Search

Solid-state vibronic lasers can now be made whose pulses tune in time as well as in frequency. "Pulse stretching," or control of temporal pulse duration and shape, is effected by newly developed techniques involving intra-resonator electro-optic feedback. The physics of electro-optic feedback controlled lasers is discussed, and an alexandrite laser employing a very fast electro-optic feedback system is described. Experimental characterization of this system has demonstrated broadly tunable (720-800 nm), variable duration (100 ns–5 μs) pulses with pulse energies of several hundred millijoules. Although a number of factors affect pulse temporal and amplitude stability as well as pulse shape, temporal fluctuations of only a few percent have been achieved. These pulses are sufficiently stable and reproducible to be reliably upconverted into the UV by second harmonic generation (SHG). The characteristics of microsecond duration "stretched" pulses are reported and a number of interesting new applications in science and medicine are described. These include applications in sodium lidar, lithotripsy, and dentistry.

Two-color multiphoton adsorptions to very high lying (n = 12-41) Rydberg states in diazabicyclooctane and azabicyclooctane are analyzed. The problem of spectral congestion close to the ionization limit is overcome by choosing an appropriate two-photon intermediate electronic state, the lowest-lying 3s Rydberg, to introduce vibrational selectivity in the sequential adsorption process. The subsequent one-photon absorption to the higher lying Rydberg preserves the vibrational selectivity leading to extremely simple, uncongested spectra. The sequential absorptions are monitored by production of ions during subsequent collisions in a low-pressure cell. Analyses of the extended Rydberg series have provided experimental values of ionization potentials to particular vibrationally excited cores to +/-5-cm/sup -1/ resolution. Several series, with differing quantum defects, are seen to converge to each limit. These series are correlated with different symmetry molecular orbitals. Finally, these results suggest that large molecular ions may be produced optically in specific vibrationally excited states by using modifications of this technique.

The picosecond geminate rebinding of molecular oxygen was monitored in a variety of different human, reptilian, and fish hemoglobins. The fast (100 to 200 picoseconds) component of the rebinding is highly sensitive to protein structure. Both proximal and distal perturbations of the heme affect this rebinding process. The rebinding yield for the fast process correlates with the frequency of the stretching motion of the iron-proximal histidine mode (VFe-His) observed in the transient Raman spectra of photodissociated ligated hemoglobins. The high-affinity R-state species exhibit the highest values for VFe-His and the highest yields for fast rebinding, whereas low affinity R-state species and T-state species exhibit lower values of VFe-His and correspondingly reduced yields for this geminate process. These findings link protein control of ligand binding with events at the heme.

The two‐photon resonant n→3s Rydberg transitions in CH3CHO, CH3CDO, CD3CHO, and CD3CDO have been studied by multiphoton ionization in a parallel plate cell with 0.4 cm−1 resolution. Vibrational analysis confirms that the molecule remains planar in the resonant state, and that the methyl group torsional barriers have values of 750±25, 770±25, 750±25, and 780±25 cm−1 in the respective molecules compared to ground state values of 413, 413, 410, and 408 cm−1. The methyl group has an s–cis configuration in both the ground and excited states. Experiments using a supersonic expansion of the sample show that hot bands other than those arising from the low frequency methyl torsion are readily suppressed. Evidence for saturation effects in the MPI spectrum are also presented.

The multiphoton ionization (MPI) spectra of the title compounds have been studied in the 3533–6000 A region in an effort to elucidate the fragmentation and ionization pathways. At wavelengths longer than 4000 A, the compounds dissociate more rapidly than they are ionized, thereby forming Cr(I) which then can be ionized in a (2+1) resonant process. The resonance wavelengths are identical in the three compounds, and only Cr+ is observed in the MPI‐produced mass spectra. At wavelengths shorter than 4000 A, both Cr+ and CrO+ appear in the mass spectra of Cr(CO)6 and Cr(CO)3C6H6. Surprisingly, however, no ionization of the unbound ligands was observed in any of the complexes studied. Electron time‐of‐flight measurements in Cr(CO)6 show that at wavelengths shorter than 4000 A ionization of the parent–molecule resonant state and fragmentation to neutrals with eventual ionization of Cr(I) are important ionization paths. The observation of severe ac Stark broadening of the Cr(I) MPI resonances implies that a rate‐equation approach to ion production will be inadequate, except at very low flux. Additionally, the exclusive production of Cr+ indicates extremely rapid dissociation of the intermediate molecular fragments. The breakdown of the rate‐equation approach leads to effective power exponents which are smaller than those required by the known Cr+ mass‐spectrometric appearance potentials; this indeed is found experimentally. The branching ratios for production of various excited states of Cr(I) are dependent upon the precursor molecule and the incident laser flux.

