Showing papers on "Buffer gas published in 2020"

Buffer gas cooling of a trapped ion to the quantum regime

Abstract: Great advances in precision measurements in the quantum regime have been achieved with trapped ions and atomic gases at the lowest possible temperatures1–3. These successes have inspired ideas to merge the two systems4. In this way, we can study the unique properties of ionic impurities inside a quantum fluid5–12 or explore buffer gas cooling of a trapped-ion quantum computer13. Remarkably, in spite of its importance, experiments with atom–ion mixtures have remained firmly confined to the classical collision regime14. We report a collision energy of 1.15(±0.23) times the s-wave energy (or 9.9(±2.0) μK) for a trapped ytterbium ion in an ultracold lithium gas. We observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum effects in the atom–ion collisions. Our results open up numerous opportunities, such as the exploration of atom–ion Feshbach resonances15,16, in analogy to neutral systems17. Cooling an atom–ion hybrid system and bringing it into the quantum regime is challenging owing to the unavoidable heating caused by atom–ion collisions. Here a new record low is achieved in such a system, and the quantum effect starts to manifest.

Buffer-gas cooling, high-resolution spectroscopy, and optical cycling of barium monofluoride molecules

Abstract: We demonstrate buffer-gas cooling, high-resolution spectroscopy and cycling fluorescence of cold barium monofluoride (BaF) molecules. Our source produces an intense and internally cold molecular beam containing the different BaF isotopologues with a mean forward velocity of 190 m/s. For a well-collimated beam of 138BaF we observe a flux of more than 1e10 molecules/sr/pulse in the X2Sigma, N=1 state in our downstream detection region. Studying the absorption line strength of the intermediate A'Delta state we infer a lifetime of 790+\-346 ns, significantly longer than previously estimated. Finally, highly-diagonal Franck-Condon factors and magnetic remixing of dark states allow us to realize a quasi-cycling transition in 138BaF that is suitable for future laser cooling of this heavy diatomic molecule.

Beam thermalization in a large gas catcher

Abstract: Thermalization of fast ions in a buffer gas provides a method to transform the high-energy, exotic beams produced by projectile fragmentation at the National Superconducting Cyclotron Laboratory (NSCL) into low-energy beams. The process includes slowing down the fast exotic beams in solid degraders combined with momentum compression and removal of the remaining kinetic energy by collisions with a buffer gas. The original beam thermalization area for mass measurements at the NSCL was reconfigured to accommodate a new momentum compression beam line, a large radio-frequency (RF) gas catcher constructed by Argonne National Lab and a low-energy beam transport system. A large variety of exotic isotopes produced by projectile fragmentation and selected by the A1900 fragment separator was thermalized in the 1.2 m long gas catcher filled with helium at approximately 100 mbar. The ions were guided to an extraction nozzle with a combination of electrostatic and RF potentials and ejected by the gas flow. A novel RF ion guide was used in a differential pumping system to remove the helium and transport the ions into ultrahigh vacuum. Finally, the ions were accelerated to 30 kV for transport to various experiments. The distribution of the thermalized ions among chemical adducts is one of the operational challenges. The important steps implemented to minimize the production of the chemical adducts in the gas catcher are discussed. The operational status of the facility and some example results from characterization of the gas catcher operation with 37 K and 47 K beams are presented.

The laser power effect on the performance of gas leak detector based on laser photo-acoustic spectroscopy

Abstract: The use of a homemade gas leak detector based on laser photo-acoustic spectroscopy for fast and sensitive trace gas detection is reported. To obtain optimal acoustic resonator dimensions, resonant frequency variations in terms of resonator's length and radius were simulated, and then a gas leak detector based on photo-acoustic theory was designed and fabricated. The system limit of detection to trace NO2, SO2 and SF6 was 92, 270 and 8 ppb respectively. Therefore, this system can be used as a sensitive gas sensor for detecting partial leakage of some environmental and industrial pollutants. Variations of the resonant frequency versus various concentrations of NO2, SO2 and SF6 in the presence of different buffer gases were studied, and results showed that the lighter buffer gas gives a higher resonance frequency. Also, variations of the photo-acoustic signal and SNR of these gas samples in terms of various CO2 laser powers in the presence of different buffer gases were investigated, and the saturation laser power for each experiment was obtained. Finally, it was shown that experimental results are in good agreement with photo-acoustic theory.

