Buffer Gas Mixture Research Articles

Quadratic dependence on temperature of Cs 0–0 hyperfine resonance frequency in single Ne buffer gas microfabricated vapour cell

Presented is the observation of a quadratic temperature dependence of the Cs 0-0 ground state hyperfine resonance frequency in a single Neon (Ne) buffer gas vapour microcell. The inversion temperature, expected to be theoretically independent of the buffer gas pressure, is measured to be about 80°C for two different samples. A proposal to develop chip scale atomic clocks with improved long-term frequency stability, simpler configuration (a single buffer gas instead of a buffer gas mixture) and then relaxed constraints on pressure accuracy during the cell filling procedure is presented.

Impact of collisional quenching on the detection of HgCl_2 via photofragment emission

The effects of collisional quenching on photofragment emission (PFE) detection of vapor-phase HgCl(2) in combustion flue gas constituents are investigated. Exciting HgCl(2) via the 1(1)Pi(u)<--1(1)Sigma(g)+ transition, time-resolved measurements of emission from the Hg(6(3)P(1)) daughter in buffer-gas mixtures of N(2), O(2), and CO(2) indicate that the fragmentation pathway passes through a long-lived intermediate species, which we assign to Hg(6(3)P(2)). Total quenching rate coefficients of Hg(6(3)P(1)) by N(2), O(2), and CO(2) are consistent with values reported in the literature. In addition, total quenching rate coefficients for the intermediate Hg(6(3)P(2)) state are determined to be 1.72(+/-0.08)x10(-10) cm(3) molecule(-1) s(-1) and 2.90(+/-0.37)x10(-10) cm(3) molecule(-1) s(-1) for N(2) and O(2), respectively. An analysis of the impact of the collisionally dependent energy-transfer process that precedes the formation of Hg(6(3)P(1)) on the use of PFE to measure HgCl(2) concentration is presented.

Laser Noise Cancellation in Single-Cell CPT Clocks

We demonstrate a new technique for the suppression of noise associated with the laser source in atomic clocks based on coherent population trapping (CPT). The technique uses differential detection of the transmission of linearly and circularly polarized beams that propagate through different parts of a single rubidium vapor cell filled with a buffer gas mixture. The common-mode noise associated with the laser frequency and amplitude noise is suppressed by the differential detection of the two laser beams. The CPT signal, which is present only in the circularly polarized laser beam, is unaffected. The implementation of the technique requires only a change of the polarization of part of the laser beam and an additional photodiode. The technique is simple and applicable to CPT frequency references where a major source of noise is the laser, such as compact and chip-scale devices.

Hydrocarbon-free potassium laser

A hydrocarbon-free alkali laser operating on a mixture of potassium vapour and helium buffer gas has been demonstrated. Hydrocarbon-free alkali lasers can operate at higher temperatures without risk of cell contamination caused by chemical reactions between the alkali atoms and the hydrocarbon buffer gas.

Pulsed laser photofragment emission for detection of mercuric chloride

The viability of pulsed laser photofragment emission (PFE) is evaluated for the in situ measurement of vapor-phase mercuric chloride (HgCl(2)) concentration in combustion flue gas. Dispersed emissions from both the Hg (6(3)P(1)) and HgCl (B(2)Sigma(+)) photoproducts are presented, and the dependence of the HgCl(2) PFE signal originating from Hg (6(3)P(1)) on the collisional environment is examined for buffer-gas mixtures of N(2), O(2), and CO(2). Integrated PFE intensity measurements as a function of buffer gas pressure support the assumption that the primary effect of the relevant flue gas constituents is to quench emission from Hg (6(3)P(1)). The quenching rate constants for PFE from HgCl(2) were measured to be 1.37 (+/-0.16) x 10(5) Torr(-1) s(-1) for N(2), 9.35 (+/-0.25) x 10(6) Torr(-1) s(-1) for O(2), and 1.49 (+/-0.29) x 10(6) Torr(-1) s(-1) for CO(2). These values are in good accord with literature values for the quenching of Hg (6(3)P(1)). The emission cross section for Hg (6(3)P(1)) generated by photodissociation of HgCl(2) in 760 Torr N(2) is found to be 1.0 (+/-0.2) x 10(-25) m(2) by comparing the PFE signal to N(2) Raman scattering.

