Wim Ubachs

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Verified

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• PhD
• Professor at Vrije Universiteit Amsterdam

We present a coherent mid-infrared (mid-IR) light source based on a periodically poled lithium niobate optical parametric oscillator (OPO) that is capable of producing arbitrary pulse shapes in the nanosecond regime. By controlling the shape of the 1064-nm wavelength pump, we compensate for the cavity dynamics that enable temporal pulse shaping of...

The binding energies of the five bound rotational levels $J=0-4$ in the highest vibrational level $v=14$ in the X$^1\Sigma_g^+$ ground electronic state of H$_2$ were measured in a three-step ultraviolet-laser experiment. Two-photon UV-photolysis of H$_2$S produced population in these high-lying bound states, that were subsequently interrogated at h...

We demonstrate a method to determine the longitudinal phase-space distribution of a cryogenic buffer gas cooled beam of barium-fluoride molecules based on a two-step laser excitation scheme. Temporal resolution is achieved by a transversely aligned laser beam that drives molecules from the ground state X2Σ+ to the A2Π1/2 state around 860 nm, while...

The water molecule, crucial to the chemical composition and dynamics of the universe, is typically identified in its gas phase via radio and submillimeter transitions, with frequencies up to a few THz. To understand the physicochemical behavior of astronomical objects, accurate transition frequencies are required for these lines. From a set of 26 n...

An electrostatic hexapole lens is used to manipulate the transverse properties of a beam of barium fluoride molecules from a cryogenic buffer gas source. The spatial distribution of the beam is measured by recording state-selective laser-induced fluorescence on an emccd camera, providing insight into the intensity and transverse position spread of...

We present results on simultaneous observations of Class I methanol masers at 25, 36, and 44 GHz towards 22 Galactic targets carried out with the Effelsberg 100-m telescope. The study investigates relations between the hyperfine (HF) structure of the torsion-rotation transitions in CH3OH and maser activity. By analyzing the radial velocity shifts b...

We present results on simultaneous observations of Class~I methanol masers at 25, 36, and 44 GHz towards 22 Galactic targets carried out with the Effelsberg 100-m telescope. The study investigates relations between the hyperfine (HF) structure of the torsion-rotation transitions in CH3OH and maser activity. By analyzing the radial velocity shifts b...

We demonstrate a spin-precession method to observe and analyze multilevel coherence between all hyperfine levels in the X2Σ+,N=0 ground state of barium monofluoride (Ba138F19). The signal is sensitive to the state-preparation Rabi frequency and external electric and magnetic fields applied in searches for a permanent electric dipole moment (EDM). I...

Cavity-enhanced spectroscopic techniques are highly sensitive laser-based methods for interrogating the atomic and molecular constituents of any gaseous medium that is confined into an optical resonator. A first advantage over conventional absorption spectroscopy comes from the extremely long path length of the laser radiation inside the stable, hi...

Recently, we have demonstrated a method to record the longitudinal phase-space distribution of a pulsed cryogenic buffer gas cooled beam of barium fluoride molecules with high resolution. In this paper, we use this method to determine the influence of various source parameters. Besides the expected dependence on temperature and pressure, the forwar...

Saturated absorption measurements of transitions in the (2-0) band of radioactive tritium hydride are performed with the ultrasensitive noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy intracavity absorption technique in the range 1460–1510 nm. The hyperfine structure of rovibrational transitions of tritium hydride, in contras...

Network theory has fundamentally transformed our comprehension of complex systems, catalyzing significant advances across various domains of science and technology. In spectroscopic networks, hubs are the quantum states involved in the largest number of transitions. Here, utilizing network paths probed via precision metrology, absolute energies hav...

We demonstrate highly efficient 2 μm-wavelength laser-produced plasma light sources for nanolithographic applications, reaching efficiencies of up to 5%.

We introduce a spectroscopic absorption imaging method in the UV regime (225–400 nm) to study tin vapor created by irradiating a thin film with a low intensity 108 W cm−2 nanosecond laser pulse, a case inspired by current developments around “advanced target shaping” in industrial laser-produced plasma sources for extreme ultraviolet light. The 4-n...

