Showing papers on "Buffer gas published in 2014"

Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas

Abstract: In combination with sympathetic cooling of translational degrees of freedom (leading to Coulomb crystallization), cooling of the rotational degrees of freedom of magnesium hydride ions using a helium buffer gas leads to temperatures in a tunable range from 60 kelvin down to about 7 kelvin for a single ion, the lowest such temperature so far recorded. Cold molecules in quantities sufficient for study are in demand in a number of fields, ranging from fundamental physics to astrochemistry. Anders Hansen et al. demonstrate how a combination of techniques — helium buffer gas cooling (for internal rotational states) and 'sympathetic' collision cooling (for translational states) — can be used to achieve efficient cooling of molecular ions in all degrees of freedom. Most notable is the effectiveness of the technique for rotational cooling, which enhances the prospects for undertaking such studies on larger molecules. The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations1,2, high-resolution spectroscopy of complex molecules3,4,5, cold chemistry6,7 and astrochemistry8. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion9,10 or a cryogenic trap environment11,12. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation6,7,13,14,15,16,17,18,19,20,21,22,23. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH+ molecular ions, which have been trapped and sympathetically cooled13 in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second—that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings—we have cooled a single molecular ion to a rotational temperature of kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions24. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy25 of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest4,5.

Neutral gas sympathetic cooling of an ion in a Paul trap.

Abstract: A single ion immersed in a neutral buffer gas is studied. An analytical model is developed that gives a complete description of the dynamics and steady-state properties of the ions. An extension of this model, using techniques employed in the mathematics of economics and finance, is used to explain the recent observation of non-Maxwellian statistics for these systems. Taken together, these results offer an explanation of the long-standing issues associated with sympathetic cooling of an ion by a neutral buffer gas.

Cooling, spectroscopy and non-sticking of trans-stilbene and Nile Red.

Abstract: We create and study trans-Stilbene and Nile Red in a cryogenic (7 K) cell with a low density helium buffer gas. No molecule-helium cluster formation is observed, indicating limited atom-molecule sticking in this system. We place an upper limit of 5 % on the population of clustered He-trans-Stilbene, consistent with a measured He-molecule collisional residence time of less than 1 μs. With its very low energy torsional modes, trans-Stilbene is less rigid than any molecule previously buffer-gas-cooled into the Kelvin regime. We also report cooling and gas phase visible spectroscopy of Nile Red, a much larger molecule. Our data suggest that buffer gas cooling will be feasible for a variety of small biological molecules.

Internal state thermometry of cold trapped molecular anions

Abstract: Photodetachment spectroscopy of OH- and H3O2- anions has been performed in a cryogenic 22-pole radiofrequency multipole trap. Measurements of the detachment cross section as a function of laser frequency near threshold have been analysed. Using this bound-free spectroscopy approach we could demonstrate rotational and vibrational cooling of the trapped anions by the buffer gas in the multipole trap. Below 50 K the OH- rotational temperature shows deviations from the buffer gas temperature, and possible causes for this are discussed. For H3O2- vibrational cooling of the lowest vibrational quantum states into the vibrational ground state is observed. Its photodetachment cross section near threshold is modelled with a Franck-Condon model, with a detachment threshold that is lower, but still in agreement with the expected threshold for this system.

Buffer gas loaded magneto-optical traps for Yb, Tm, Er and Ho

Abstract: Direct loading of lanthanide atoms into magneto-optical traps (MOTs) from a very slow cryogenic buffer gas beam source is achieved, without the need for laser slowing. The beam source has an average forward velocity of 60– and a velocity half-width of , which allows for direct MOT loading of Yb, Tm, Er and Ho. Residual helium background gas originating from the beam results in a maximum trap lifetime of about 80 ms (with Yb). The addition of a single-frequency slowing laser applied to the Yb in the buffer gas beam increases the number of trapped Yb atoms to with a loading rate of . Decay to metastable states is observed for all trapped species and decay rates are measured. Extension of this approach to the loading of molecules into a MOT is discussed.

A second-generation constrained reaction volume shock tube

Abstract: We have developed a shock tube that features a sliding gate valve in order to mechanically constrain the reactive test gas mixture to an area close to the shock tube endwall, separating it from a specially formulated non-reactive buffer gas mixture. This second-generation Constrained Reaction Volume (CRV) strategy enables near-constant-pressure shock tube test conditions for reactive experiments behind reflected shocks, thereby enabling improved modeling of the reactive flow field. Here we provide details of the design and operation of the new shock tube. In addition, we detail special buffer gas tailoring procedures, analyze the buffer/test gas interactions that occur on gate valve opening, and outline the size range of fuels that can be studied using the CRV technique in this facility. Finally, we present example low-temperature ignition delay time data to illustrate the CRV shock tube's performance.

