assumes a Gaussian-shaped initial density profile.

physics.plasm-ph↗

arXiv2018

Power-law scaling of plasma pressure on laser-ablated tin microdroplets

The measurement of the propulsion of metallic microdroplets exposed to nanosecond laser pulses provides an elegant method for probing the ablation pressure in dense laser-produced plasma. We present the measurements of the propulsion velocity over three decades in the driving Nd:YAG laser pulse energy, and observe a near-perfect power law dependence. Simulations performed with the RALEF-2D radiation-hydrodynamic code are shown to be in good agreement with the power law above a specific threshold energy. The simulations highlight the importance of radiative losses which significantly modify the power of the pressure scaling. Having found a good agreement between the experiment and the simulations, we investigate the analytic origins of the obtained power law and conclude that none of the available analytic theories is directly applicable for explaining our power exponent.

physics.plasm-ph↗

arXiv2017

Search for varying constants of nature from astronomical observation of molecules

The status of searches for possible variation in the constants of nature from astronomical observation of molecules is reviewed, focusing on the dimensionless constant representing the proton-electron mass ratio $μ=m_p/m_e$. The optical detection of H$_2$ and CO molecules with large ground-based telescopes (as the ESO-VLT and the Keck telescopes), as well as the detection of H$_2$ with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope is discussed in the context of varying constants, and in connection to different theoretical scenarios. Radio astronomy provides an alternative search strategy bearing the advantage that molecules as NH$_3$ (ammonia) and CH$_3$OH (methanol) can be used, which are much more sensitive to a varying $μ$ than diatomic molecules. Current constraints are $|Δμ/μ| < 5 \times 10^{-6}$ for redshift $z=2.0-4.2$, corresponding to look-back times of 10-12.5 Gyrs, and $|Δμ/μ| < 1.5 \times 10^{-7}$ for $z=0.88$, corresponding to half the age of the Universe (both at 3$σ$ statistical significance). Existing bottlenecks and prospects for future improvement with novel instrumentation are discussed.

astro-ph.CO↗

arXiv2017

C$_{60}^+$ and the Diffuse Interstellar Bands: An Independent Laboratory Check

In 2015, Campbell et al. (Nature 523, 322) presented spectroscopic laboratory gas phase data for the fullerene cation, C$_{60}^+$, that coincide with reported astronomical spectra of two diffuse interstellar band (DIB) features at 9633 and 9578 Å. In the following year additional laboratory spectra were linked to three other and weaker DIBs at 9428, 9366, and 9349 Å. The laboratory data were obtained using wavelength-dependent photodissociation spectroscopy of small (up to three) He-tagged C$_{60}^+-$He$_n$ ion complexes, yielding rest wavelengths for the bare C$_{60}^+$ cation by correcting for the He-induced wavelength shifts. Here we present an alternative approach to derive the rest wavelengths of the four most prominent C$_{60}^+$ absorption features, using high resolution laser dissociation spectroscopy of C$_{60}^+$ embedded in ultracold He droplets. Accurate wavelengths of the bare fullerene cation are derived based on linear wavelength shifts recorded for He$_n$C$_{60}^+$ species with $n$ up to 32. A careful analysis of all available data results in precise rest wavelengths (in air) for the four most prominent C$_{60}^+$ bands: 9631.9(1) Å, 9576.7(1) Å, 9427.5(1) Å, and 9364.9(1) Å. The corresponding band widths have been derived and the relative band intensity ratios are discussed.

astro-ph.GA↗

arXiv2017

Probing the Gravitational Dependence of the Fine-Structure Constant from Observations of White Dwarf Stars