We demonstrate the efficient generation of extreme ultraviolet (EUV) light from laser-produced plasma (LPP) driven by 2 μm wavelength laser light as an alternative for 10 μm CO2 gas LPP currently employed in EUV lithography machines for high-volume manufacturing of semiconductor devices. High conversion efficiencies of laser light into “in-band” EU...

The ionization energy of HD has been determined to be EI(HD)=124568.48466(7)cm−1 by two-photon Doppler-free vacuum-ultraviolet pulsed laser spectroscopy, near-infrared continuous-wave laser spectroscopy, and Rydberg-series extrapolation by multichannel quantum-defect theory (MQDT). From this value, the dissociation energy of HD is deduced to be D0(...

The saturated absorption spectrum of the hyperfineless S(0) quadrupole line in the (2-0) band of H2 is measured at λ=1189 nm, using the NICE-OHMS technique under cryogenic conditions (72 K). It is the first time that a Lamb dip of a molecular quadrupole transition has been recorded. At low (150–200 W) saturation powers a single narrow Lamb dip is o...

We present simultaneous 1D imaging and broadband spectroscopy of a laser-produced plasma (LPP) source of extreme ultraviolet light, using a tapered zone plate that is matched to the dispersion of a transmission grating. We describe the design and fabrication of the zone plates in the 5–80 nm wavelength regime with designed spatial resolution of ∼10...

A laser system generating high-energy pulses at 2-µm wavelength with pulse widths tunable from 10–24 ns is described. It comprises an optical parametric oscillator that generates mJ-level signal seed radiation and an optical parametric amplifier that boosts the output to 800 mJ of combined signal and idler when pumped with 2 J pulses of 1064-nm las...

We present simultaneous 1D imaging and broadband spectroscopy of a laser-produced plasma source of extreme ultraviolet light, using a tapered zone plate that is matched to the dispersion of a transmission grating. We describe the design and fabrication of the zone plates in the 5-80 nm wavelength regime with designed spatial resolution of ~10 μm an...

We present simultaneous 1D imaging and broadband spectroscopy of a laser-produced plasma source of extreme ultraviolet light, using a tapered zone plate that is matched to the dispersion of a transmission grating. We describe the design and fabrication of the zone plates in the 5-80 nm wavelength regime with designed spatial resolution of ~10 μm an...

The saturated absorption spectrum of the hyperfine-less S(0) quadrupole line in the (2-0) band of H$_2$ is measured at $\lambda=1189$ nm, using the NICE-OHMS technique under cryogenic conditions (72~K). It is for the first time that a Lamb dip of a molecular quadrupole transition is recorded. At low (150-200 W) saturation powers a single narrow Lam...

We demonstrate a spin-precession method to observe and analyze multi-level coherence between all hyperfine levels in the $X ^2\Sigma^+,N=0$ ground state of barium monofluoride ($^{138}$Ba$^{19}$F). The signal is sensitive to the state-preparation Rabi frequency and external electric and magnetic fields applied in searches for a permanent electric d...

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the \X\ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H$_2$S, HDS and D$_2$S are used to produce vibrationally hot H$_2$, HD and D$_2$. The wave functi...

A non-linear spectroscopic study of the HDO molecule is performed in the wavelength range of 1.36–1.42 $\mathrm{\mu}$ m using noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy (NICE-OHMS). More than 100 rovibrational Lamb dips are recorded, with an experimental precision of 2–20 kHz, related to the first overtone of the O–H str...

Lamb-dips of R(0), R(2) and P(2) rovibrational transitions in the (2-0) overtone band of the HD molecule are measured at cryogenic temperatures using the technique of NICE-OHMS spectroscopy. Resulting transition frequencies are 214905335240(100) kHz for R(0), 219042856794(150) kHz for R(2) and 206898802150(150) kHz for P(2). Small, but systematic d...

A Grüneisen relationship is defined for gases, following the formulation of the original microscopic Grüneisen ratio γ = (dlnω)/(dlnV ) for solids. In the case of gases acoustic excitations represent the modes at frequency ω to be considered. By comparing to measured Brillouin shifts in various gases (SF 6 , N 2 O, and CO 2 ) and under various cond...

