Steven M. Beck

Mixed metal–silicon clusters formed by chemical reaction in a supersonic molecular beam: Implications for reactions at the metal/silicon interface

We report observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom–silicon clusters. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and F2 laser photoionization time‐of‐flight mass spectrometry. Three group‐VIB transition metals and copper have been investigated. The dominant product cluster peaks observed in the mass spectra obtained for all three group VIB metals corresponds to identical but remarkable cluster stoichiometries. The dominant product peaks have formulas given by MSin where n=16. Copper results are different than the other three metals, indicating the importance of the metal valence electronic structure to the chemistry. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interfa...

Synthetic-aperture imaging laser radar: laboratory demonstration and signal processing.

The spatial resolution of a conventional imaging laser radar system is constrained by the diffraction limit of the telescopes aperture. We investigate a technique known as synthetic-aperture imaging laser radar (SAIL), which employs aperture synthesis with coherent laser radar to overcome the diffraction limit and achieve fine-resolution, long-range, two-dimensional imaging with modest aperture diameters. We detail our laboratory-scale SAIL testbed, digital signal-processing techniques, and image results. In particular, we report what we believe to be the first optical synthetic-aperture image of a fixed, diffusely scattering target with a moving aperture. A number of fine-resolution, well-focused SAIL images are shown, including both retroreflecting and diffuse scattering targets, with a comparison of resolution between real-aperture imaging and synthetic-aperture imaging. A general digital signal-processing solution to the laser waveform instability problem is described and demonstrated, involving both new algorithms and hardware elements. These algorithms are primarily data driven, without a priori knowledge of waveform and sensor position, representing a crucial step in developing a robust imaging system.

Studies of silicon cluster–metal atom compound formation in a supersonic molecular beam

The first observation of a reaction between a metal atom and silicon in a supersonic jet to form metal atom silicon clusters is reported. Using the technique of laser vaporization supersonic expansion with metal carbonyl seeded carrier gas, clusters of the form MSin have been detected by ArF and KrF laser photoionization time‐of‐flight mass spectrometry. Three transition metals have been investigated, Cr, Mo, and W. The dominant product cluster peaks observed in the mass spectra obtained for all three metals corresponds to identical but remarkable cluster stoichiometries. The dominant peaks have formulas given by MSin where n=15 and n=16. The metal–semiconductor clusters are relatively more stable towards photofragmentation than the bare silicon cluster of the same size. The observation of these new species may be relevant to reactions which occur at the interface between a silicon wafer and deposited metals.

Picosecond transient reflectivity of unpinned gallium arsenide (100) surfaces

Surface recombination was measured for photowashed and unwashed GaAs using picosecond transient photoreflectance methods. The results for the washed surfaces clearly demonstrate slow surface recombination that is accurately described by an ambipolar diffusion model. The fast decay observed for unwashed samples implies rapid surface recombination involving a more complex mechanism.

Cell coatings to minimize sample (NH 3 and N 2 H 4 ) adsorption for low-level photoacoustic detection

A photoacoustic signal rise time technique is used to evaluate the suitability of several surface materials for minimizing sample adsorption loss in a trace level toxic vapor monitor. Four materials, 304 stainless steel, gold, paraffin wax, and Teflon, are tested using ammonia as a sample. Teflon is also tested using hydrazine. Results show that both metals interact strongly with the sample. Teflon coating is found to provide the resonant photoacoustic cell with accurate real time response for both hydrazine and ammonia sample flows.

Multiwatt continuous-wave and Q-switched Er:YAG lasers at 1645 nm: performance issues

The cw and Q-switched performance of Er:YAG lasers operating at 1645 nm were measured. Guided by previous work in the literature, we sought to improve efficiency at low pulse repetition frequencies by decreasing the doping level from 0.5 to 0.25 at.% to reduce upconversion losses. Only a small improvement was obtained with this first-time-tested lower-doped material. Measurements of the fluorescence due to upconversion directly indicated that loss due to this process could not account for the observed power loss at low pulse repetition frequencies. Enhanced green emission during Q-switched operation, resulting from two-photon absorption of 1645 nm intracavity laser light, is reported for what we believe to be the first time. Measurements indicated that the output loss from this process is negligible.

Photofragmentation of mass‐selected (C6H6)+n clusters: Measurement of monomer–cluster binding energy for n=7–15

Benzene cluster cations up to 15 molecular units in size, produced in the throat of a supersonic nozzle by laser‐induced plasma formation, are studied by mass‐selected photofragmentation spectroscopy. All of the clusters exhibit a strong, broad absorption which extends at least from 690 to 1064 nm, centered near 900 nm. This absorption is assigned to the dimer based intervalence transition observed in solid state spectra of the dimer ions. Excitation into this band produces prompt ejection of neutral benzene molecules from the cluster ion. At low fragmentation laser intensities, a single daughter ion species is observed. We observe a simple dependence of the number of neutral molecules ejected on the fragmentation laser wavelength, strongly suggesting a sequential evaporation process of neutral benzene molecules. For clusters larger than six, an average bond energy ranges from 0.37–0.34 eV/molecule, quite close to the heat of vaporization for liquid benzene. The 14 molecule cluster is found to have an ano...

Demonstration of synthetic aperture imaging ladar

The spatial resolution of a conventional imaging LADAR system is constrained by the diffraction limit of the telescope aperture. The purpose of this work is to investigate Synthetic Aperture Imaging LADAR (SAIL), which employs aperture synthesis with coherent laser radar to overcome the diffraction limit and achieve fine-resolution, long range, two-dimensional imaging with modest aperture diameters. This paper details our laboratory-scale SAIL testbed, digital signal processing techniques, and image results. A number of fine-resolution, well-focused SAIL images are shown including both retro-reflecting and diffuse scattering targets. A general digital signal processing solution to the laser waveform instability problem is described and demonstrated, involving both new algorithms and hardware elements. These algorithms are primarily data-driven, without a priori knowledge of waveform and sensor position, representing a crucial step in developing a robust imaging system. These techniques perform well on waveform errors, but not on external phase errors such as turbulence or vibration. As a first step towards mitigating phase errors of this type, we have developed a balanced, quadrature phase, laser vibrometer to work in conjunction with our SAIL system to measure and compensate for relative line of sight motion between the target and transceiver. We describe this system and present a comparison of the vibrometer-measured phase error with the phase error inferred from the SAIL data.

Narrowband Er:YAG nonplanar ring oscillator at 1645 nm

We report 1645 nm narrowband operation of a monolithic Er:YAG nonplanar ring oscillator resonantly pumped at 1532 nm. Unidirectional cw power up to 0.5 W was obtained with a measured linewidth of 21 kHz.

Phase transition behavior observed in small silicon cluster ions

Small silicon clusters produced by laser vaporization supersonic expansion are heated to different internal temperatures during a subsequent photoionization step, depending upon the ionization laser wavelength. Following ionization and mass selection the cluster is photofragmented by a 532 nm pulse from a Nd:YAG laser, and the charged fragments are collected and mass analyzed by a time‐of‐light mass spectrometer. It is found that the fragmentation patterns for clusters larger than Si+10 have a dependence upon the cluster temperature. This dependence is ascribed to clusters which are fairly rigid structures at low temperatures, undergoing a transition to a fluxional state at higher temperatures. Such a transition is analogous to a phase transition in larger systems.