Mark J. Dyer

Two-dimensional imaging of OH laser-induced fluorescence in a flame

A method for obtaining a two-dimensional image of laser-induced fluorescence of a naturally occurring flame radical is described. A tunable laser beam is focused by a cylindrical lens into a sheet that passes through the flame, exciting OH lying within the plane defined by the laser. The fluorescence at right angles is imaged onto an intensified vidicon tube, forming a map of the ground-state OH concentration within the flame. For a sheet 0.5 mm thick, up to 2800 counts/mm(2) of flame area are obtained on a single laser pulse for an OH concentration of 700 parts in 10(6). The method holds considerable promise for imaging in time-varying systems, such as reactive turbulent combustion.

Rotational‐level‐dependent quenching of A 2Σ+ OH and OD

Rate constants kQ for collisional quenching of A 2Σ+, v’=0, OH and OD have been measured for specific rotational levels N’ of the radical and a wide variety of collision partners. Through measurements of the time‐dependent laser‐induced fluorescence in a low pressure discharge flow at room temperature, we observe a decrease in kQ with increasing rotational quantum number for most quenchers. The internal levels of the collision pairs appear unimportant from experiments involving deuterium substitution. A comparison of rotationless rates for different colliders [kQ(N=0)] with calculations based on collision complex formation indicate that attractive forces play a role in the quenching process.

Wavelength dependence of the absolute Raman gain coefficient for the Q(1) transition in H 2

The absolute Raman gain for the Q(1) vibrational Raman transition in H2 at 4155 cm−1 has been measured at pump (Stokes) wavelengths of 532 (683), 477 (594), and 307 (351) nm using a pulsed pump and a cw probe laser, both of which were single frequency. Good agreement is found between the measured and calculated Raman steady-state gain coefficients. Empirical formulas are derived to calculate the Raman gain as a function of wavelength, density, and temperature.

Time-resolved CH (A 2 Δ and B 2 ∑ − ) laser-induced fluorescence in low pressure hydrocarbon flames

Total collisional removal rate constants for the CH A(2)Delta and B(2)Sigma(-) electronic states are obtained in low pressure (<20-Torr) hydrocarbon flames. The B state is consistently removed ~70% faster than the A state. Variations of +/-50% are observed for different rotational levels and positions in the flame. For these flames, A-state emission following excitation of the B state indicates a rapid electronic-to-electronic energy transfer pathway that is insensitive to collision environment. Upper limits to the collider specific cross sections are obtained for H(2)O, N(2), and CO(2). The CH concentration and temperature profiles are measured and parametrized using a unique method.

The influence of catalytic activity on the ignition of boundary layer flows Part I: Hydroxyl radical measurements

Abstract The effect of catalytic activity on gas phase ignition in an ethane-air boundary layer was investigated by measuring OH concentration profiles over heated platinum and quartz surfaces. Platinums catalytic influence on gas phase ignition was found to depend strongly on the fuel-air ratio. Experiments were performed in a flat plate boundary layer configuration at 1 atm pressure and surface temperatures of 1200–1650 K. The hydroxyl measurements were made by planar and pointwise laser-induced fluorescence. Inhibition of ethane-air ignition was observed at equivalence ratios greater than 0.35, with enhancement seen below this value. Depletion of fuel near the catalytic surface by mass transfer-limited oxidation reactions can account for the catalysts inhibiting effect. Catalytic promotion of gas phase ignition was evidenced by significantly higher OH concentration levels over platinum plates (as compared to quartz) for the leanest equivalence ratio tested (Φ = 0.079). The results indicate that catalytic surface-induced production of active intermediates can promote gas phase ignition and offset the effect of reactant depletion near the surface.