Assessing the Impact of Drift Gas Polarizability in Polyatomic Ion Mobility Experiments.

Abstract: Due to the core assumptions of kinetic theory and the drive toward realizing reproducible gas-phase measurements, ion mobility experiments are commonly conducted in the presence of an inert, neat buffer gas, usually nitrogen or helium. Mixing drift gases in defined, static ratios can provide useful information not only for optimizing the separation of analytes but also for defining the interaction between the ion and neutral particle. In a foundational effort, we seek to validate the role of the drift gas polarizability on the observed mobility of the ions by systematically mixing drift gases to discretely access a range of bulk gas polarizabilities not given by pure drift gases. Compared to historical efforts to probe the role of polarizability on the ion-neutral collisional cross section where a linear relationship was assumed, the data collected in the present effort clearly illustrate a quadratic dependency of the ion-neutral particle collision cross section and polarizability (R2 > 0.999). When translating these data into the mobility dimension, we illustrate that the gas-phase mobility of polyatomic ions conforms to Blanc's law. These observations combined with considerations related to Langevin's polarization limit provide an experimental mechanism to estimate to what degree an ion-neutral interaction conforms to either the hard-sphere or induced-dipole model. To support these observations, additional comparisons are made with the respective reduced masses, polarizabilities, and mobilities of ions in mixtures where different degrees of hard-sphere interactions are present.

Rabi Resonances in Buffer-Gas-Filled Cs-Vapor Cells for SI-Traceable Microwave Magnetic Field Detection

Abstract: We describe a microwave (MW) magnetic-field detection technique based on the Rabi resonance of a Cs-buffer gas vapor cell. Inside the vapor cell, alkali atoms interact with MWs radiated from an antenna through an open-ended rectangular waveguide. We obtained the Rabi resonance line shape, which determines precisely the Rabi frequency of the MW magnetic field with a phase modulation frequency. For various vapor cells, a detailed characterization of several experimental parameters and a theoretical analysis were performed. The optimal laser intensity and the cell temperature that maximize the Rabi resonance amplitude are obtained. Moreover, the cell-temperature-based frequency shift of the Rabi resonance was investigated. These experimental results are important in developing future International System of Units-traceable MW magnetic field sensors and an MW power standard.

Diagnostics of CO concentration in gaseous mixtures at elevated pressures by resonance enhanced multi-photon ionization and microwave scattering

Abstract: In this work, a novel diagnostic technique for carbon monoxide (CO) number density measurements in a nitrogen buffer mixture at elevated pressures up to 5 bar was developed and tested. The technique utilizes 2 + 1 resonance enhanced multi-photon ionization (REMPI) of CO induced by a femtosecond laser pulse at 230.1 nm, followed by detection of the number of REMPI-induced electrons using the microwave scattering (MS) method (REMPI-MS technique). Dependences of the number of REMPI-generated electrons on CO number density and laser energy were measured and analyzed in conjunction with a four energy level model of the CO molecule. The number of REMPI-induced electrons scaled linearly with CO number density up to about 5 × 1018 cm−3 and was independent of the buffer gas pressure up to 5 bar. Higher CO number densities caused saturation onset associated with laser beam energy loss while travelling through the gaseous mixture due to two-photon absorption and photoionization. The number of REMPI-induced electrons was found to scale cubically with the laser pulse energy for the tested energy range of 8–20 μJ (intensity in the focal region about 7–18 GW/cm2), which is consistent with the operation regime where the number density of excited CO molecules increases throughout the laser pulse duration and does not saturate in time. The linear scaling region of the REMPI-MS signal can be used for a CO number density diagnostic after appropriate calibration of the system.