A fully quantal derivation of the isolated line broadening parameters applicable to gas phase EPR experiments is presented. Errors inherent in the use of zero field operators are discussed. Under this simplifying approximation line shifts are shown to be identically zero, while the linewidth may be expressed in terms of either Grawert coefficients or isotropically averaged population transfer cross sections. Connection is made to a previously derived semiclassical density matrix formulation. Application of the results to He+F(2P3/2, 2P1/2) broadening will be made in a following paper.

Focused, pulsed, tunable dye lasers have been used to deposit chromium films by multiphoton dissociation of Cr(CO)6. The multiphoton dissociation of Cr(CO)6 is strongly wavelength dependent. Large increases in deposition rate occur below 350 nm, where the threshold between initial 1 and 2 photon absorption to the lowest lying dissociative state are reached in Cr(CO)6. Lack of correlation of deposition rate with multiphoton ionization (MPI) resonances, and polarity of a biased electrode in the cell, suggests multiphoton absorption leads to decomposition to Cr atoms followed by ionization. Power dependence of the deposition rates and MPI signals indicate saturation in the focal region for wavelengths 280

Previous experimental work on gas phase EPR pressure broadening of F atom lines by He has been extended to investigate temperature effects in the range ∼150–350 °K. The qualitative features of the previous room temperature work persist with unequal line broadening cross sections for the three 2P3/2 Zeeman transitions and with the 2P1/2 Zeeman transition cross section remaining a factor of 50–100 smaller. Considerable variation in the cross sections as a function of temperature is evident, with neither the 2P1/2 nor 2P3/2 cross sections exhibiting a simple temperature dependence. Comparisons with available theoretical predictions indicate that the previous treatments are not adequate to describe the present experimental results.

The ratio of signals for circularly and linearly polarized light obtained by multiphoton ionization of acetaldehyde at 3633.8 A has been measured under a wide variety of experimental conditions. Laser flux was found to strongly affect the ratio, with measured values ranging from 0.02 to 0.77. Pressures up to 50 atm of added second gas at low acetaldehyde pressure and low flux increased the ratio by a factor of two. At absorber pressures of several hundred Torr, there is a dramatic increase in the polarization ratio compared to that at

Details of the positive ion fragmentation patterns of acetaldehyde produced by collisionless multiphoton ionization are presented. Fragments are generated in the ion collection region of a quadrupole mass spectrometer and analyzed with unit mass resolution. Daughter ions are found corresponding to the absorption of up to five photons. All fragment ions show the same excitation spectrum, which closely resembles that obtained with total ion collection in both the mass spectrometer and a conventional parallel plate ionization cell. Individual fragment ion production depends strongly on laser power, going approximately as Ip, where p=1.17±0.07 for CH3CHO+, up to p=3.54±0.09 for CH+3. Total ion production varies as p=2.19±0.08. Considerable deviation from a simple power law dependence can be found for some fragments, however. A rate equation approach is used to explain the experimental results.

Grain growth habits and the structure of grain boundaries have been investigated using transmission electron microscopy and scanning Auger microscopy of bulk sintered samples of orthorhombic Ba 2 YCu 3 O 7– x . This material exhibits typical transport critical currents of 40–500 A/cm 2 at 77 K and R = 0 T c ' s near 90 K. All grains of the correct phase have the characteristic reflection twin lamellar structure, but exhibit one of two growth habits: (1) small (1–5 μm diameter) equiaxed grains, and (2) larger (3–40 μm diameter) platelet-shaped grains with an aspect ratio between the twinned a – b plane dimension and the short c -axis dimension on the order of 6. Grain boundary structure falls into one of three classes: (1) tight boundaries with only a network of dislocations at the interface, characteristic of equiaxed grains and boundaries of platelet grains not involving the c -axis boundary, (2) basal-plane-faced boundaries of platelets involving introduction of dislocation loops on the c -axis side of the boundary only, with the depth of the penetration of the dislocation loops (50–1500 A) varying with degree of local strain, and (3) basal-plane-faced boundaries of platelets where severe local strain has produced substantial plastic deformation and generation of small voids, again with the depth of the boundary region a function of local strain. We propose a model for these observations based on the highly anisotropic thermal contraction of the material during cooling from the high sintering temperatures. Scanning Auger microscopy of in situ fractured surfaces reveals segregation of carbon at the grain boundaries at 1–4 monolayer level. These structures span the spatial range likely to cause weak-link coupling between grains as the source of low transport J c in polycrystalline bulk material.