Nascent NO vibrational distribution from 2485 Å NO2 photodissociation

The initial NO vibrational level distribution has been determined for NO2 photodissociation at 2485 A. Excitation spectra of the NO vibrational levels were measured by using both the NO A 2Σ+←X 2Π and B 2Π←X 2Π transitions, the latter being somewhat stronger due to saturation effects. It was determined that the NO population was strongly inverted, with most of the nascent NO being in v=6–8; the thermodynamic limit is v=8. Injection locking of the KrF laser output permitted study of the 2491 A NO2 band, and it was evident that the increased absorption in this region gave greatly enhanced signal levels in the excitation spectra, at those wavelengths where NO2 and NO absorption lines coincide. It was demonstrated that in the 2640–2850 A wavelength region, NO2 can be detected by use of a single dye laser, simultaneously dissociating NO2 and electronically exciting the resultant vibrationally hot NO. Deactivation of NO(v=8) by NO2 was found to proceed with a rate coefficient of 1.1×10−11 cm3 molecule−1 s−1, wh...

Structure and mechanical flexibility of carbon nanotube ribbons: An atomic-force microscopy study

Collapsed multiwalled carbon nanotube (MWCNT) sections and segments (carbon nanotube ribbons) on surfaces were observed by atomic-force microscopy. The collapsed form of these MWCNTs allowed the determination of the layer number and the inner and outer diameters for their uncollapsed form. The results show that such carbon nanotube ribbons are extremely flexible and readily conform to the underlying substrate morphology. The formation of the collapsed form of these nanotubes is consistent with recent theoretical analysis on the mechanics of nanotubes.

Magnetic separation of GdC2 encapsulated in carbon nanoparticles

Abstract Single crystal particles of α-GdC2 have been encapsulated into carbon polyhedral shells by the carbon arc-discharge technique. These particles range in size from 20 to 50 nm and are paramagnetic in nature. The phase and stoichiometry of these crystals have been determined using a combination of techniques, such as X-ray diffraction and high-resolution electron microscopy. As reported for α-LaC2 earlier, the crystal structure of bulk α-GdC2 is also tetragonal, and it is metallic and undergoes hydrolysis. However, the encapsulation of the nanoscale crystals in carbon shells prevents their hydrolysis indefinitely. These nanometer-sized particles, which can be preferentially extracted using powerful magnetic fields, may have useful applications in several fields of science.

Laser double‐resonance study of the collisional removal of O2 (A 3Σ+u, ν=6, 7, and 9) with O2, N2, CO2, Ar, and He

The collisional removal of O2 molecules prepared in selected vibrational levels of the A 3Σ+u state is studied using a two‐laser double‐resonance technique. The output of the first laser excites the O2 to A 3Σ+u, ν=6, 7, or 9, and the ultraviolet output of the second laser monitors these levels via resonance‐enhanced ionization through either the ν=5 level of the C 3Πg Rydberg state, or the valence state or states tentatively associated with the 5 3Πg state. The temporal evolution of the A 3Σ+u state vibrational level is observed by scanning the time delay between the two pulsed lasers. Collisional removal rate constants are obtained for A 3Σ+u, ν=7 and 9 colliding with O2, N2, CO2, Ar, and He; and for ν=6 colliding with O2 and N2. We find the collisional removal of the A 3Σ+u state to be fast (k≥10−11 cm3 s−1) for all colliders studied. The rate constants vary by about an order of magnitude from the fastest collisional deactivator, CO2 to the slowest studied, the rare gases Ar and He. The rate constants ...

Rapidly sequenced pair of two-dimensional images of OH laser-induced fluorescence in a flame

A Nd:YAG laser was Q switched twice during one flashlamp pulse and pumped a dye laser, whose frequency-doubled output, tuned to an OH absorption line, illuminated a planar region through a flame. The OH laser-induced fluorescence was focused onto an oscillating mirror, which directed the successive two-dimensional images onto different halves of a vidicon tube. Usable pair separation times of 40 to 180 microsec were obtained. The method has promise for studying changes in small-scale structure in turbulent reactive flows and for instantaneous measurement of the spatial distribution of two observables.