Buffer-gas cooling of molecules in the low-density regime: comparison between simulation and experiment

Abstract: Cryogenic buffer gas cells have been a workhorse for the cooling of molecules in the last decades. The straightforward sympathetic cooling principle makes them applicable to a huge variety of different species. Notwithstanding this success, detailed simulations of buffer gas cells are rare, and have never been compared to experimental data in the regime of low to intermediate buffer gas densities. Here, we present a numerical approach based on a trajectory analysis, with molecules performing a random walk in the cell due to collisions with a homogeneous buffer gas. This method can reproduce experimental flux and velocity distributions of molecules emerging from the buffer gas cell for varying buffer gas densities. This includes the strong decrease in molecule output from the cell for increasing buffer gas density and the so-called boosting effect, when molecules are accelerated by buffer-gas atoms after leaving the cell. The simulations provide various insights which could substantially improve buffer-gas cell design.

Optical isolator based on highly efficient optical pumping of Rb atoms in a miniaturized vapor cell

Abstract: We demonstrate both numerically and experimentally the important role of buffer gas in achieving optical isolator based on efficient hyperfine-structure optical pumping of Rubidium (87Rb) atoms in miniaturized vapor cells. It is observed that the increase in the buffer gas pressure allows achieving highly efficient optical pumping. Specifically, at a buffer gas pressure of 40 Torr, it is possible to pump about 85% of the atoms out of the pumped state into the other ground state. While optical pumping has been reported for conventional centimeter-scale cells, we show that in miniaturized cells the role of buffer gas is crucial, as the higher collision rate with the walls diminishes the effect of optical pumping, posing a stringent limitation on the applicability of optical pumping in miniaturized systems. Following the results reported herein, we demonstrate an optical isolator with isolation ratio better than 20 dB at a relatively low temperature of about 〖80〗^ C, and with a low magnetic field (~800 Gauss). The obtained results provide another step forward in the quest for miniaturizing quantum devices. The demonstrated device shows that such miniaturized vapor cells can be considered for applications such as all-optical switching, optical quantum memory, frequency references, and magnetometry to name a few.

Design and performance of an E-band chirped pulse spectrometer for kinetics applications: OCS – He pressure broadening

Abstract: Product channel-specific reaction kinetics at low temperature presents a significant challenge for both experiment and theory but provides essential inputs for astrochemical models of cold interstellar environments. Reaction kinetics studies using chirped pulse Fourier transform microwave (CP-FTMW) spectroscopy provide a potential solution but require the use of a buffer gas to thermalize the reactants and products; however, collisions with the buffer gas reduce the length of the detected signal, the free induction decay, through pressure broadening. The effect of this on time domain signals is largely unexplored using CP-FTMW spectrometers. A new E-band (60–90 GHz) spectrometer has been constructed using previously unavailable equipment in order to maximize its performance for detecting molecules in collisional environments. The design of this spectrometer is described in detail. The pressure broadening of OCS in He was used to test the spectrometer under collisional conditions similar to those that are routinely used to perform reaction kinetics measurements. The corresponding pressure broadening coefficients for transitions in this frequency range were determined at room temperature and the performance of the spectrometer was assessed in relation to recently reported chirped pulse in uniform flow experiments.

Simulation of Cryogenic Buffer Gas Beams

Abstract: The cryogenic buffer gas beam (CBGB) is an important tool in the study of cold and ultracold molecules. While there are known techniques to enhance desired beam properties, such as high flux, low velocity, or reduced divergence, they have generally not undergone detailed numerical optimization. Numerical simulation of buffer gas beams is challenging, as the relevant dynamics occur in regions where the density varies by orders of magnitude, rendering standard numerical methods unreliable or intractable. Here, we present a hybrid approach to simulating CBGBs that combines gas dynamics methods with particle tracing. The simulations capture important properties such as velocities and divergence across an assortment of designs, including two-stage slowing cells and de Laval nozzles. This approach should therefore be a useful tool for optimizing CBGB designs across a wide range of applications.

In Situ Monitoring of Heterogeneous Catalytic Hydrogenation via 129Xe NMR Spectroscopy and Proton MRI

Abstract: The ability to use molecular hydrogen, H2, as a buffer gas in spin exchange optical pumping of noble gases enables the production of hydrogen gas containing a low percentile (5%) of hyperpolarized ...