The cryogenic gas stopping cell of SHIPTRAP

Abstract: The overall efficiency of the Penning-trap mass spectrometer SHIPTRAP at GSI Darmstadt, employed for high-precision mass measurements of exotic nuclei in the mass region above fermium, is presently mostly limited by the stopping and extraction of fusion-evaporation products in the SHIPTRAP gas cell. To overcome this limitation a second-generation gas cell with increased stopping volume was designed. In addition, its operation at cryogenic temperatures leads to a higher gas density at a given pressure and an improved cleanliness of the helium buffer gas. Here, the results of experiments with a 219 Rn recoil ion source are presented. An extraction efficiency of 74(3)% was obtained, a significant increase compared to the extraction efficiency of 30% of the present gas stopping cell operated at room temperature. The optimization of electric fields and other operating parameters at room as well as cryogenic temperatures is described in detail. Furthermore, the extraction time of 219 Rn ions was determined for several operating parameters.

Production of cold beams of ND3 with variable rotational state distributions by electrostatic extraction of He and Ne buffer-gas-cooled beams.

Abstract: The measurement of the rotational state distribution of a velocity-selected, buffer-gas-cooled beam of ND3 is described In an apparatus recently constructed to study cold ion-molecule collisions, the ND3 beam is extracted from a cryogenically cooled buffer-gas cell using a 215 m long electrostatic quadrupole guide with three 90° bends (2+1) resonance enhanced multiphoton ionization spectra of molecules exiting the guide show that beams of ND3 can be produced with rotational state populations corresponding to approximately Trot = 9–18 K, achieved through manipulation of the temperature of the buffer-gas cell (operated at 6 K or 17 K), the identity of the buffer gas (He or Ne), or the relative densities of the buffer gas and ND3 The translational temperature of the guided ND3 is found to be similar in a 6 K helium and 17 K neon buffer-gas cell (peak kinetic energies of 692(013) K and 590(001) K, respectively) The characterization of this cold-molecule source provides an opportunity for the first ex

Kennard-Stepanov relation connecting absorption and emission spectra in an atomic gas.

Abstract: The Kennard-Stepanov relation describes a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. Here we examine absorption and emission spectra of rubidium atoms in a dense argon buffer gas environment. We demonstrate that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled in the collisionally broadened atomic gas system. Our experimental findings are supported by a simple theoretical model.

Improving Ion Mobility Measurement Sensitivity by Utilizing Helium in an Ion Funnel Trap

Abstract: Ion mobility instruments that utilize nitrogen as buffer gas are often preceded by an ion trap and accumulation region that also uses nitrogen, and for different inert gases, no significant effects upon performance are expected for ion mobility spectrometry (IMS) of larger ions. However, we have observed significantly improved performance for an ion funnel trap upon adding helium; the signal intensities for higher m/z species were improved by more than an order of magnitude compared to using pure nitrogen. The effect of helium upon IMS resolving power was also studied by introducing a He/N2 gas mixture into the drift cell, and in some cases, a slight improvement was observed compared to pure N2. The improvement in signal can be largely attributed to faster and more efficient ion ejection into the drift tube from the ion funnel trap.

Mass selectivity of dipolar resonant excitation in a linear quadrupole ion trap.

Abstract: RATIONALE For mass analysis, linear quadrupole ion traps operate with dipolar excitation of ions for either axial or radial ejection. There have been comparatively few computer simulations of this process. We introduce a new concept, the excitation contour, S(q), the fraction of the excited ions that reach the trap electrodes when trapped at q values near that corresponding to the excitation frequency. METHODS Ion trajectory calculations are used to calculate S(q). Ions are given Gaussian distributions of initial positions in x and y, and thermal initial velocity distributions. To model gas damping, a drag force is added to the equations of motion. The effects of the initial conditions, ejection Mathieu parameter q, scan speed, excitation voltage and collisional damping, are modeled. RESULTS We find that, with no buffer gas, the mass resolution is mostly determined by the excitation time and is given by R~dβdqqn, where β(q) determines the oscillation frequency, and n is the number of cycles of the trapping radio frequency during the excitation or ejection time. The highest resolution at a given scan speed is reached with the lowest excitation amplitude that gives ejection. The addition of a buffer gas can increase the mass resolution. The simulation results are in broad agreement with experiments. CONCLUSIONS The excitation contour, S(q), introduced here, is a useful tool for studying the ejection process. The excitation strength, excitation time and buffer gas pressure interact in a complex way but, when set properly, a mass resolution R0.5 of at least 10,000 can be obtained at a mass-to-charge ratio of 609. Copyright © 2014 John Wiley & Sons, Ltd.