The R(1), R(3), and P(3) rovibrational transitions in the (2-0) overtone band of the HD molecule are measured in Doppler-free saturation using the technique of NICE-OHMS spectroscopy. For the P(3) line, hitherto not observed in saturation, we report a frequency of 203821936805(60) kHz. The dispersive line shapes observed in the three spectra show s...

A spectroscopic study of atomic oxygen (O i ) is performed using the Fourier-transform vacuum ultraviolet (VUV) spectrometer at the Dichroïsme Et Spectroscopie par Interaction avec le Rayonnement Synchrotron beamline of the SOLEIL synchrotron. Transition frequencies of O i lines in the range between 76,000 and 151,000 cm ⁻¹ , corresponding to wavel...

We present an investigation of the performance of the relativistic multireference Fock-space coupled-cluster (FSCC) method for predicting molecular hyperfine structure (HFS) constants, including a thorough computational study to estimate the associated uncertainties. In particular, we considered the F19 HFS constant in the ground and excited states...

The R(1), R(3) and P(3) ro-vibrational transitions in the (2-0) overtone band of the HD molecule are measured in Doppler-free saturation using the technique of NICE-OHMS spectroscopy. For the P(3) line, hitherto not observed in saturation, we report a frequency of $203\,821\,936\,805\,(60)$ kHz. The dispersive line shapes observed in the three spec...

Ro-vibronic spectra of the ¹³C¹⁸O carbon monoxide isotopologue were obtained with (i) emission spectroscopy in the visible region using a Bruker IFS 125HR spectrometer (University of Rzeszów) and (ii) vacuum-ultraviolet absorption spectroscopy using the wave-front-division spectrometer on the DESIRS beamline of the SOLEIL synchrotron. A deperturbat...

Precision measurement of anti-protonic Helium provides a test of physical laws.

The high-resolution B$^1\Sigma^+$ - A$^1\Pi$ (0, 0) and (0, 3) emission bands of the less-abundant $^{13}$C$^{17}$O isotopologue have been investigated by Fourier-transform spectroscopy in the visible region using a Bruker IFS 125HR spectrometer at an accuracy 0.003 cm$^{-1}$. These spectra are combined with high-resolution photoabsorption measurem...

Plasma produced from molten-tin microdroplets generates extreme ultraviolet light for state-of-the-art nanolithography. Currently, CO2 lasers are used to drive the plasma. In the future, solid-state mid-infrared lasers may instead be used to efficiently pump the plasma. Such laser systems have promise to be more compact, better scalable, and have h...

Accepted values of the masses of several subatomic particles have been under debate since recent measurements in Penning traps produced more precise yet incompatible results, implying possible inconsistencies in closely related physical constants like the proton-electron and deuteron-proton mass ratios. These quantities also influence the predicted...

Precise measurement of an atomic hydrogen transition resolves the proton size puzzle.

The technique of Noise-Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) is employed to detect rovibrational transitions of H216O at wavelengths of 1.4 μm. This intracavity-saturation approach narrows down the typical Doppler-broadened linewidths of about 600 MHz to the sub-MHz domain. The locking of the spectroscopy lase...

We present the results of the calibration of a channeltron-based electrostatic analyzer operating in time-of-flight mode (ESA-ToF) using tin ions resulting from laser-produced plasma, over a wide range of charge states and energies. Specifically, the channeltron electron multiplier detection efficiency and the spectrometer resolution are calibrated...

The ionization energy of D2 has been determined experimentally from measurements involving two-photon Doppler-free vacuum-ultraviolet pulsed laser excitation and near-infrared continuous-wave laser excitation to yield EI(D2)=124745.393739(26) cm−1. From this value, the dissociation energy of D2 is deduced to be D0(D2)=36748.362282(26) cm−1, represe...

The ionization energy of D$_2$ has been determined experimentally from measurements involving two-photon Doppler-free vacuum-ultraviolet pulsed laser excitation and near-infrared continuous-wave laser excitation to yield $E_\mathrm{I}(\mathrm{D}_2)=124\,745.393\,739(26)$ \wn. From this value, the dissociation energy of D$_2$ is deduced to be $D_0$(...

We present an investigation of the performance of the relativistic multi-reference Fock-space coupled cluster (FSCC) method for predicting molecular hyperfine structure (HFS) constants, including a thorough computational study to estimate the associated uncertainties. In particular, we considered the $^{19}$F HFS constant in the ground and excited...

