Gas Cell Experiments Research Articles

A Transcutaneous Carbon Dioxide Monitor Based on Time-Domain Dual Lifetime Referencing.

The partial pressure of arterial carbon dioxide plays a critical role in assessing the acid-base and respiratory status of the human body. Typically, this measurement is invasive and can only be taken momentarily when an arterial blood sample is drawn. Transcutaneous monitoring is a noninvasive surrogate method that provides a continuous measure of arterial carbon dioxide. Unfortunately, current technology is limited to bedside instruments mainly used in intensive care units. We developed a first-of-its-kind miniaturized transcutaneous carbon dioxide monitor that utilizes a luminescence sensing film and a time-domain dual lifetime referencing method. Gas cell experiments confirmed the monitor's ability to accurately identify changes in the partial pressure of carbon dioxide within the clinically significant range. Compared to the luminescence intensity-based technique, the time-domain dual lifetime referencing method is less prone to measurement errors caused by changes in excitation strength, reducing the maximum error from ∼40% to ∼3% and resulting in more reliable readings. Additionally, we analyzed the sensing film by investigating its behavior under various confounding factors and its susceptibility to measurement drift. Finally, a human subject test demonstrated the effectiveness of the applied method in detecting even slight changes in transcutaneous carbon dioxide, as small as ∼0.7%, during hyperventilation. The prototype, which consumes 30.1 mW of power, is a wearable wristband with compact dimensions of 37 mm× 32 mm.

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

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 → 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 [Pérez-Ríos et al., 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 ≲ T ≲ 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 the magnitude independent 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.

Validation of spectral gas radiation models under oxyfuel conditions. Part A: Gas cell experiments

Abstract Combustion of hydrocarbon fuels with pure oxygen results in a different flue gas composition as combustion with air. Standard CFD spectral gas radiation models for air combustion are out of their validity range. The series of three articles provides a common spectral basis for the validation of new developed models. In part A of the series gas cell transmissivity spectra in the spectral range of 2.4–5.4 μm of water vapor and carbon dioxide in the temperature range from 727 to 1500 ° C and at different concentrations were compared at a nominal resolution of 32 cm −1 to line-by-line models from different databases, two statistical-narrow-band models and the exponential wide band model. The two statistical-narrow-band models EM2C and RADCAL showed a good agreement with a maximal band transmissivity deviation of 3%. The exponential-wide-band model showed a deviation of 6%. The new line-by-line database HITEMP2010 had the lowest band transmissivity deviation of 2.2% and was recommended as a reference model for the validation of simplified CFD models.

Optically pumped atoms with velocity- and spin-changing collisions at low gas pressures

We discuss optical pumping when (a) the collision rates of optically pumped atoms with atoms or molecules of the background gas are small enough that individual velocity groups can be preferentially excited by a monochromatic light beam, (b) the collision rates are still fast enough to partially transfer the spin polarization to other velocity groups, and (c) there are nonnegligible losses of polarization due to collisional spin relaxation and Larmor precession. These conditions lead to a strong correlation between the velocity and the spin polarization of the atoms---that is, to ``spin-tagging'' of the different velocity groups. This regime is similar to that of optically pumped $^{23}\mathrm{Na}$ atoms of the Earth's upper atmosphere, but it is seldom encountered in laboratory experiments. For cooling and trapping experiments, the collision rates with background gas are negligible. For gas-cell experiments the velocity-changing rates are normally so fast compared to spin relaxation or Larmor precession rates that the atoms have a Maxwellian velocity distribution with negligible correlation between the spin polarization and the velocity. We analyze the limiting cases of strong and weak collisions, which change the velocity by a large or small fraction, respectively, of the mean thermal velocity. The Keilson-Storer model [J. Keilson and A. E. Storer, Q. Appl. Math. 10, 243 (1952)] is used to discuss strong collisions, with memory parameter $\ensuremath{\alpha}=0$ and weak collisions with $\ensuremath{\alpha}\ensuremath{\rightarrow}1$. For weak collisions, the physics can be modeled by coupled Fokker-Planck equations, identical to those for forced diffusion in a harmonic-oscillator potential well. In this limit there are solutions analogous to the quantum mechanical coherent states of a harmonic oscillator.

