Jan P. Hessler

Rate coefficient for the reaction H+O2→OH+O: Results at high temperatures, 2000 to 5300 K

The tunable‐laser flash‐absorption technique has been used to study the high‐temperature behavior of the reaction H+O2→OH+O by monitoring the absorption of the hydroxyl radical. Sensitivity analysis of a detailed reaction mechanism shows that for fuel rich mixtures only two reactions are sensitive when hydroxyl is monitored: H2+M→2H+M and H+O2→OH+O. Rate coefficients for these reactions have been determined by least‐squares analysis of measured absorption profiles. For the rate of dissociation of H2 in krypton we obtain k1(T)=(8.86±0.88)×10−10 exp[−48321/T(K)] cm3 s−1 from 3450 to 5300 K. For the H+O2 reaction we combined our results with previous low temperature measurements and recommend k2(T)=(1.62±0.12)×10−10 exp[−(7474±122)/T(K)] cm3 s−1 from 960 to 5300 K. The uncertainties are at the 95% confidence level. Measured cross sections for rotational and vibrational energy transfer in O2 and OH have been used to show that relaxation effects do not influence the results. We compare our results to recent tr...

Energy level analysis of Np3+:LaCl3 and Np3+:LaBr3

The polarized absorption and fluorescence spectra of 0.01–5% Np3+ doped into single‐crystal LaCl3 were measured at moderate and high resolution in the range to 50 000 cm−1 at 298, 77, and 4 K. The 150 crystal field components identified were fit by a parametrized model which has previously been shown to provide an excellent account of the energy level structures observed for lanthanides doped into LaCl3. This constitutes the first successful evaluation of the crystal field interaction for an actinide ion in LaCl3 in which J mixing effects were explicitly included. The crystal field parameters for Np3+:LaCl3, B20=163, B40=−632, B60=−1625, and B66=1028, all in cm−1, were determined to be approximately twice as large as those for the analogous lanthanide, Pm3+:LaCl3. From analysis of Zeeman patterns in the fluorescence spectra, the ground state was found to be doubly degenerate, having crystal quantum numbers ±2 and a parallel splitting factor 0.17 LU (Lorentz unit).

Solar Neutrinos: Proposal for a New Test

The predicted flux on the earth of solar neutrinos has eluded detection, confounding current ideas of solar energy production by nuclear fusion. The dominant low-energy component of that flux can be detected by mass-spectrometric assay of the induced tiny concentration of 1.6 x 107 year lead-205 in old thallium minerals. Comments are solicited from those in all relevant disciplines.

Efficient generation of 200-nm light in β-BaB 2 O 4

Efficient generation of 200-nm light is achieved in ..beta..-BaB/sub 2/O/sub 4/ through third harmonic generation of pulsed dye-laser radiation. (AIP)

Spatially resolved small-angle x-ray scattering studies of soot inception and growth

The high spectral brilliance of x-rays produced at the Basic Energy Sciences Synchrotron Radiation Center of Argonnes Advanced Photon Source allows us to perform small-angle x-ray scattering (SAXS) measurements of the distributions of soot particles in flames. SAXS provides an in situ probe of the size and distribution of particles in the region between 1 and 100 nm. Detailed measurements on a propylene/air diffusion flame allow us to extract a spatially dependent background, which occurs in gas-phase combustion systems, and to perform Abel inversions, which provide the radial dependence of the scattering intensity. A bimodal distribution of soot particles is needed to describe our results. The radial behavior of the two modes of this distribution implies that the chemistry and fluid dynamics are strongly coupled in this simple diffusion flame. The larger particles of this distribution correspond to the previously observed primary particles, which have a relatively complex radial dependence. Midway between the fuel source and the widest part of the flame, the primary particles have a mean radius of 6 nm or less and their concentration is symmetrically distributed about the flame front. At the widest part of the flame, two distinct distributions of primary particles are observed. Near the center of the flame, the particles have a mean radius of 10 nm and a polydispersity of 0.3 and beyond a transition region they have a mean radius of 21 nm and a polydispersity of 0.2. The smaller particles, which require additional experiments before they can be identified, correspond to soot nuclei, polycyclic aromatic hydrocarbon species such as naphthalene, and/or disordered carbons with graphitic basal planes.

Small-angle x-ray scattering studies of soot inception and growth.

The high spectral intensity of X-rays produced by the undulator at the Basic Energy Sciences Synchrotron Radiation Center of Argonnes Advanced Photon Source has allowed us to perform small-angle X-ray scattering (SAXS) studies of the initial distribution of soot particles formed by various fuels. SAXS provides an in situ probe of the morphology of soot in the region between 1 and 100 nm and complements the ex situ technique of electron microscopy. The basic aspects of SAXS and its potential are illustrated with measurement on a laminar flame of acetylene in air. The more complex fuel toluene has been studied in a flat-flame burner that supports a CH4/H2/air or CO/H2/air diffusion flame stabilized by N2 co-flow. This burner produces a nearly constant temperature region above the flame where the pyrolysis and combustion of the heavier fuels occurs. Kinetic information is obtained by performing measurements of the scattered intensity profile as a function of the height above the burner. These profiles have been reduced to give the mean radius and dispersion of a distribution of spherical particles. Mean radii between 0.8 and 18 nm have been observed. The smallest of these is a factor of ten smaller than previously detected with Lorentz-Mie scattering. Near 1550 K, the soot distribution found in toluene shows a distinct step behavior that is consistent with model calculations.

The terbium chloride–aluminum chloride vapor system. III. Spectral intensity analysis

The intensities of Tb3+ ground state absorption and 5D4 fluorescence transitions in the TbCl3(AlCl3)x(g) complex system have been analyzed according to the Judd–Ofelt theory of forced electric dipole transitions. This represents the first successful attempt to utilize observations of both absorption and fluorescence spectra in establishing the parameters of the theory. The positions of 19 Tb3+ energy levels were determined from the spectra and used to perform a parametric analysis of the Hamiltonian matrix for the ion’s 4f8 configuration. The wave functions obtained in diagonalization of the Hamiltonian were used to calculate unit tensor operator matrix elements required in the intensity analysis. A good fit to the measurements was obtained with intensity parameters W2 = 20.5, W4 = 2.3, and W6 = 3.0 pm2. These results were then used to compute intensities of fluorescence transitions from the higher lying 5D3, 5G6, and 5L10 levels, yielding additional qualitative agreement between theory and experiment. So...

Efficient monochromator to isolate VUV light generated by four-wave mixing techniques

The lateral chromatic aberration from a LiF lens, which is positioned off-axis with respect to the incident light beams, is used to isolate the VUV light generated by the four-wave mixing technique from the intense pump light. The transmittance of the monochromator at the VUV wavelength is limited by the transmittance of a single lens. The ratio of transmitted-to-incident pump energy is 3.3 x 10(-6). A single monochromator with two interchangeable lenses can be used for either tightly focused or collimated pump beams.

Deactivation on the Nd3+, 4F32 level in neodymium chloride - aluminum chloride vapor complexes

Abstract The first fluorescence lifetime studies in a trivalent rare earth-metal vapor complex NdCl 3 · (AlCl 3 ) χ are reported. The observed, temperature-dependent, nonradiative deactivation and the inferred radiative lifetime are discussed in terms of various models. At 803 K, corresponding to a density of 4.6 × 10 17 Nd 3+ ions/cm 3 and a total system pressure ≈5 atm, the measured lifetime is 46 μs.

Probing dynamics in the Franck–Condon and exit channel regions of dissociating H2S: Emission spectra upon tunable excitation from 199–203 nm

This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excita...