Hot white dwarf stars are the ideal probe for a relationship between the fine-structure constant and strong gravitational fields, providing us with an opportunity for a direct observational test. We study a sample of hot white dwarf stars, combining far-UV spectroscopic observations, atomic physics, atmospheric modelling and fundamental physics, in the search for variation in the fine structure constant. This variation manifests as shifts in the observed wavelengths of absorption lines, such as quadruply ionized iron (FeV) and quadruply ionized nickel (NiV), when compared to laboratory wavelengths. Berengut et al. (Phys. Rev. Lett. 2013, 111, 010801) demonstrated the validity of such an analysis using high-resolution Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) spectra of G191-B2B. We have made three important improvements by: (a) using three new independent sets of laboratory wavelengths, (b) analysing a sample of objects, and (c) improving the methodology by incorporating robust techniques from previous studies towards quasars (the Many Multiplet method). A successful detection would be the first direct measurement of a gravitational field effect on a bare constant of nature. Here we describe our approach and present preliminary results from nine objects using both FeV and NiV.

astro-ph.CO↗

arXiv2016

Rayleigh-Brillouin scattering in SF$_6$ in the kinetic regime

Rayleigh-Brillouin spectral profiles are measured with a laser-based scatterometry setup for a 90 degrees scattering angle at a high signal-to-noise ratio (r.m.s. noise below 0.15 \% w.r.t. peak intensity) in sulphur-hexafluoride gas for pressures in the range 0.2 -- 5 bar and for a wavelength of $λ=403.0$ nm. The high quality data are compared to a number of light scattering models in order to address the effects of rotational and vibrational relaxation. While the vibrational relaxation rate is so slow that vibration degrees of freedom remain frozen, rotations relax on time scales comparable to those of the density fluctuations. Therefore, the heat capacity, the thermal conductivity and the bulk viscosity are all frequency-dependent transport coefficients. This is relevant for the Tenti model that depends on the values chosen for these transport coefficients. This is not the case for the other two models considered: a kinetic model based on rough-sphere interactions, and a model based on fluctuating hydrodynamics. The deviations with the experiment are similar between the three different models, except for the hydrodynamic model at pressures $p \lesssim 2\;{\rm bar}$. As all models are in line with the ideal gas law, we hypothesize the presence of real gas effects in the measured spectra.

physics.flu-dyn↗

arXiv2016

A molecular fountain

The resolution of any spectroscopic or interferometric experiment is ultimately limited by the total time a particle is interrogated. We here demonstrate the first molecular fountain, a development which permits hitherto unattainably long interrogation times with molecules. In our experiments, ammonia molecules are decelerated and cooled using electric fields, launched upwards with a velocity between 1.4 and 1.9\,m/s and observed as they fall back under gravity. A combination of quadrupole lenses and bunching elements is used to shape the beam such that it has a large position spread and a small velocity spread (corresponding to a transverse temperature of $<$10\,$μ$K and a longitudinal temperature of $<$1\,$μ$K) when the molecules are in free fall, while being strongly focused at the detection region. The molecules are in free fall for up to 266\,milliseconds, making it possible to perform sub-Hz measurements in molecular systems and paving the way for stringent tests of fundamental physics theories.

physics.chem-ph↗

arXiv2016

A Spectroscopic Survey of Electronic Transitions of C$_6$H, $^{13}$C$_6$H, and C$_6$D

Electronic spectra of C$_6$H are measured in the $18\,950-21\,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2Π$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $ν_6$ mode associated with the C-C bond vibration and the $ν_4$ and $ν_3$ modes associated with C$\equiv$C triple bonds. For the two lowest $ν_{11}$ and $ν_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $μ^2Σ$($ν_{11}$) and both $μ^2Σ$($ν_{10}$) and $κ^2Σ$($ν_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $μ^2Σ$($ν_9$) and $μ^2Σ$($ν_8$) levels. In the excitation region below the first non-degenerate vibration ($ν_6$), some $^2Π-^{2}Π$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2Π-^{2}Π$ transition found above the $ν_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2Π$ state of C$_6$H is derived and presented.

physics.chem-ph↗

arXiv2016

Plasma Propulsion of a Metallic Micro-droplet and its Deformation upon Laser Impact

