Peter K. Schenck

Galvanic detection of optical absorptions in a gas discharge

An electrical signal, resulting from discrete optical absorptions, has been observed for a variety of elements, including several for which such an effect had not been previously reported. In the present case, the effect is observed as a change in the voltage across a gas discharge tube produced by irradiation with a laser tuned to the wavelength of a transition of a species in the discharge. This signal may be used—without optical detection apparatus—for spectroscopic investigations or analytical determinations of materials in the discharge. Signals were obtained for transitions of lithium, sodium, calcium, barium, uranium, neon, and helium, in commercial hollow cathode lamps, and neon and helium in conventional discharge tubes.

Spectroscopy and photophysics of the CF2Ã1B1-X̃1A1 system

Abstract Laser excited single vibronic level (SVL) fluorescence and SVL fluorescence excitation spectra of the low pressure vapor phase CF2 A - X transition are reported. The spectral origin is at 268.74 nm (37 197 cm−1); extensive progressions in the bending mode ν″2 = 666 ± 5 cm−1 dominate the spectra; weaker combination bands involving the symmetric stretch ν″1 = 1186 ± 15 cm−1 are observed. Fluorescence bands appear that may be assigned as originating either from the 112n or the 2n31 vibronic levels, giving ν′1 = 976 ± 24 cm−1 or ν′3 = 900 ± 20 cm−1, respectively. Measured vibronic band intensities 〈v′|v″〉2 are given for all transitions from the upper states 2n for 0 ≤ n′2 ≤ 6. The collision-free A (0,0,0) state radiative lifetime is 61 ± 3 nsec. This same lifetime is observed even for vibronic states containing 8000 cm−1 excess vibrational energy (i.e., the n′2 = 16 level). SVL fluorescence spectra and radiative lifetimes were used to calculate the transition dipole moment R e = 1.22 D for this system, and a low resolution absorption cross section σ(200; 300 K) = 6.7 × 10−19 cm2 for the A - X origin at band maximum (268.74 nm).

Investigation of the structure of barium titanate thin films by Raman spectroscopy

Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal‐organic chemical vapor deposition (MOCVD) and laser‐assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non‐BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and transmission electron microscopy. The film‐to‐film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity‐phase features, was qualitatively consistent with the x‐ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser‐deposited film varies significantl...

Opto-galvanic spectroscopy of a neon discharge: Mechanism studies

Irradiation of gaseous discharges with a tunable laser produces easily observed voltage changes at wavelengths which correspond to electronic transitions for species in the discharge. This work reports observations made in the 572–654 nm region on a neon glow discharge, which is operated at 5 torr (670 Pa) pressure and 2–25 mA current. A wide variety of absorption transitions have been detected which originate in the 1sn (3P2,1,0, 1P1) and 2pn excited states. The resulting voltage signals are both positive and negative, and some transitions exhibit sign changes as a function of discharge current. Some of the results can be interpreted in terms of processes which directly affect the neon metastable atom concentration (3P2, 3P0) and states also produce voltage changes which can be explained in terms of processes which either enhance or decrease ionization.

Self-assembled multiferroic nanostructures in the CoFe2O4-PbTiO3 system

The effect of substrate orientation on the morphologies of epitaxial self-assembled nanostructures was demonstrated using multiferroic 0.67PbTiO3-0.33CoFe2O4 thin films. The two-phase composite films were grown by pulsed laser deposition on single crystal SrTiO3 substrates having (001) and (110) orientations. The nanostructures of both orientations consisted of vertical rod- or platelet-like columns of CoFe2O4 dispersed in a PbTiO3 matrix. For the (001) orientation the platelet habits were parallel to the {110} planes, whereas for the (110) orientation the platelets were parallel to the {111} planes. The differences were explained using a thermodynamic theory of heterophase structures.

Absorption spectra of metal oxides using optogalvanic spectroscopy

The absorption spectra of neutral flame species may be detected by measuring current changes induced by photon irradiation at wavelengths corresponding to electronic transitions. This paper presents the first results using this optogalvanic spectroscopy to study molecular absorption. Numerous transitions have been detected for ScO, YO, and LaO in the 360–630 nm wavelength region. The most extensive data have been obtained for LaO, where we observe a total of at least four electronic states and 18 sequences. Five of the sequences have not been seen in previous emission studies.