Synthesizing Germanium Nanotubes in an Electric Arc Plasma

Abstract: An original technique for the synthesis of germanium nanostructures in an electric arc argon plasma is described. The plasma–chemical synthesis of germanium nanotubes up to 100 μm long and up to 1 μm in diameter is performed by selecting the strength of the electric current, the pressure of the buffer gas, and the interelectrode distance. Individual nanotubes reach lengths of several mm. Unlike germanium nanotubes grown via CVD, the germanium nanotubes synthesized in this work have distinct shapes and sizes. They are separated from each other and can be easily manipulated. It is shown that this technique allows both germanium nanotubes and germanene to be grown.

Experimental setup for studying an ultracold mixture of trapped Yb + – Li 6

Abstract: We describe and characterize an experimental apparatus that has been used to study interactions between ultracold lithium atoms and ytterbium ions. The preparation of ultracold clouds of Li atoms is described as well as their subsequent transport and overlap with ${\mathrm{Yb}}^{+}$ ions trapped in a Paul trap. We show how the kinetic energy of the ion after interacting with the atoms can be obtained by laser spectroscopy. We analyze the dynamics of the buffer-gas-cooled ion after releasing the atoms, which indicates that background heating, due to electric-field noise, limits attainable buffer gas cooling temperatures. This effect can be mitigated by increasing the density of the Li gas in order to improve its cooling power. Imperfections in the Paul trap lead to so-called excess micromotion, which poses another limitation to the buffer gas cooling. We describe in detail how we measure and subsequently minimize excess micromotion in our setup. We measure the effect of excess micromotion on attainable ion temperatures after buffer gas cooling and compare this to molecular dynamics simulations, which describe the observed data very well.

Buffer-gas cooling of molecules in the low-density regime: Comparison between simulation and experiment

Abstract: Cryogenic buffer gas cells have been a workhorse for the cooling of molecules in the last decades. The straightforward sympathetic cooling principle makes them applicable to a huge variety of different species. Notwithstanding this success, detailed simulations of buffer gas cells are rare, and have never been compared to experimental data in the regime of low to intermediate buffer gas densities. Here, we present a numerical approach based on a trajectory analysis, with molecules performing a random walk in the cell due to collisions with a homogeneous buffer gas. This method can reproduce experimental flux and velocity distributions of molecules emerging from the buffer gas cell for varying buffer gas densities. This includes the strong decrease in molecule output from the cell for increasing buffer gas density and the so-called boosting effect, when molecules are accelerated by buffer-gas atoms after leaving the cell. The simulations provide various insights which could substantially improve buffer-gas cell design.

Towards a He-buffered laser ablation ion source for collinear laser spectroscopy

Abstract: Laser ablation opens a material-independent method to produce ions from transition metals for laser spectroscopy. To overcome some drawbacks of this process, an ion source is under development at TU Darmstadt. A distinctive feature of this source is that ions are produced via laser ablation in presence of helium buffer gas where they stop and cool in the process of their collisions with the buffer gas atoms and are then extracted by the gas flow into low-pressure conditions through the supersonic nozzle. The compact RF-only funnel ion guide placed on the axis behind the nozzle exit allows for effective extraction of high-quality ion beams into a pressure region below 10− 4 mbar. The extraction is realized by using the gas flow trough a supersonic nozzle and an RF-only funnel ion guide, followed by a second nozzle and an RF+DC funnel representing two differential pumping stages. The technical details of this laser ablation ion source are described and the results of the first tests with the RF-only funnel are presented.

On the formation of van der Waals complexes through three-body recombination

Abstract: In this work, we show that van der Waals molecules X-RG (where RG is the rare gas atom) may be created through direct three-body recombination collisions, i.e., X + RG + RG $\rightarrow$ X-RG + RG. In particular, the three-body recombination rate at temperatures relevant for buffer gas cell experiments is calculated via a classical trajectory method in hyperspherical coordinates [J. Chem. Phys. 140, 044307 (2014)]. As a result, it is found that the formation of van der Waals molecules in buffer gas cells (1 K $\lesssim T \lesssim 10$ K) is dominated by the long-range tail (distances larger than the LeRoy radius) of the X-RG interaction. For higher temperatures, the short-range region of the potential becomes more significant. Moreover, we notice that the rate of formation of van der Walls molecules is of the same order of magnitude independently of the chemical properties of X. As a consequence, almost any X-RG molecule may be created and observed in a buffer gas cell under proper conditions.