The multiphoton ionization spectrum of acetaldehyde vapor, two photon resonant at the n→3s Rydberg transition, has been studied at 1 cm−1 resolution. Vibrational analysis shows that the molecule remains planar in the resonant state, and that the methyl group torsional barrier has a value of 755±10 cm−1 in that state, compared to 413 cm−1 in the ground state. GTO calculations in the ground state and in the lowest ionic state are only partly successful in explaining the barrier. The Rydberg nature of the resonant state has been verified by the extreme broadening of its two photon absorption when pressurized by ?80 atm of argon. Due to the near resonance of the first absorbed photon with the (n,π*) state, the multiphoton transition is extraordinarily intense; the origin band is at 55 039 cm−1, and displays a circular‐to‐linear polarization ratio varying from 0.08 to 0.8 depending upon the laser power and sample pressure.

Decomposition of Au metallopolymer thin films induced by a scanning cw Ar+ laser leads to metallic features that, under a broad range of experimental conditions, exhibit marked periodic structure. Preliminary mechanistic studies point to a thermal decomposition pathway in which the heat released in the exothermic reaction drives the reaction front ahead of the laser‐irradiated area. The spatial period is determined by laser power, scan speed, and film thickness. We argue that this mechanism is analogous to laser‐initiated explosive phenomena observed in the recrystallization of amorphous semiconductors.

Transmission electron microscopy is used to characterize the defects in high Tc superconductor, Ba2YCu3O6.9 produced by two different ceramic processing procedures. In both samples the microstructure is dominated by lamellar domains. These have been identified as reflection twins by observations both of rotations of the diffraction patterns and of δ‐fringe contrast. The reflection plane is (110). The characteristic width associated with the twins correlates with annealing time. Occasionally within the twinned domain, substructure on a 50‐A scale is visible, which is attributed to further subdivision into twinned and matrix domains. The twin structure does not interrupt the continuity of the Cu‐O planes between Y and Ba layers, but does terminate Cu‐O chains in the inter‐Ba Cu‐O planes. Convergent beam electron diffraction results are consistent with a Pmmm space group. Microcracks and disloction loops are also observed in the (001) basal plane, indicating that the (001) plane is both the slip and cleavage plane in this material.

Correlations between observed defect structures and the orientation of planes involved in grain boundary formation in single phase, nearly ideal density Ba2YCu3O7−x are presented. Three types of grain boundary structures are observed: (1) coherent boundaries with only a low‐energy network of dislocations at the interface, which are associated with boundaries not involving a basal plane face, (2) semicoherent boundaries surrounded by a 50–1500 A wide band of dislocation loops on the c‐axis side of the basal‐plane‐faced boundary, and (3) incoherent boundaries where severe local strain has produced substantial plastic deformation and generation of small voids, again associated with basal‐plane‐faced boundaries. These results are interpreted using a model for local stress based on the known highly anisotropic thermal contraction of the material during cooling from sintering temperatures.

Transient absorption has been used as a probe of the reaction kinetics in the laser‐initiated thermal decomposition of Au metallopolymers. Feature sizes are considerably larger than the dimensions of the laser spot due to the propagation of a reaction front driven by the heat release from the highly exothermic reaction. Front velocities in the range of 30–50 cm/s are measured using spatially separated generating and probe lasers. These effects should be generalizable to any laser‐driven thermal reaction in the condensed phase with sufficient heat release, and, as such, constitute a major limitation on feature size obtainable in these systems.

A technique is described for preparing superconducting cuprate thin films using wet chemical precursors. This new method is simple and inexpensive, and allows facile manipulation of film stoichiometry. A solution of metal acetates in aqueous acetic acid is spun on a substrate, pyrolyzed, and heat treated. A 1600‐A film of Ba2YCu3O7 prepared by this method had a room‐temperature resistivity of 500 μΩ cm, Tc(onset) of 90 K, and R=0 at 58 K. It showed preferential orientation with the orthorhombic c axis perpendicular to the film, and random azimuthal orientation. The preparation and electrical and microstructural characterization of these films are discussed.