A high-throughput thermoelectric power-factor screening tool for rapid construction of thermoelectric property diagrams

The authors have developed a high-throughput screening tool that maps out thermoelectric power factors of combinatorial composition-spread film libraries. The screening tool allows one to measure the electrical conductivity and Seebeck coefficient of over 1000 sample points within 6h. Seebeck coefficients of standard films measured with the screening tool are in good agreement with those measured by traditional thermoelectric measurement apparatus. The rapid construction of thermoelectric property diagrams is illustrated for two systems: (Zn, Al)–O binary composition-spread film on Al2O3 (0001) and (Ca,Sr,La)3Co4O9 ternary composition-spread film on Si (100).

INFLUENCE OF THE MICROSTRUCTURE OF PT/SI SUBSTRATES ON TEXTURED GROWTH OF BARIUM TITANATE THIN FILMS PREPARED BY PULSED LASER DEPOSITION

Pt-coated silicon substrates with strong (111) Pt texture were annealed in an oxidizing atmosphere at temperatures from 500 °C to 750 °C. BaTiO 3 thin films were deposited by pulsed laser ablation on the substrates. Observation by transmission electron microscopy showed that the substrate anneal caused the formation of TiO 2 in the Pt layer, accompanied by the formation of a high density of faceted protrusions on the Pt surface, particularly at the higher anneal temperatures. The Pt protrusions had (111) facets, parallel to the substrate surface, on which (100)-oriented BaTiO 3 grains were observed. BaTiO 3 grains with an epitaxial relationship to the Pt lattice were observed on inclined facets of the Pt protrusions [which were not (111) planes], and also on the nonplanar regions of the Pt surface. These epitaxial BaTiO 3 grains had (111) preferred orientation relative to the substrate surface. Thus, the BaTiO 3 films displayed bimodal growth behavior, with both (100) texture and (111) epitaxy. We propose a model for this behavior based on surface energy considerations.

Gas-dynamic effects in the laser-pulse sputtering of AlN: is there evidence for phase explosion?

Abstract The overall light intensity (fluorescence) of the sputtered atoms, ions, and molecules has been measured for polycrystalline AlN which was bombarded with 248 nm laser pulses in the presence of a background pressure of N 2 . AlN is unusual in that, in spite of a ∼6 eV band gap, it is easily rendered highly absorptive of 248, 308, or 694 nm laser pulses. In fact, since it is well established in other work that ∼1.5 J/cm 2 (308 nm) brings AlN to the melting temperature, ∼3050 K, we will assume that the fluence used here (∼20 J/cm 2 ) was more than enough to bring the target surface first to a temperature sufficient for normal vaporization but finally to the vicinity of the thermodynamic critical temperature, T tc . As a result a significant quantity of particles can be assumed to have been expelled by phase explosion. The tentativeness in the argument rests in the problem that some part of the incident fluence beyond ∼1.5 J/cm 2 will have been consumed in laser–plume interaction. Nevertheless there is evidence in work by Pedraza et al. [A.J. Pedraza, J-Y. Zhang, H. Esrom, Mater. Res. Soc. Symp. Proc. 285 (1993) 209] that both AlN and Al respond linearly to the fluence up to at least 6 J/cm 2 . It was found that the assumed phase-exploded particles decelerated rapidly, possibly due to their encounter with the normally vaporized particles, or possibly due to an electric field arising from positive charging of the target surface. The fluorescence maximum (which can be safely assumed to be also a density maximum) was then nearly stationary, a situation which characterized the lowest background pressures of N 2 (≤3.5 Pa). At higher pressures (≥3.5 Pa) a second fluorescence maximum appeared nearer the contact front and was found to move. Following the suggestion of Horwitz 1 we take this feature as being an artifact of electrons near the contact front diffusing (or scattering) backwards and causing fluorescence which is unrelated to the particle density. From the velocity of the contact front one obtains explicit information on the mean kinetic energies ( E 4 ) of the particles in the plume (1.5–2 eV). Another estimate of E 4 follows from the initial expansion observed from 0–200 ns (1.5–3 eV). Such energies suggest, independently of the fact that the fluence was high (∼20 J/cm 2 ), that a temperature near T tc was reached and that phase explosion may have occurred. We finally note that, however tentative is the claim for phase explosion, it is certain that a close relative of phase explosion, due to subsurface heating, was not involved. This is because the numerical demonstrations of subsurface heating have been flawed.

Lead Zirconate-Titanate Thin Films Prepared by the Laser Ablation Technique

Lead zirconate-titanate (PZT) thin films were prepared by the laser ablation technique. The PZT (Zr/Ti=53/47) target was irradiated using a focused q-switched Nd:YAG laser (15 ns, 100 mJ at 1.064 μ;m). The as-deposited films were amorphous as indicated by X-ray powder patterns, but crystallized readily with brief annealing above 650°C. The dielectric constant and the resistivity of the crystallized films were studied using a parallel-plate type capacitor structure.