High-precision measurements of krypton and xenon isotopes with a new static-mode Quadrupole Ion Trap Mass Spectrometer.

Abstract: Measuring the abundance and isotopic composition of noble gases in planetary atmospheres can answer fundamental questions in cosmochemistry and comparative planetology. However, noble gases are rare elements, a feature making their measurement challenging even on Earth. Furthermore, in space applications, power consumption, volume and mass constraints on spacecraft instrument accommodations require the development of compact innovative instruments able to meet the engineering requirements of the mission while still meeting the science requirements. Here we demonstrate the ability of the quadrupole ion trap mass spectrometer (QITMS) developed at the Jet Propulsion Laboratory (Caltech, Pasadena) to measure low quantities of heavy noble gases (Kr, Xe) in static operating mode and in the absence of a buffer gas such as helium. The sensitivity reaches 1E13 cps Torr-1 (about 1011 cps/Pa) of gas (Kr or Xe). The instrument is able to measure gas in static mode for extended periods of time (up to 48 h) enabling the acquisition of thousands of isotope ratios per measurement. Errors on isotope ratios follow predictions of the counting statistics and the instrument provides reproducible results over several days of measurements. For example, 1.7E-10 Torr (2.3E-8 Pa) of Kr measured continuously for 7 hours yielded a 0.6 permil precision on the 86Kr/84Kr ratio. Measurements of terrestrial and extraterrestrial samples reproduce values from the literature. A compact instrument based upon the QITMS design would have a sensitivity high enough to reach the precision on isotope ratios (e.g. better than 1 percent for 129,131-136Xe/130Xe ratios) necessary for a scientific payload measuring noble gases collected in the Venus atmosphere.

MnCl2 laser with pulse repetition frequency up to 125 kHz

Abstract: This paper considers the operating parameters of 24 cm3 active volume MnCl2-laser in a wide range of PRF (pulse repetition frequency). The maximum average power reaches 1 W at 17.6 kHz PRF and 0.2% efficiency. The laser generation on manganese metal at 125 kHz PRF has been obtained for the first time. The pumping conditions under which the output power of the visible spectral range starts exceeding the output power at IR lines have been determined. In particular, the electric field strength is calculated for the given GDT (gas discharge tube) parameters and buffer gas pressure. The dependence between the output power and pumping power is shown to be linear. The condition provides the change in IR and VIS output power relation is determined.

Analytical Calculation of the Composition of Thermal Dusty Plasma with Metal Particles

Abstract: For a thermal dusty plasma with metal particles, the relations of the particle charge to the temperature of the system and the particle size have been obtained using a two-plasma model and the approximation of a rectangular potential well. The number densities of electrons and ions in the plasma have been determined for various temperatures and particle sizes of the system. It has been shown that in the case of metal particles, the ionization of the buffer gas leads to underestimation of the charge in comparison with the case when the ionization of the gas is negligible.

Collision Cross Section Calculations Using HPCCS.

Abstract: A technical overview of the High Performance Collision Cross Section (HPCCS) software for accurate and efficient calculations of collision cross sections for molecular ions ranging from small organic molecules to large protein complexes is presented. The program uses helium or nitrogen as buffer gas with considerable gains in computer time compared to publicly available codes under the Trajectory Method approximation. HPCCS is freely available under the Academic Use License at https://github.com/cepid-cces/hpccs .

Time-Resolved Measurements and Master Equation Modelling of the Unimolecular Decomposition of CH3OCH2

