Thomas A. Germer

Picosecond Time-Resolved Adsorbate Response to Substrate Heating: Spectroscopy and Dynamics of CO/Cu(100)

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm−1 (ν1) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν1 is anharmonically coupled to the frustrated translation near 32 cm−1 (ν4), so that transient shifts in ν1 indicate population changes in ν4. The ν1 response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.

Angular dependence and polarization of out-of-plane optical scattering from particulate contamination, subsurface defects, and surface microroughness.

The angular dependence and the polarization of light scattered by a small particle a distance d inside and outside a reflecting surface is calculated in the Rayleigh limit. This calculation yields expressions for the polarized bidirectional reflectance distribution function matrices for in-plane and out-of-plane scattering. The results are compared with those obtained from microroughness-induced scattering. For the p-in/p-out configuration with oblique incidence, there exist out-of-plane angles for which scattering that is due to one of the mechanisms vanishes, whereas that from the others does not. By exploiting this knowledge, we can make improvements in the detection of small particles or subsurface defects. It is also shown that one must take care when differentiating subsurface-defect-induced scattering from microroughness-induced scattering using in-plane scattering and wavelength scaling laws.

Picosecond measurement of substrate‐to‐adsorbate energy transfer: The frustrated translation of CO/Pt(111)

The transient infrared response of CO/Pt(111) following picosecond visible excitation is reported. A spectrally broad decrease in reflectivity correlates with heating of the Pt lattice, and an observed shift in the CO(v=0→1) transition is interpreted as heating of the 60 cm−1 in‐plane frustrated translational mode. A phenomenological three temperature model that assumes the adsorbate vibrational temperature Tads exclusively couples to either the electronic temperature Te (with a time constant τe) or to the lattice temperature Tlat (with a time constant τlat) describes the temporal response of the adsorbate vibrations. The lattice phonon temperature Tlat(z,t) and measured temperature dependence of the optical constants predict the observed spectrally broad reflectivity change. Density matrix methods model the infrared response of the transiently heated molecule. Limits of τe=2±1 ps or τlat<1 ps are established by comparison of predicted spectra and the data.

Polarization of light scattered by microrough surfaces and subsurface defects

The polarization of light scattered into directions out of the plane of incidence for 532-nm light incident at 45° with p polarization was measured from rough silicon, rough titanium nitride, polished fused silica and glass ceramic, and ground and incompletely polished black glass. Models for polarized light scattering from microroughness, subsurface defects, and facets are reviewed. The measurements demonstrate the validity of the models and the utility of polarized light scattering measurements for distinguishing between roughness and defects.

Anisotropic, hierarchical surface patterns via surface wrinkling of nanopatterned polymer films.

By combining surface wrinkling and nanopatterned polymer films, we create anisotropic, hierarchical surfaces whose larger length-scale (wrinkling wavelength) depends intimately on the geometry and orientation of the smaller length-scale (nanopattern). We systematically vary the pattern pitch, pattern height, and residual layer thickness to ascertain the dependence of the wrinkling wavelength on the nanopattern geometry. We apply a composite mechanics model to gain a quantitative understanding of the relationship between the geometric parameters and the anisotropy in wrinkling wavelength. Additionally, these results shed light on the effect of surface roughness, as represented by the nanopattern, on the metrology of thin films via surface wrinkling.

Detection of circular polarization in light scattered from photosynthetic microbes

The identification of a universal biosignature that could be sensed remotely is critical to the prospects for success in the search for life elsewhere in the universe. A candidate universal biosignature is homochirality, which is likely to be a generic property of all biochemical life. Because of the optical activity of chiral molecules, it has been hypothesized that this unique characteristic may provide a suitable remote sensing probe using circular polarization spectroscopy. Here, we report the detection of circular polarization in light scattered by photosynthetic microbes. We show that the circular polarization appears to arise from circular dichroism of the strong electronic transitions of photosynthetic absorption bands. We conclude that circular polarization spectroscopy could provide a powerful remote sensing technique for generic life searches.

Polarized light scattering by microroughness and small defects in dielectric layers.

The polarization of light scattered by the surface of a material contains information that can be used to identify the sources of that scatter. Theories for light scattering from interfacial roughness of a dielectric layer and from defects in that dielectric layer are reviewed. Methods for calculating the Mueller matrix or the Stokes vector for scatter from multiple sources and for decomposing a Stokes vector into contributions from two nondepolarizing scattering sources are derived. The theories are evaluated for a specific sample and geometry. Results show that some incident polarizations are more effective than others at discriminating among scattering sources, with s-polarized light being least effective. The polarization of light scattered from interfacial roughness depends upon the relative roughness of the two interfaces and the degree of correlation between the two interfaces. The scattering from defects in the film depends on the depth of the defect and differs from that from any one of the cases of interfacial roughness. The scattering from defects randomly distributed in the film and for small dielectric permittivity variations in the film is also calculated. Experimental results are presented for a 52-nm SiO2 film thermally grown on microrough silicon.

Influence of particle oxide coating on light scattering by submicron metal particles on silicon wafers

We report the effect of room-temperature oxidation on scattering of 633 nm light by copper particles deposited on a silicon wafer. The results provide a validation for a theory of light scattering by coated particles on a substrate and establish the lifetime of these particles as light scattering standards to be on the order of a few months. The results also suggest that the room-temperature oxidation of copper particles proceeds in a continuous manner, rather than approaching an asymptotic thickness as found by Cabrera and Mott [Rep. Prog. Phys. 12, 163 (1948)] on copper films.

Cosolvent-assisted Spray Pyrolysis for the Generation of Metal Particles

A cosolvent-assisted spray pyrolysis process was developed for the formation of phase-pure metal particles from metal salt precursors without the direct addition of hydrogen or other reducing gas. Generation of phase-pure copper and nickel particles from aqueous solutions of copper acetate, copper nitrate, and nickel nitrate over the temperature range of 450 to 1000 °C was demonstrated. Addition of ethanol as a cosolvent plays a crucial role in producing phase-pure metal powders. Results of a modeling study of ethanol decomposition kinetics suggest that cosolvent decomposition creates a strong reducing atmosphere during spray pyrolysis via in situ production of hydrogen and carbon monoxide.

Polarization of out-of-plane scattering from microrough silicon

The polarization of light scattered by silicon with a small degree of microroughness was measured out of the plane of incidence. First-order vector perturbation theory for scattering from a rough surface predicts the behavior well. The data and the theory show Brewster-like angles where p?p scattering from surface microroughness vanishes, as well as a deterministic polarization in other directions.