Micron‐sized gold features have been produced by the cw Ar+ laser‐initiated decomposition of thin organogold polymer films. These features may exhibit pronounced periodic structure under a wide variety of experimental conditions, resulting from propagation of the reaction front ahead of the laser by the heat of reaction associated with the chemical transformation. Gold features formed by irradiation of gold chloride‐doped polymer films further demonstrate these thermal effects. Thermoanalytical studies provide pertinent data for selection of suitable precursor materials.

Islanding and surface diffusion for Ge on Si(111)7×7 and Si(100)2×1 surfaces were examined in a UHV apparatus with in situ scanning Auger/SEM capabilities. At room-temperature uniform growth is observed, while elevated temperatures lead to Stranski-Krastanov growth with complex island size distributions. Extensive surface diffusion is observed on Si(100)2×l; however, surface diffusion is demonstrated to be extremely sensitive to contamination with carbon on the order of ≈0.05 ML, as well as to e-beam irradiation.

The transport Jc of the polycrystalline YBa2Cu3O7-δ superconductor seems to be dominated by weak links between high Jc regions as evidenced by low Jc values and their strong field dependence. The possible effects of thermal expansion-contraction and the tetragonal-orthorhombic transformation on the weak links and the Jc values were investigated by repeated thermal cycling of sintered pellets between -196°C and various high temperatures (600–850°C) using a furnace heating and cooling in an oxygen atmosphere. While more than a five-fold decrease in Jc from -400 to -70 A/cm2 (at 77K in zero field) is observed after 5 temperature cycles between -196'C and 850°C, only a slight decrease (to -370 Atm2) is noticed after 5 cycles between -196*C to 600°C, the temperature span of which is not all that much smaller than the former cycles. Transmission electron microscopy analysis clearly indicates that the drastic deterioration in Jc by repeated phase transformation is caused by increased amount of microcracks on (001) basal planes near the grain boundaries. The results will be discussed in terms of the large thermal expansion anisotropy of this layer-structured compound.

Although the fate of molecules undergoing non-linear absorptions in intense IR radiation fields has been studied for some time, it is only in the past four years that attention has focused on multiphoton absorption of visible/UV photons.

Thin films of high Tc cuprate superconductors have been prepared by spinning on metal organic precursors from solution, followed by pyrolysis to oxide films. This new technique allows easy manipulation of film stoichiometry, and gives excellent compositional uniformity throughout the films. A 1600 A film of Ba2YCu3O7 on 〈100〉 MgO had a room temperature resistivity of 500 μΩcm, Tc(onset) of 90K, and R=0 at 58K. The film morphology was essentially a single planar array of grains with lateral dimensions of 2–5μm, oriented with the orthorhombic c axis perpendicular to the substrate. The preparation and electrical and microstructural characterization of thin films of Ba2YCu3O7 and other high Tc cuprates will be discussed.

Group IV-IV heterostructures with Sn as one constituent have potentially important applications. We report on an investigation of the initial stages of interface formation for deposition of Sn on Si(100)2×1 and Si(111)7×7. We find that simple growth occurs up to a critical coverage θc. (α1.25×1015 cm-2), independent of temperature. Beyond θc. growth continues to be laminar for deposition at 150 K only, while island formation is observed at temperatures at and above room temperature. The Si(111)7×7 reconstruction seems unperturbed by Sn deposition at room temperature while the Si(100)2×1 begins to order. However, the reordering, a necessary condition for perfect growth, is incomplete.

Low defect density masks are necessary to obtain the high chip yields required in VLSI technology. This paper will outline the boron nitride mask fabrication process that has been developed at AT&T Bell Laboratories, and discuss the nature and distribution of normally encountered defects. The advantages and disadvantages of various repair techniques will be addressed and specific examples of repairs made on silicon integrated circuit X-ray masks will be shown.

Ba2YCu3O7 films produced by co-evaporation of BaF2, Cu and Y onto S1TiO3 substrates in an O2 ambient followed by post-annealing were analyzed using RBS, X-ray diffraction and Auger microscopy. These films exhibit Tc's (R=0) of =90K and a best Jc of > 1.0×106 A/cm2 at 81K for a =2500A thick film. RBS and Auger depth profiling limit the level of F present in the post-annealed films to

Correlations between observed defect structures and the orientation of planes involved in grain boundary (GB) formation in single phase, near ideal density orthorhombic Ba2YCu3O7−x are presented. Coherent boundaries involving only a low energy network of dislocations at the interface are associated with boundaries not involving the (001) basal plane, while basal-plane-faced GBs are highly defective. These results are interpreted using a model for local stress based on the known anisotropie thermal contraction of the material during cooling from sintering temperatures. Scanning Auger microscopy of in situ fractured surfaces demonstrates that there is preferential segregation of carbon at the GBs as well.