Abstract: The rate coefficient for the unimolecular decomposition of CH3OCH2, k1, has been measured in time-resolved experiments by monitoring the HCHO product. CH3OCH2 was rapidly and cleanly generated by 248 nm excimer photolysis of oxalyl chloride, (ClCO)2, in an excess of CH3OCH3, and an excimer pumped dye laser tuned to 353.16 nm was used to probe HCHO via laser induced fluorescence. k1(T,p) was measured over the ranges: 573–673 K and 0.1–4.3 × 1018 molecule cm−3 with a helium bath gas. In addition, some experiments were carried out with nitrogen as the bath gas. Ab initio calculations on CH3OCH2 decomposition were carried out and a transition-state for decomposition to CH3 and H2CO was identified. This information was used in a master equation rate calculation, using the MESMER code, where the zero-point-energy corrected barrier to reaction, ΔE0,1, and the energy transfer parameters, ⟨ΔEdown⟩ × Tn, were the adjusted parameters to best fit the experimental data, with helium as the buffer gas. The data were combined with earlier measurements by Loucks and Laidler (Can J. Chem.1967, 45, 2767), with dimethyl ether as the third body, reinterpreted using current literature for the rate coefficient for recombination of CH3OCH2. This analysis returned ΔE0,1 = (112.3 ± 0.6) kJ mol−1, and leads to k∞1(T)=2.9×1012 (T/300)2.5 exp(−106.8 kJ mol−1/RT). Using this model, limited experiments with nitrogen as the bath gas allowed N2 energy transfer parameters to be identified and then further MESMER simulations were carried out, where N2 was the buffer gas, to generate k1(T,p) over a wide range of conditions: 300–1000 K and N2 = 1012–1025 molecule cm−3. The resulting k1(T,p) has been parameterized using a Troe-expression, so that they can be readily be incorporated into combustion models. In addition, k1(T,p) has been parametrized using PLOG for the buffer gases, He, CH3OCH3 and N2.

Modeling characterisation of a bipolar pulsed discharge

Abstract: We apply particle based kinetic simulations to explore the characteristics of a low-pressure gas discharge driven by high-voltage (similar to kV) pulses with alternating polarity, with a duty cycle of approximate to 1% and a repetition rate of 5 kHz. The computations allow tracing the spatio-temporal development of several discharge characteristics, the potential and electric field distributions, charged particle densities and fluxes, the mean ion energy at the electrode surfaces, etc. As such discharges have important surface processing applications, e.g. in the treatment of artificial bones, we analyse the time-dependence of the flux and the mean energy of the ions reaching the electrode surfaces, which can be both conducting and dielectric. Our investigations are conducted for argon buffer gas in the 40-140 Pa pressure range, for 1-5 cm electrode gaps and voltage pulse amplitudes ranging between 600 V and 1200 V.

Electromagnetically induced absorption resonances in Hanle-configuration prepared in a paraffin coated 87Rb cell

Abstract: We present an experimental investigation of electromagnetically-induced absorption (EIA) at the D1 line of 87Rb contained in an anti-relaxation coated vacuum optical cell. The configuration includes a pump and a probe beam propagating in opposite directions and having mutually orthogonal linear polarizations; the probe beam absorption is registered depending on the value of a magnetic field scanned around zero and applied collinearly to the laser beams. The advantages of this scheme have been recently evidenced in vapor cells filled with a buffer gas. In the present work we studied the width and the contrast of the EIA resonances obtained in a coated cell for different values of the pump power. The results are compared with those obtained in a buffer gas cell for the same transition of 87Rb. The theoretical calculations are in good agreement with the experiment.

Making ions cooler

Abstract: Cooling of trapped ions with a neutral buffer gas makes the study of atom–ion hybrid systems possible in the quantum regime. The new record low achieved opens the door to numerous opportunities, including full control over the atom–ion interactions.

Velocity dependence of the performance of flowing-gas K DPAL with He and He/CH4 buffer gases: 3D CFD modeling and comparison with experimental results

Abstract: Accurate 3D computational fluid dynamics (CFD) modeling of flowing-gas K DPAL is presented, taking into account ionization and ion–electron recombination processes, ambipolar diffusion of K ions, and electron heating. Whereas in a static K DPAL with He buffer gas, the neutral K atoms in the lasing medium are depleted by these processes, the depletion can be mitigated by application of gas flow. The lowest gas velocity necessary for effective operation of a laser with He buffer is ∼500m/s, and is much higher than previously estimated [Opt. Express25, 30793 (2017)OPEXFF1094-408710.1364/OE.25.030793]. The predictions of the model for different He/CH4 mixtures are presented and verified by comparing them with experimental results obtained at the Air Force Institute of Technology [“Kinetics of higher lying potassium states after excitation of the D2 transition in the presence of helium,” dissertation (Air Force Institute of Technology, 2018)].