Extreme ultraviolet (EUV) nanolithography relies on CO 2 -lasers to drive EUV-emitting tin plasma at 10.6-micrometer wavelength. We will present research using instead solid-state laser light, at 2-micrometer wavelength, to efficiently drive plasma from carefully preshaped targets.

The light source in extreme ultraviolet (EUV) lithography tools is a hot and dense laser-driven plasma. We will give an overview of our recent work on modeling the radiative and expansion characteristics of these plasmas.

A laser system based on three-wave mixing is used to generate laser-light at 2-micometer wavelength. This system is used for in-depth studies of the utility of 2-micrometer-driven sources of extreme ultraviolet (EUV) light in Nanolithography.

Active frequency-chirp compensation for a narrowband pulsed Titanium-Sapphire laser system is demonstrated using an intra-cavity electro-optic modulator resulting in improved spectral resolution and stability. With referencing to an optical frequency comb and further residual frequency chirp detection from shot-to-shot measurements, the resulting l...

Quasi-bound resonances of H2 are produced via two-photon photolysis of H2S molecules as reactive intermediates or transition states and detected before decay of the parent molecule into three separate atoms. As was previously reported [K. F. Lai et al., Phys. Rev. Lett. 127, 183001 (2021)] four centrifugally bound quantum resonances with lifetimes...

We present a calibrated spectrum in the 5.5–265.5 nm range from a microdroplet-tin Nd:YAG-laser-produced plasma under conditions relevant for the production of extreme ultraviolet (EUV) light at 13.5 nm for nanolithography. The plasma emission spectrum obtained using a custom-built transmission grating spectrometer results from a careful calibratio...

Quasi-bound resonances of H$_2$ are produced via two-photon photolysis of H$_2$S molecules as reactive intermediates or transition states, and detected before decay of the parent molecule into three separate atoms. As was previously reported [K.F. Lai et al., Phys. Rev. Lett. 127, 183001 (2021)] four centrifugally bound quantum resonances with life...

Shape resonances in H2, produced as reaction intermediates in the photolysis of H2S precursor molecules, are measured in a half-collision approach. Before disintegrating into two ground state H atoms, the reaction is quenched by two-photon Doppler-free excitation to the F electronically excited state of H2. For J=13, 15, 17, 19, and 21, resonances...

We report on the electrostatic trapping of neutral SrF molecules. The molecules are captured from a cryogenic buffer-gas beam source into the moving traps of a 4.5-m-long traveling-wave Stark decelerator. The SrF molecules in X^{2}Σ^{+}(v=0,N=1) state are brought to rest as the velocity of the moving traps is gradually reduced from 190 m/s to zero....

We present the results of a joint experimental and theoretical study of plasma expansion arising from Nd:YAG laser ablation (laser wavelength λ = 1.064 μm) of tin microdroplets in the context of extreme ultraviolet lithography. Measurements of the ion energy distribution reveal a near-plateau in the distribution for kinetic energies in the range 0....

Shape resonances in H$_2$, produced as reaction intermediates in the photolysis of H$_2$S precursor molecules, are measured in a half-collision approach. Before desintegrating into two ground state H atoms, the reaction is quenched by two-photon Doppler-free excitation to the F electronically excited state of H$_2$. For $J=13,15,17,19$ and 21, reso...

Huge efforts have recently been taken in the derivation of accurate compilations of rovibrational energies of water, one of the most important reference systems in spectroscopy. Such precision is desirable for all water isotopologues, although their investigation is challenged by hyperfine effects in their spectra. Frequency-comb locked noise-immun...

A measurement of the magnitude of the electric dipole moment of the electron (eEDM) larger than that predicted by the Standard Model (SM) of particle physics is expected to have a huge impact on the search for physics beyond the SM. Polar diatomic molecules containing heavy elements experience enhanced sensitivity to parity (P) and time-reversal (T...

The rotational structure of the A¹Π(v = 2) level of ¹²C¹⁸O is re-examined using high-accuracy experimental data comprised of 541 molecular lines obtained by two complementary Fourier-transform techniques. The far ultraviolet absorption spectrum of the A¹Π – X¹Σ⁺(2, 0) band, in the range 66,500 - 67,650 cm–1, was recorded by the vacuum-ultraviolet F...