A key requirement for many practical applications of the new class of high Tc superconductors is the ability to sustain high current densities in the presence of magnetic fields. The new materials present two substantially different behaviors. For Ba2YCu3O7 critical current behavior of polycrystalline material is dominated by intergranular transport. We will review a model for transport involving the effects of the correlation of defects with grain boundary orientation, the presence of impurities at the grain boundaries, measurements of tunneling characteristics through boundaries, and the effect of grain alignment on critical current. The case is substantially different in the 84 K superconducting phase Bi2.2Sr2Ca0.8Cu2O8+δ where there is a very strong dependence of critical current densities at 30–84 K on magnetic field for single crystals. There is a lack of threshold behavior in the I‐V characteristics in finite applied fields larger than Hc1(T) which is attributable to flux motion. This suggests that...

In this paper, we discuss the application of polarization selective Raman microprobe spectroscopy to the detailed, non-destructive analysis of the local crystal orientation of a polysilicon sample grown over SiO 2 pads and laser annealed. Intensity measurements taken as a function of input polarization angle are fit to an expression derived from the Raman scattering selection rules to calculate the angles by which the crystal structure is twisted within the original substrate plane, as well as the degree of tipping of the crystal plane away from the plane of the Si substrate. The results give some indication as to the direction of seeding during recrystallization.

Laser induced etching has been used to pattern 2000–7000 A thick films of the superconducting perovskite Ba2YCu3O7 and its amorphous precursor. A 5145 A Ar+ laser beam, focused to a 1.0 μm beam diameter, with incident intensities of 5–250 mW, scanned at rates of 20–200 μm/s in air locally heated the optically absorbing films to induce reaction. Local melting was observed during the laser scans, indicating temperatures in excess of 950°C induced in predominantly perovskite phase films. Ablated grooves 1.5–3.0 μm wide were generated, with destruction of the superconducting phase verified by optical microscopy over a wide range of laser parameters. Scanning Auger and energy dispersive x‐ray analyses were used to monitor local chemical composition near the cuts. A wide range of behavior was found for etching of the amorphous precursor films, ranging from simple ablation, to multiple phase segregation, to generation of periodic morphological structure characteristic of local plasma formation in the ejected plu...

The technique of Raman microprobe spectroscopy has been extended to allow polarization sensitive characterization of local crystal quality with lateral mapping capability. Computer minimization algorithms have been utilized to invert Raman scattering intensity versus incident laser polarization information to local crystal orientation. This microprobe method has been applied to investigate the quality of laser annealed polysilicon over oxide films. During the course of laterally seeded epitaxial regrowth of silicon over the oxide film, it was found that the annealed crystal tends to develop large grains of varied size and orientation. Measurements of the local phonon frequency shift, which monitors strain in the material, show large variations across the structures. It is demonstrated that the strain inferred from phonon frequency shifts must be interpreted in terms of the local crystal orientation.

Molecular beam epitaxy (MBE) is an important technique for the creation of new, non-equilibrium semiconductor materials and structures exhibiting novel physical phenomena. Surface diffusion plays an important role in the growth of these structures, influencing such fundamental growth processes and constants as islanding, critical thickness and epitaxial temperatures. Two approaches to the general problem of surface diffusion and islanding, using the SiGe system as a prototypical semiconductor heterostructure, are discussed: The time evolution of patterned deposits, and kinetic studies of nucleation and growth. While disordered laminar growth occurs for deposition at 300 K, elevated temperatures lead to Stranski-Krastanow (SK) growth (uniform coverage theta SK with excess Ge in islands). Diffusion coefficients for Ge on Si(100) have been determined for coverages below theta SK and show a significant coverage dependence. They are extremely sensitive to contamination with carbon on the order of approximately 0.05 ML, as well as to e-beam irradiation. In situ annealing experiments were performed to study the islanding process in real time. Provided the initial coverage exceeds the thickness of the SK layer, theta SK approximately 3 ML on Si(100)2 x 1, the initially uniform but disordered layer begins to collapse into a SK-type morphology at about 250 degrees C. At a ramping rate of 0.1 degrees C/s this process is completed at approximately 400 degrees C. A temperature dependence of the SK-layer thickness has been discovered for the first time. It is in excellent agreement with theoretical predictions.