Fine-tuning of ceramic-based chemical sensors via novel microstructural modification: Part II: Low level CO sensing by molybdenum oxide, MoO 3

Employing a buffer gas mixture of CO and CO 2, the equilibrium oxygen partial pressure was varied across the Mo/MoO 3 proximity line by varying the ratio of concentration of the two gases, as described in an earlier communication. This led to the reduction of MoO 3 in one case and oxidation of the reduced phase (Mo or MoO 2) in the other with a concomitant variation in the microstructural features of the resulting species. The formation and growth of the new oxide surface under conditions of oxygen potential modulation has been found to impart enhanced sensing characteristics to the MoO 3-based chemical sensors for low levels of carbon monoxide (14–100 ppm CO) in the ambient.

Sub-ppm multi-gas photoacoustic sensor

A photoacoustic multi-gas sensor using tuneable laser diodes in the near-infrared region is reported. An optimized resonant configuration based on an acoustic longitudinal mode is described. Automatic tracking of the acoustic resonance frequency using a piezo-electric transducer and a servo electronics is demonstrated. Water vapour, methane and hydrogen chloride have been measured at sub-ppm level in different buffer gas mixtures. The importance of the system calibration in presence of several diluting gases is discussed. Finally, trace gas measurements have been assessed and detection limits (signal-to-noise ratio = 3) of 80 ppb at 1651.0 nm for CH 4, 24 ppb at 1368.6 nm for H 2O and 30 ppb at 1737.9 for HCl have been demonstrated.

All-optical atomic clock based on coherent population trapping in ^85Rb

An all-optical microwave frequency standard based on coherent population trapping (CPT) in 85Rb is developed. The CPT resonances are detected by an ordinary edge-emitting diode laser in a simple optical setup. A buffer-gas mixture is carefully optimized to yield a narrow linewidth and a reduced temperature dependence of the resonance frequency. With the developed system we are able to measure ultranarrow optically induced hyperfine CPT resonances at &lt;20 Hz, which is in good agreement with the linewidth calculated from experimental parameters. The frequency of an RF-signal generator has been stabilized to the CPT resonance between the two mF=0 magnetic sublevels. The relative frequency stability (square root of Allan variance) follows a slope of 3.5×10-11 τ-1/2(1 s&lt;τ&lt;2000 s). The best stability of 6.4×10-13 is reached at an integration time of τ=2000 s. This stability is sufficient for many high-precision applications. Frequency-shift measurements were made to evaluate the frequency dependencies on the operation parameters.

Excitation dynamics during the multiphoton absorption in SF 6+buffer-gas mixtures

A detailed analysis of the total average number of absorbed photons 〈 n〉 total during Infrared multiphoton absorption processes in mixtures of SF 6–Ar, N 2 and CH 4 buffer gases is presented. The results for 〈 n〉 total are deduced using pulsed photoacoustic technique in collisional regime. Complete analysis is based on the theoretical generalized coupled two-level model (GCT) and its application to different gas mixtures. Evaluation of partial 〈 n〉 coll. values is presented too, obtained using the results from time-resolved optoacoustic (TROA) and time-resolved absorption (TRA) methods for V–T relaxation times ( τ V–T) and the saturable absorber (SA) method for R,R–T relaxation times ( τ rot.–rel.), and applying them directly to the GCT model. All methods (TROA and SA) and the GCT model use the same photoacoustic results from our experiment under identical experimental conditions.

Manifestations of collisions in the laser SRS — CARS diagnostics of hydrogen in rarefied gas mixtures

CARS diagnostics of hydrogen in rarefied gas mixtures is performed using the biharmonic laser pump based on the SRS by the vibrational hydrogen transition. The diagnostics is shown to be substantially affected by collisions, which result in variations in the linewidth and the frequency of the Raman-active transition. This influence is distinctly observed at pressures higher than 0.05 atm and depends on the composition of the buffer gas mixture.

