Ron C. Estler

Effects of beam parameters on excimer laser deposition of YBa2Cu3O7−δ

Broad angular distributions have been observed for XeCl laser ablation plumes used in the deposition of YBa2Cu3O7−δ thin films. Distributions (inferred from film thickness) and film stoichiometry were measured as a function of laser fluence, beam shape, and oxygen pressure. Parallel to the long axis of the laser spot, plumes exhibited a cos3.5(θ) spread and composition varied with angle; in the perpendicular direction, more diffuse [cos1.5(θ)] plume distributions were associated with stoichiometric deposition. The observed phenomena are consistent with formation of a Knudsen layer near the target surface.

Effect of reagent orientation and rotation upon product state distribution in the reaction Sr+HF (v=1,J) →SrF(v′, J′) +H

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Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Effect of deposition rate on properties of YBa2Cu3O7−δ superconducting thin films

Superconducting thin films of YBa2Cu3O7−δ on (100)u2009SrTiO3 are prepared in situ by a pulsed laser deposition technique at deposition rates from 1 to 145 A/s. Crystallinity of the films is examined by Rutherford backscattering in the channeling mode using a 2.2 MeV He+ ion beam. The backscattering minimum yield (χmin) increases with the deposition rate. A χmin of 3% is observed in the films deposited at the lowest deposition rate. Even at a deposition rate of 145 A/s, the films show good crystallinity with χmin of 15%, indicating epitaxial growth. The x‐ray diffraction measurements show that the films have strong c‐axis orientation normal to the substrates. The films have metallic resistance versus temperature behavior with zero resistance temperatures of 90 K. The results indicate that the pulsed laser deposition technique could be used to deposit large‐area films efficiently with adequate substrate movement.

Large critical current densities in YBa2Cu3O7−x thin films made at high deposition rates

Critical current densities (Jc) in YBa2Cu3O7−x films made at deposition rates from 0.1 to 14.5 nm/s (∼50 μm/h) were measured using a direct transport method. As the deposition rate was increased by two orders of magnitude, the films exhibited no marked degradation in current carrying capability with Jc of ∼4×106 A/cm2 at 77 K and zero field. Jc for all the films showed similar behavior under a magnetic field up to 8 T, although extra structural defects were found in the films deposited at the higher rates. The results from this experiment indicate the feasibility for coating wires, tapes, and other macroscopic shapes with high Tc superconductors using proper vapor deposition techniques.

Mass spectroscopic identification of wavelength dependent UV laser photoablation fragments from polymethylmethacrylate

Samples of polymethylmethacrylate have been irradiated with laser pulses of 266 nm wavelength along with the wavelengths generated via hydrogen‐shifted stimulated Raman scattering. A quadrupole mass spectrometer monitors in real time the photoablation products produced during the irradiation. At wavelengths of 266 nm and above, the products are dominated by monomer, CO2, and CO. At wavelengths below 266 nm, a dramatic change of ablation products is observed, with methyl formate appearing as a major photochemical product.

The dynamics of the reaction Sr+HF(v = 1)→SrF+H: Effect of rotation

In order to gain insight into the effects of rotation in chemical reactions, the Sr+HF(v = 1)→SrF+H reaction has been studied under single collision conditions as a function of reagent HF rotation. The HF rotational state is selected via HF laser excitation. Product internal state distributions and relative total reactive cross sections have been measured using laser‐induced fluoresence. For each initial rotational level examined (J = 1→3), a nearly statistical distribution of product vibrational energy is observed. For each initial HF rotational state, the vibrational distribution peaks at v = 0, decreasing monotonically thereafter, while increasing the initial rotational state of HF increases the population of the higher vibrational states of SrF product. Cross section measurements indicate that the reaction is inhibited by HF rotation: the cross section decreases to less than one half its value on comparing Sr+HF(v = 1, J = 1) to Sr+HF(v = 1, J = 3). A large uncertainty in the J = 4 measurement could p...

Determinations of bond energies by time-of-flight single-collision chemiluminescence

Abstract A pulsed beam of metastable atoms traverses a scattering chamber filled with oxidant gas at low pressures (beam + gas arrangement); the resulting chemiluminescence is spectroscopically resolved as a function of time to yield a time-of-flight (TOF) spectrum for different internal states. From this data, the initial relative translational energy distribution is derived for the reactants that populate the excited internal state observed. Lower bounds are placed on the barium halide (BaX) dissociation energies, using the reactions Ba( 3 D) + X 2 → BaX* + X, where X = Br, I. Arguments are presented to show that these lower bounds represent measurements of the true bond energies: it is concluded that D 0 0 (BaBr) = 85.8 ± 2 kcal/mole and D 0 0 (BaI) = 72.9 ± 2 kcal/mole. The present work corrects previous determinations of bond energies from single-collision chemiluminescent studies which were in error because of unrecognized metastable contamination in the high-temperature atomic beam.

Upper and lower bounds on the F5S–F bond energy

The upper and lower bounds for the F5S‐F bond energy based on the chemiluminescent reaction of SF6 with metastable Ca and Sr atoms are reported. These observations were made under single‐collision conditions. AIP

Low-energy electron impact study of acetone *

A low‐energy, high resolution, electron impact spectrometer with a molecular beam target has been constructed and used to study the electronic energy levels of acetone vapor. Similar spectra were also taken with a second electron spectrometer having a static gas target. Both sets of spectra are essentially identical and show the expected singlet states at 4.37, 6.35, and 7.45 eV, the first triplet state at 4.16 eV, and a very broad second triplet process from 5.3 to 6.1 eV with an intensity maximum at 5.88 eV. Attempts to resolve vibrational structure in the latter band were unsuccessful, even though vibrational spacings of 0.14 eV were easily resolved elsewhere in the spectrum.