The propulsion of a liquid indium-tin micro-droplet by nanosecond-pulse laser impact is experimentally investigated. We capture the physics of the droplet propulsion in a scaling law that accurately describes the plasma-imparted momentum transfer, enabling the optimization of the laser-droplet coupling. The subsequent deformation of the droplet is described by an analytical model that accounts for the droplet's propulsion velocity and the liquid properties. Comparing our findings to those from vaporization-accelerated mm-sized water droplets, we demonstrate that the hydrodynamic response of laser-impacted droplets is scalable and independent of the propulsion mechanism.

physics.plasm-ph↗

arXiv2015

Search for a drifting proton--electron mass ratio from H$_2$

An overview is presented of the H$_2$ quasar absorption method to search for a possible variation of the proton--electron mass ratio $μ=m_p/m_e$ on a cosmological time scale. Details of the analysis of astronomical spectra, obtained with large 8--10 m class optical telescopes, equipped with high-resolution echelle grating based spectrographs, are explained. The methods and results of the laboratory molecular spectroscopy of H$_2$, in particular the laser-based metrology studies for the determination of rest wavelengths of the Lyman and Werner band absorption lines, are reviewed. Theoretical physics scenarios delivering a rationale for a varying $μ$ will be discussed briefly, as well as alternative spectroscopic approaches to probe variation of $μ$, other than the H$_2$ method. Also a recent approach to detect a dependence of the proton-to-electron mass ratio on environmental conditions, such as the presence of strong gravitational fields, will be highlighted. Currently some 56 H$_2$ absorption systems are known and listed. Their usefulness to detect $μ$-variation is discussed, in terms of column densities and brightness of background quasar sources, along with future observational strategies. The astronomical observations of ten quasar systems analyzed so far set a constraint on a varying proton-electron mass ratio of $|Δμ/μ| < 5 \times 10^{-6}$ (3-$σ$), which is a null result, holding for redshifts in the range $z=2.0-4.2$. This corresponds to look-back times of 10--12.4 billion years into cosmic history. Attempts to interpret the results from these 10 H$_2$ absorbers in terms of a spatial variation of $μ$ are currently hampered by the small sample size and their coincidental distribution in a relatively narrow band across the sky.

astro-ph.GA↗

arXiv2015

Physics beyond the Standard Model from hydrogen spectroscopy

Spectroscopy of hydrogen can be used for a search into physics beyond the Standard Model. Differences between the absorption spectra of H$_2$ as observed at high redshift and those measured in the laboratory can be interpreted in terms of possible variations of the proton-electron mass ratio. Investigation of some ten of such absorbers in the redshift range $z= 2.0-4.2$ yields a constraint of $|Δμ/μ|< 5 \times 10^{-6}$ at 3$σ$. Observation of H$_2$ from the photospheres of white dwarf stars inside our Galaxy delivers a constraint of similar magnitude on a dependence of $μ$ on a gravitational potential $10^4$ times as strong as on the Earth's surface. Laser-based precision measurements of dissociation energies, vibrational splittings and rotational level energies in H$_2$ molecules and their deuterated isotopomers HD and D$_2$ produce values for the rovibrational binding energies fully consistent with quantum ab initio calculations including relativistic and quantum electrodynamical (QED) effects. Similarly, precision measurements of high-overtone vibrational transitions of HD$^+$ ions, captured in ion traps and sympathetically cooled to mK temperatures, also result in transition frequencies fully consistent with calculations including QED corrections. Precision measurements of inter-Rydberg transitions in H$_2$ can be extrapolated to yield accurate values for level splittings in the H$_2^+$-ion. These comprehensive results of laboratory precision measurements on neutral and ionic hydrogen molecules can be interpreted to set bounds on the existence of possible fifth forces and of higher dimensions, phenomena describing physics beyond the Standard Model.

physics.atom-ph↗

arXiv2015

Rayleigh-Brillouin Scattering in Binary Gas Mixtures

Precise measurements are performed on spectral lineshapes of spontaneous Rayleigh-Brillouin scattering in mixtures of the noble gases Ar and Kr, with He. Admixture of a light He atomic fraction results in marked changes of the spectra, although in all experiments He is merely a spectator atom: it affects the relaxation of density fluctuations of the heavy constituent, but its contribution to the scattered light intensity is negligibly small. The results are compared to a theory for the spectral lineshape without adjustable parameters, yielding excellent agreement for the case of binary mono-atomic gases, signifying a step towards modeling and understanding of light scattering in more complex molecular media.