Submillimeter-Wave Spectroscopy of TiCl in the Ground Electronic State

The submillimeter-wave rotational transitions of TiCl in the ground state were observed using a double-modulation technique. TiCl was generated in a DC-discharge of a mixture of TiCl4 vapor (less than 1 mTorr) and Ar buffer gas (80 mTorr) at a current of 200 mA. The 4Φ3/2, 4Φ5/2, 4Φ7/2, and 4Φ9/2 spin components of Ti35Cl (v=0, 1, 2) and Ti37Cl (v=0) were detected. The data were analyzed using effective rotational constants for each spin component as well as with the usual N2 reduced Hamiltonian. Recent Fourier transform and laser data were included in our fits and we confirm that the ground state of TiCl is a 4Φr electronic state.

Simulation of the Collisional Cooling Effect for Binary and Ternary Buffer Gas Mixtures in a Quadrupole Ion Trap Mass Spectrometer

Simulation of the collisional cooling effect using binary and ternary buffer gas mixtures in the ion trap mass spectrometer was performed using the SIMION 6.0 program, in which a 3D collision model was used to describe the ion–buffer gas collisions. Computation of the ion kinetic energy based on the Langevin theory under various conditions was used to account for the effects of mixed buffer gases on ion-collisional cooling. The addition of small amounts (8–9%) of one or two heavier target gases (nitrogen, argon or SF6) to the 1 mtorr helium buffer gas in the ion trap can greatly improve the collisional cooling effects. The kinetic energy of ions can be reduced to close to 0 eV. Although the use of ternary buffer gas mixtures can also effectively improve the collisional cooling effects, the use of binary buffer gas mixtures is the better choice due to convenience and lower cost in real ion trap applications.

Effect of argon buffer and the UV preionization regime on the electric-discharge XeCl laser parameters

The gas discharge and output radiation parameters were studied in XeCl lasers with various buffer gas compositions (argon vs. neon) and different types of excitation regimes (fast vs. quasi-stationary) in the preionization source. The intervals of specific energy for the neon-and argon-based buffer gas mixtures are 10–125 and 45–360 J/(l atm), respectively. The specific parameters of radiation in the neon-and argon-based buffer gas mixtures depend on the energy required for the UV preionization. For high-power UV preionization sources, the output parameters of lasers using argon-based gas mixtures may exceed those obtained with neonbased mixtures.

Modeling the plasma kinetics in a kinetically enhanced copper vapor laser utilizing HCl+H/sub 2/ admixtures

A detailed computer model has been used to simulate the plasma kinetics and lasing characteristics in a kinetically enhanced copper vapor laser (KE-CVL) which utilizes Ne-H/sub 2/-HCl buffer gas mixtures. The model reproduces key features of the observed operating characteristics of the KE-CVL-in particular, relating to the electrical characteristics of the plasma tube, time evolution of Cu 4s/sup 2/S/sub 1/2/ ground state density, and formation of the laser output. It is shown that the principal role of the HCl additive is to increase the electron loss rate during the interpulse period via dissociative attachment reactions between free electrons and vibrationally excited HCl (/spl nu/=1,2) molecules. This leads to a reduction of the prepulse electron density establishing more favorable prepulse conditions for laser action during the subsequent excitation phase. In the KE-CVL, the plasma skin effect governing the development of the radial electric field is greatly reduced compared to conventional CVL's, altering the spatio-temporal evolution of the optical gain and laser field intensities to substantially enhance high-beam-quality output. Comparisons between model results and experimental data for the decay rate of the Cu 4s/sup 2/ /sup 2/D/sub 3/2/ metastable lower laser level in the early afterglow suggest that there may be an additional de-excitation mechanism for the /sup 2/D/sub 3/2,5/2/ levels in the KE-CVL plasma which has yet to be identified.