Generalization ofGrawert parameters - quantum close-coupling calculationsfor Ba (6s6p, 3P2 1P1) + Ar inelastic collision

The conventional and coherence cross sections that can bemeasured in collisional experiments with atomic beams areexpressed via quantities dependent on the dynamics only andindependent of geometry. The quantities introduced are thegeneralized Grawert parameters describing the collisionalprocesses that occur in gas cell experiments (average crosssections). The generalization of Grawert parameters allows us to obtain direct relationships between the average crosssections and the conventional and coherence cross sections. Theformulae introduced are applied to Ba-Ar inelastic collisionsin order to discuss the characteristics of the multistructurecross sections.

Thermal Decomposition of Ammonium Dinitramide at Moderate and High Temperatures

Temperature-controlled gas cell and thin film laser pyrolysis techniques have been used to investigate the condensed phase thermal decomposition of ammonium dinitramide (ADN). Gas cell experiments have been performed at heating rates of 10-2−10-1 °C s-1 and in a temperature region 20−250 °C. Pulsed CO2 laser heating of thin ADN films has allowed heating rates of 107 °C s-1 and temperatures of about 630 °C to be reached. The thermal decomposition products have been monitored by FTIR spectroscopy. Both experimental techniques show that the primary decomposition products are N2O and NO2. Nitric oxide is produced at a later stage in the reaction. No evidence has been found for participation of dinitramidic acid in the reaction mechanism. The fact that the same initial products are observed over a wide range of temperatures and heating rates shows that this mechanism can be used to model the initial stages of combustion of ADN.

Reactive collisions with excited-state atoms

In the context of molecular reaction dynamics, the study of elementary reactions with excited species has developed rapidly in the last decades. This article reviews recent developments in the study of reactive collisions with excited-state atoms, under single-collision conditions provided by crossed-beam and low-pressure gas cell experiments. Application of high-resolution laser techniques allows selective electronic excitation of atomic reagents. This excitation is used to identify the key features of state-selected and spin-orbit effects, on both the reaction cross-sections and product state distributions. The polarized nature of the laser field is used to gain a detailed understanding of the reaction dynamics and stereodynamics, through the alignment of atomic orbitals. Several examples are discussed in the light of current theoretical models, a detailed analysis of the data leading to the identification of non-adiabatic processes taking place on excited potential-energy surfaces. New experimental developments will allow for an even deeper understanding of these elementary processes.

Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br

An opto-thermal molecular beam study has been carried out to investigate the multiple-photon laser excitation of SF 6 and CF 3 Br. The molecular beam was produced by means of a supersonic expansion through a nozzle at variable temperature. The opto-thermal signal was measured by means of a high-sensitivity superconducting bolometer. The multiple-photon excitation of SF 6 has been measured as a function of the initial ro-vibrational population of the molecule. The experimental results have been compared with both previously published data of molecular beam and gas cell experiments and theoretical calculations. A satisfactory agreement has been found between some of our experimental results and the theoretical spectra obtained by means of the heat-bath feed-back model.

Resonance Fluorescence and Photon Trapping of Two Atoms on a Metallic Surface

ABSTRACTSurface-dressed optical Bloch equations are derived for the purpose of evaluating the resonance fluorescence spectrum of two interacting identical atoms near or adsorbed on a metal surface. The derivation takes into account the influence of reflected photons, dephasing due to atomic collisions, the linewidth of the driving laser field and the resonance excitation of surface plasmons. A unique behavior of the surfacemodified fluorescence, not seen in analogous gas cell experiments, is predicted. Under the appropriate circumstance, a photon emitted from one of the two atoms can be trapped by the two-adatom-surface system, and this is studied by means of a theory which treats the atoms and their surface images on the same footing.