Zeyad T. Alwahabi

Rotational transfer, an angular momentum model

We have re‐examined critical experiments on collision induced rotational transfer (RT) and conclude that the probability of RT is controlled by the factors that control the probability of angular momentum (AM) change. The probability of energy change seems less important in this respect. In the light of this we suggest a model for RT in which the probability of AM change is calculated directly and present a formalism for this purpose. We demonstrate that such a calculation leads to an exponential‐like fall of RT probabilities with transferred AM, a consequence of the radial dependence of the repulsive part of the intermolecular potential. Thus in this AM model, the exponential gap law has a simple physical origin. The AM model we describe may be used as the basis of an inversion routine through which it is possible to convert RT data into a probability density of the repulsive anisotropy. Through this model therefore it is possible to relate experimental RT data directly to the forces that are responsible...

Development of temperature imaging using two-line atomic fluorescence

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.

Sodium and Potassium Released from Burning Particles of Brown Coal and Pine Wood in a Laminar Premixed Methane Flame Using Quantitative Laser-Induced Breakdown Spectroscopy:

A quantitative point measurement of total sodium ([Na]total) and potassium ([K]total) in the plume of a burning particle of Australian Loy Yang brown coal (23 ± 3 mg) and of pine wood pellets (63 ± 3 mg) was performed using laser-induced breakdown spectroscopy (LIBS) in a laminar premixed methane flame at equivalence ratios (Φ) of 1.149 and 1.336. Calibration was performed using atomic sodium or potassium generated by evaporation of droplets of sodium sulfite (Na2SO3) or potassium sulfate (K2SO4) solutions seeded into the flame. The calibration compensated for the absorption by atomic alkalis in the seeded flame, which is significant at high concentrations of solution. This allowed quantitative measurements of sodium (Na) and potassium (K) released into the flame during the three phases of combustion, namely devolatilization, char, and ash cooking. The [Na]total in the plume released from the combustion of pine wood pellets during the devolatilization was found to reach up to 13 ppm. The maximum concentration of total sodium ([Na]maxtotal) and potassium ([K]maxtotal) released during the char phase of burning coal particles for Φ = 1.149 was found to be 9.27 and 5.90 ppm, respectively. The [Na]maxtotal and [K]maxtotal released during the char phase of burning wood particles for Φ = 1.149 was found to be 15.1 and 45.3 ppm, respectively. For the case of Φ = 1.336, the [Na]maxtotal and [K]maxtotal were found to be 13.9 and 6.67 ppm during the char phase from burning coal particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning wood particles. The concentration of alkali species was higher during the ash phase. The limit of detection (LOD) of sodium and potassium with LIBS in the present arrangement was estimated to be 29 and 72 ppb, respectively.

Solvent effects on two-line atomic fluorescence of indium

We aim to investigate the potential of four different organic solvents, namely, acetone, ethanol, methanol, and isopropanol, and the organic-solvent-water mixtures as a seeding medium for the two-line atomic fluorescence technique. Water is used as the reference case. Indium, which has been previously shown to have suitable spectroscopic attributes, is chosen as the thermometry species in the present study. Acetone and methanol are shown to enhance the fluorescence signal intensity the most (approximately threefold to fivefold at stoichiometric conditions) when used. Acetone and methanol are shown to improve the fluorescence emission over the entire stoichiometric envelope of flame, most significantly in the rich combustion region, as well as a twofold enhancement in the signal-to-noise ratio.

Stereochemical influences in atom–triatomic collisions

High-resolution dispersed fluorescence spectra form NH2 in the electronically excited A2A1 state have been obtained. Single quantum-state excitation using a tunable ring dye laser was employed and rotational energy transfer (RET) in the excited vibronic manifold was studied. Analysis shows that angular momentum based propensity rules are much more successful at predicting RET relaxation rates than are those based on the energy transfer. The results are interpreted with reference to the molecule-fixed axis system, and this allows us to work backwards to the incoming particle trajectory. Thus we obtain information about the stereochemistry of the NH2(2A1)+ H interaction.

Instantaneous Temperature Imaging of Diffusion Flames Using Two-Line Atomic Fluorescence

This work investigates the first demonstration of nonlinear regime two-line atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is ∼6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.

Dynamical angular momentum models for rotational transfer in polyatomic molecules

We propose a model for collision‐induced rotational transfer (RT) in polyatomic molecules based on the angular momentum (AM) sphere, a classical representation of the dynamical motion of the rotational AM vector in the molecular frame. The model develops further that proposed by us [AlWahabi et al., J. Chem. Soc., Faraday Trans. 85, 1003 (1989)] in which RT probabilities are related to the AM gap linking initial and final Nkakc states. The AM sphere representation embodies the full internal motion of the molecule via its effect on the inertial axes and the trajectory of the individual rotational state vectors. In this representation there is no unique AM gap for a particular transition between states of nominally well‐defined Nkakc and here we propose and test several models for obtaining the distance in AM space between initial and final trajectories. Models are evaluated from their ability to fit data on NH2–H collisions. We find that even the simplest approximations, such as shortest distance in AM spa...

New Seeding Methodology for Gas Concentration Measurements

This paper presents the first demonstration of the pulsed laser ablation technique to seed a laminar non-reacting gaseous jet at atmospheric pressure. The focused, second harmonic from a pulsed Nd:YAG laser is used to ablate a neutral indium rod at atmospheric pressure and temperature. The ablation products generated with the new seeding method are used to seed the jet, as a marker of the scalar field. The neutral indium atoms so generated are found to be stable and survive a convection time of the order of tens of seconds before entering the interrogation region. The measurements of planar laser-induced fluorescence (PLIF) with indium and laser nephelometry measurements with the ablation products are both reported. The resulting average and root mean square (RMS) of the measurements are found to agree reasonably well although some differences are found. The results show that the pulsed laser ablation method has potential to provide scalar measurement for mixing studies.

Quantitative detection of metallic traces in water-based liquids by microwave-assisted laser-induced breakdown spectroscopy

The enhancement of laser-induced breakdown spectroscopy (LIBS) assisted with microwave radiation is demonstrated for an aqueous solution of indium using the 451.13 nm emission line. Microwave power was delivered via a near-field applicator to the LIBS measurement volume where the indium aqueous solution was presented as a liquid jet. The microwave enhancement effect was observed to decrease with increasing laser pulse fluence at 532 nm resulting in a maximum emission intensity occurring at a laser pulse fluence of 85.2 J∙cm(-2), independent of the microwave power used. The detection limits of indium in an aqueous solution were determined to be 10.8 ± 0.7 and 124 ± 5 ppm for the cases of microwave enhanced and standard LIBS, respectively. The 11.5-fold detection limit enhancement obtained in the liquid phase is of the same order of magnitude as that reported for other elements in solid samples, but lower than that obtained in solid phase utilizing a similar experimental setup. This establishes microwave enhancement as an effective technique for the detection of metals in aqueous solutions. In addition, the temporal evolution of plasma emission intensity was investigated and was found to be qualitatively similar to that of plasma produced from solid phase samples, which reveals the same coupling mechanism between laser generated plasma and microwave radiation.

Orthogonal planar laser polarization spectroscopy.

Planar laser polarization spectroscopy has recently been used to image the hydroxyl radical in combustion for small intersection angles of pump and probe beams. We report an experimental configuration that allows planar laser polarization imaging for perpendicular intersection of pump and probe beams. We demonstrate what to our knowledge is the first planar laser polarization spectroscopy imaging at a 90 degree intersection of pump and probe beams for both linearly and circularly polarized pump beams.