A 60-W high-beam-quality single-oscillator copper vapor laser

We report dramatic improvements (up to four times) in the generation of high-beam-quality (HBQ) output power from copper vapor lasers (CVLs) by using tailored HCl-H/sub 2/-Ne buffer gas mixtures. Up to 60 W of HBQ power was obtained from a 38-mm bore-diameter CVL at a pulse repetition frequency of 12 kHz and 50 W at 22 kHz. This improvement in power capability is attributable to the modification of the spatio-temporal gain characteristics of the laser when using the new buffer gas mixture.

Influence of the pre-pulse plasma electron density on the performance of elemental copper vapour lasers

A detailed computer model for the kinetics in an elemental copper vapour laser (Cu–Ne–H 2) has been used to investigate the importance of the pre-pulse electron density on the performance and lasing characteristics of such a device. The results show that the laser output power and operating efficiency are increased by 65% and 100%, respectively, at a pulse repetition frequency of 17 kHz, if the pre-pulse electron density is reduced by a factor of 5–10. Modelling of the plasma kinetics during the afterglow period suggests that such a reduction is brought about when trace quantities (∼0.3%) of HCl are introduced into the plasma to increase the electron density decay rates via dissociative attachment reactions. The predicted improvements in laser performance which occur as a result of a reduced pre-pulse electron density are consistent with the observed operating characteristics of Kinetically Enhanced copper vapour lasers which use HCl–H 2–Ne buffer gas mixtures.

Alkali-metal-atom polarization imaging in high-pressure optical-pumping cells

We present a detailed experimental analysis of Rb-polarization imaging in high-pressure gas cells. The Rb vapor in these cells is optically pumped by high-power diode-laser arrays. We present images for high (35 G) and low (4 G) magnetic fields and for different He and Xe buffer-gas mixtures. We demonstrate that high-field imaging provides an absolute measurement of the Rb-polarization distribution in the cell, based on the fact that a spin-temperature distribution of the hyperfine magnetic sublevels is established in high-pressure buffer gases. A survey of various mechanisms that broaden the Rb magnetic-resonance lines is presented. These broadening mechanisms determine the limits of the spatial resolution achievable for images of the Rb-polarization distribution.

Kinetically enhanced copper vapour lasers employing H 2–HCl–Ne buffer gas mixtures

The output powers from 25 mm (volume 0.49 l) and 40 mm (volume 1.9 l) diameter copper vapour lasers (nominally 20 and 55 W devices) are approximately doubled to >50 and >100 W respectively by adding small partial pressures of both H 2 and HCl to the neon buffer gas. Our studies show that this gas mixture performs better than a H 2–HBr admix, a result we attribute to more efficient reassociation of HCl in the plasma region.

Enhanced performance of elemental copper-vapor lasers by use of H_2–HCl–Ne buffer-gas mixtures

The output power from a 25-mm-diameter (volume, 0.49 L) and a 40-mm-diameter (volume, 1.9 L) copper-vapor laser (nominally 20- and 65-W devices, respectively) was approximately doubled to >50 and >100 W , respectively, by addition of small partial pressures of both H(2) and HCl to the neon buffer gas. The specific output powers of these lasers are believed to be records for any copper-vapor lasers of this size.

Reactions of size-selected positively charged nickel clusters with carbon monoxide in molecular beams

Reactions of small thermalized positively charged nickel clusters with carbon monoxide were studied in a molecular beam experiment. The nickel clusters were produced in a high intensity cluster ion source and thermalized in a large helium-filled quadrupole ion guide. The clusters were size selected by a quadrupole mass spectrometer. The mass- and charge-selected nickel clusters then passed through a linear quadrupole drift tube filled with a mixture of helium buffer gas and carbon monoxide. The reaction products were then analyzed by a quadrupole mass-spectrometer. Using this technique, saturation limits for Nin+ clusters with n=4–31 were measured and the competitive reaction channels were identified. Under certain experimental conditions carbide formation was observed in the case of the nickel tetramer, pentamer, and hexamer. The structure of the nickel carbonyl clusters is discussed within the framework of the polyhedral skeletal electron pair theory. The cluster growth may be explained by a pentagonal sequence of structures for n=4–7, capping of the pentagonal bipyramid to buildup an icosahedron at Ni13+, and further capping of this icosahedron to form a double icosahedron at Ni19+.