Magneto-optical resonance of electromagnetically induced absorption with high contrast and narrow width in a vapour cell with buffer gas

Abstract: The method for observing the high-contrast and narrow-width resonances of electromagnetically induced absorption (EIA) in the Hanle configuration under counterpropagating light waves is proposed. We theoretically analyze the absorption of a probe light wave in presence of counterpropagating one with the same frequency as the function of a static magnetic field applied along the vectors of light waves, propagating in a vapour cell. Here, as an example, we study a "dark" type of atomic dipole transition Fg=1-->Fe=1 in D1 line of 87Rb, where usually the electromagnetically induced transparency (EIT) can be observed. To obtain the EIA signal one should proper chose the polarizations of light waves and intensities. In contrast of regular schemes for observing EIA signals (in a single travelling light wave in the Hanle configuration or in a bichromatic light field consisted of two travelling waves), the proposed scheme allows one to use buffer gas to significantly enhance properties of the resonance. Also the dramatic influence of atomic transition openness on contrast of the resonance is revealed, that gives great advantage in comparison with cyclic atomic transitions. The obtained results can be interesting in high-resolution spectroscopy, nonlinear and magneto-optics.

Inert gas effects on the deposition rate of TiO2 during reactive HiPIMS

Abstract: Deposition rates have been measured during the reactive HiPIMS of Ti in the presence of oxygen and different inert gases (i.e. mixtures of X/O2 where X = Ne, Ar, Kr or Xe) by means of a quartz crystal microbalance (QCM). The QCM was positioned above the erosion racetrack directly facing the target surface at two different axial distances (50 and 100 mm). The HiPIMS discharge was operated with a pulse on-time τ = 100 μs, a pulse frequency f = 100 Hz and a constant average discharge power Pavg = 100 W (50 W for Xe/O2). The oxygen partial pressure, pO2, was maintained at a constant 0.2pt where pt is the total pressure and was maintained at a constant 0.4 Pa. Using these conditions, the discharge was operated in the so-called ‘poisoned’ mode. In contrast to the trends predicted by SRIM as well as those measured in DCMS, the power-normalized static deposition rates in reactive HiPIMS of titanium measured in gas mixtures of oxygen were observed to increase with the mass of the inert gas. The observed trend was attributed to a decreased return effect as a result of an increased average absolute target potential during the pulse on-phase when employing heavier inert gases as the buffer gas. For the case of Kr/O2, the normalized deposition rate measured in HiPIMS was found to be 87% of that measured in equivalent DCMS operation.

Sub-natural width N-type resonance in cesium atomic vapour: splitting in magnetic fields

Abstract: The sub-natural width N-type resonance in a Λ-system, on the D2 line of Cs atoms is studied for the first time in the presence of a buffer gas (neon) and the radiations of two continuous narrow-band diode lasers. A L = 1 cm long cell is used to investigate the N-type process. The N-type resonance in a magnetic field for 133Cs atoms is shown to split into seven or eight components, depending on the magnetic field and laser radiation directions. The results obtained indicate that the levels Fg = 3, 4 are the initial and final ones in the N-resonance formation. The experimental results with magnetic field agree well with theoretical descriptions.

Sensitive detection of iodine by low pressure and short pulse laser-induced breakdown spectroscopy (LIBS)

Abstract: In this study, Laser-Induced Breakdown Spectroscopy (LIBS) technology was applied to detect iodine, an essential element for human body. Iodine in buffer gases of N2 and air was detected using nanosecond and picosecond breakdowns of CH3I at reduced pressure. Compared with the conventional methods of iodine measurement, LIBS technology without sample preparation shows the merits of fast response, real-time detection and enhanced detection limit. The measurement results of iodine demonstrated that low-pressure LIBS is the favourable method for trace species measurement in analytical applications. The plasma generation processes of multi-photon ionization and electron impact ionization can be controlled by pressure and laser pulse width for the larger ionization and excitation processes of iodine, which was discussed by the intensity ratio of iodine emission at 183 nm to nitrogen emission at 174.3 nm. The detection limit of iodine measurement in N2 was 60 ppb in nanosecond breakdown at 700 Pa. Iodine in air as the buffer gas was also detected using nanosecond and picosecond breakdowns to examine the effect of oxygen.

Spectral and kinetic analysis of heterogeneous gas discharge plasma in the flow of an Al, H2O, and Ar mixture

Abstract: Plasma excited by a longitudinal pulse-periodic discharge in a mixed flow of a buffer gas (Ar), oxidizer (H2O), and aluminum dust is studied. The salient features of its kinetics are the atomic and molecular electronic channels of Al atom transformation, which were not taken into account earlier. They accelerate not only the oxidation process but also aluminum vaporization because the excitation and ionization energies are finally released on the surface of aluminum particles. This work is devoted to experimental and theoretical analysis of these channels.