physics.flu-dyn↗

arXiv2015

The B$^2Π-$X$^2Π$ electronic origin band of $^{13}$C$_6$H

The rotationally resolved spectrum of the B$^2Π-$X$^2Π$ electronic origin band transition of $^{13}$C$_6$H is presented. The spectrum is recorded using cavity ring-down spectroscopy in combination with supersonic plasma jets by discharging a $^{13}$C$_2$H$_2$/He/Ar gas mixture. A detailed analysis of more than a hundred fully-resolved transitions allows for an accurate determination of the spectroscopic parameters for both the ground and electronically excited state of $^{13}$C$_6$H.

physics.atom-ph↗

arXiv2014

A systematic study of Rayleigh-Brillouin scattering in air, N2 and O2 gases

Spontaneous Rayleigh-Brillouin scattering experiments in air, N2 and O2 have been performed for a wide range of temperatures and pressures at a wavelength of 403 nm and at a 90 degrees scattering angle. Measurements of the Rayleigh-Brillouin spectral scattering profile were conducted at high signal-to-noise ratio for all three species, yielding high-quality spectra unambiguously showing the small differences between scattering in air, and its constituents N2 and O2. Comparison of the experimental spectra with calculations using the Tenti S6 model, developed in 1970s based on linearized kinetic equations for molecular gases, demonstrates that this model is valid to high accuracy. After previous measurements performed at 366 nm, the Tenti S6 model is here verified for a second wavelength of 403 nm. Values for the bulk viscosity for the gases are derived by optimizing the model to the measurements. It is verified that the bulk viscosity parameters obtained from previous experiments at 366 nm, are valid for wavelengths of 403 nm. Also for air, which is treated as a single-component gas with effective gas transport coefficients, the Tenti S6 treatment is validated for 403 nm as for the previously used wavelength of 366 nm, yielding an accurate model description of the scattering profiles for a range of temperatures and pressures, including those of relevance for atmospheric studies. It is concluded that the Tenti S6 model, further verified in the present study, is applicable to LIDAR applications for exploring the wind velocity and the temperature profile distributions of the Earth's atmosphere. Based on the present findings, predictions can be made on the spectral profiles for a typical LIDAR backscatter geometry, which deviate by some 7 percent from purely Gaussian profiles at realistic sub-atmospheric pressures occurring at 3-5 km altitude in the Earth's atmosphere.

physics.optics↗

arXiv2014

Limits on a Gravitational Field Dependence of the Proton--Electron Mass Ratio from H$_2$ in White Dwarf Stars

Spectra of molecular hydrogen (H$_2$) are employed to search for a possible proton-to-electron mass ratio ($μ$) dependence on gravity. The Lyman transitions of H$_2$, observed with the Hubble Space Telescope towards white dwarf stars that underwent a gravitational collapse, are compared to accurate laboratory spectra taking into account the high temperature conditions ($T \sim 13\,000$ K) of their photospheres. We derive sensitivity coefficients $K_i$ which define how the individual H$_2$ transitions shift due to $μ$-dependence. The spectrum of white dwarf star GD133 yields a $Δμ/μ$ constraint of $(-2.7\pm4.7_{\rm stat}\pm 0.2_{\rm sys})\times10^{-5}$ for a local environment of a gravitational potential $ϕ\sim10^4\ ϕ_\textrm{Earth}$, while that of G29$-$38 yields $Δμ/μ=(-5.8\pm3.8_{\rm stat}\pm 0.3_{\rm sys})\times10^{-5}$ for a potential of $2 \times 10^4$ $ϕ_\textrm{Earth}$.

gr-qc↗

arXiv2014

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.

physics.optics↗