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µ m-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µ m) pulse creates...

We present a parts-per-million test of quantum electrodynamics (QED) in the HD+ molecular hydrogen ion, improving on previous tests based on vibrational and rotational transitions by factors of 76 and 1.4, respectively. The test is performed following a unified statistical approach that also produces improved constraints on physics beyond the stand...

Taking advantage of the extreme absolute accuracy, sensitivity, and resolution of noise-immune-cavity-enhanced optical-heterodyne-molecular spectroscopy (NICE-OHMS), a variant of frequency-comb-assisted Lamb-dip saturation-spectroscopy techniques, the rotational quantum-level structure of both nuclear-spin isomers of H218O is established with an av...

Context. Methanol is an important tracer to probe physical and chemical conditions in the interstellar medium of galaxies. Methanol is also the most sensitive target molecule for probing potential space-time variations of the proton-electron mass ratio, μ , a dimensionless constant of nature. Aims. We present an extensive study of the strongest sub...

Methanol is an important tracer to probe physical and chemical conditions in the interstellar medium of galaxies. Methanol is also the most sensitive target molecule for probing potential space-time variations of the proton-electron mass ratio, mu, a dimensionless constant of nature. We present an extensive ALMA study of the strongest submillimeter...

A measurement of the magnitude of the electric dipole moment of the electron (eEDM) larger than that predicted by the Standard Model (SM) of particle physics is expected to have a huge impact on the search for physics beyond the SM. Polar diatomic molecules containing heavy elements experience enhanced sensitivity to parity ($P$) and time-reversal...

Experimental spectroscopic studies are presented, in a 5.5–25.5 nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2−μm-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1−μm-wavelength laser light over a range o...

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2-$\mu$m-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2$\mu$m) pulse cre...

A supersonic beam source for SrF and BaF molecules is constructed by combining the expansion of carrier gas (a mixture of 2% SF6 and 98% argon) from an Even-Lavie valve with laser ablation of a barium/strontium metal target at a repetition rate of 10 Hz. Molecular beams with a narrow translational velocity spread are produced at relative values of...

An experimental study of laser-produced plasmas is performed by irradiating a planar tin target by laser pulses, of 4.8 ns duration, produced from a KTP-based 2-µm-wavelength master oscillator power amplifier. Comparative spectroscopic investigations are performed for plasmas driven by 1-µm- and 2-µm-wavelength pulsed lasers, over a wide range of l...

Rovibrational quantum states in the X1Σ g + electronic ground state of H2 are prepared in the v = 13 vibrational level up to its highest bound rotational level J = 7, and in the highest bound vibrational level v = 14 (for J = 1) by two-photon photolysis of H2S. These states are laser-excited in a subsequent two-photon scheme into F1Σ g + outer well...

Rovibrational quantum states in the $X^1\Sigma_g^+$ electronic ground state of H$_2$ are prepared in the $v=13$ vibrational level up to its highest bound rotational level $J=7$, and in the highest bound vibrational level $v=14$ (for $J=1$) by two-photon photolysis of H$_2$S. These states are laser-excited in a subsequent two-photon scheme into $F^1...

The spectral distribution of light scattered by microscopic thermal fluctuations in binary mixture gases was investigated experimentally and theoretically. Measurements of Rayleigh-Brillouin spectral profiles were performed at a wavelength of 532 nm and at room temperature, for mixtures of SF6−He,SF6−D2, and SF6−H2. In these measurements, the press...

Experimental spectroscopic studies are presented, in a 5.5--25.5nm extreme-ultraviolet (EUV) wavelength range, of the light emitted from plasma produced by the irradiation of tin microdroplets by 5-ns-pulsed, 2-$\mu$m-wavelength laser light. Emission spectra are compared to those obtained from plasma driven by 1-$\mu$m-wavelength Nd:YAG laser light...

The spectral distribution of light scattered by microscopic thermal fluctuations in binary mixture gases was investigated experimentally and theoretically. Measurements of Rayleigh-Brillouin spectral profiles were performed at a wavelength of 532 nm and at room temperature, for mixtures of SF$_6-$He, SF$_6-$D$_2$ and SF$_6-$H$_2$. In these measurem...