Setup for in situ investigation of gases and gas/solid interfaces by soft x-ray emission and absorption spectroscopy.

Abstract: We present a novel gas cell designed to study the electronic structure of gases and gas/solid interfaces using soft x-ray emission and absorption spectroscopies. In this cell, the sample gas is separated from the vacuum of the analysis chamber by a thin window membrane, allowing in situ measurements under atmospheric pressure. The temperature of the gas can be regulated from room temperature up to approximately 600 °C. To avoid beam damage, a constant mass flow can be maintained to continuously refresh the gaseous sample. Furthermore, the gas cell provides space for solid-state samples, allowing to study the gas/solid interface for surface catalytic reactions at elevated temperatures. To demonstrate the capabilities of the cell, we have investigated a TiO2 sample behind a mixture of N2 and He gas at atmospheric pressure.

Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping

Abstract: Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N2 gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell.

Intrinsic relaxation rates of polarized Cs vapor in miniaturized cells

Abstract: The intrinsic relaxation rates of the vector magnetization of cesium vapor enclosed in microfabricated atomic magnetometer cells are investigated. Two methods—the optically detected magnetic resonance and the ground-state Hanle effect—are used to carry out automated measurements in dependence on cell temperature and nitrogen buffer gas pressure. The experimental results are compared with expected contributions of the different relaxation processes and in this way allow the discrimination between them to help further optimization of cell design. The methods are compared in terms of basic features, data quality, and practical applicability.

MnBr vapor active medium with a built-in reactor at 100-kHz pulse repetition frequency

Abstract: Frequency and energy characteristics of a MnBr vapor laser with a built-in reactor, i.e., with gas vapors being produced within a gas discharge tube, and the dependence of the average output power on different parameters are studied in this work. The values of the GDT wall temperature and buffer gas pressure required for obtaining the maximum output power are determined. It is shown that active media of this kind are as good as media with a conventional method of gas vapor production. A pulse repetition frequency of 100 kHz is attained for the first time for a medium on Mn atom transitions. The use of this active medium as a brightness amplifier in active optical systems is described; the amplifying characteristics are analyzed.

An alkali metal vapor laser amplifier

Abstract: The operation of a transversely diode-pumped alkali metal vapor laser amplifier is theoretically studied. The amplifier operation is described by a rather intricate system of differential equations, which can be solved in the general case only numerically. In the case of intense incident radiation, an analytic solution is obtained which makes it possible to determine any energy characteristics of the laser amplifier and to find the optimal parameters of the active medium and pump radiation (temperature, buffer gas pressure, and intensity and width of the pump radiation spectrum).

The effect of active antennas on the hot-restrike of high intensity discharge lamps

Abstract: The ignition voltage of high intensity discharge (HID) lamps with mercury as the buffer gas may rise from 3 kV for the cold state up to more than 15 kV for a hot lamp. By coating a lamp burner with an electrically conductive layer, which operates as an active antenna, the ignition voltage of HID lamps can be significantly reduced. An active antenna connected to one of the lamp electrodes transports the potential from this electrode to the vicinity of the opposite electrode and generates an enhanced electric field inside the burner. On applying a symmetrically shaped ignition pulse, a weak pre-discharge within the first half-cycle produces free charge carriers initiating ignition of the lamp within the subsequent second half-cycle. The authors present a set-up for electrical and optical investigations of hot-restrike in HID lamps. The ignition voltage is measured for two different polarities as a function of the cooldown time. An analysis of its reduction is given. Furthermore, the pre-discharge is investigated by means of short-time photography. It is demonstrated that a negative polarity of the active antenna within the first half-cycle and a positive polarity within the second one is the most effective succession.

Simulations of high intensity ions beam RFQ Cooler for DESIR SPIRAL 2

Abstract: The cooling simulation of a high intensity ions beam by a new generation of a buffer gas radio-frequency cooler within the SHIRaC (SPIRAL-2 High Intensity Radiofrequency Cooler) project, installed at the SPIRAL 2/DESIR facility, is presented. Two simulation methods for the cooling process in presence of the space charge effect and the buffer gas diffusion will be studied. The beam properties degradation in terms of the transmission efficienty, the longitudinal spread energy and the transversal emittance by these effects will be discussed. Finally, a comparison in term of transmission between simulated and experimental results for Cs+ ions beam of intensity going up to 1{\mu}A, will be outlined.