Carl M. Penney

Absolute rotational Raman cross sections for N 2 , O 2 , and CO 2

We present absolute intensities for rotational Raman scattering (RRS) from N2, O2, and CO2, excited at 488.0 and 647.1 nm. The absolute scattering intensity for N2 at 488.0 nm is characterized by its differential cross section for backscattering, summed over Stokes and anti-Stokes bands and over scattered-light polarizations, which we find to be 1.64 × 10−29 cm2/sr ±8%. The ratio of the cross section for O2 to that for N2 at 488.0 nm is 2.61 ± 5%, whereas the corresponding ratio for CO2 to N2 is 10.6 ± 10%. Our values for RRS cross sections relative to the N2 vibrational Raman cross section are in reasonable agreement with corresponding ratios reported recently by Fenner et al. On the other hand, our absolute cross sections are approximately twice as large as those obtained from the results of Fenner et al., but agree closely with values calculated from recent measurements of the depolarization of Rayleigh scattering. Detailed observations of relative rotational-Raman-line intensities at temperatures of 22, 75, and 125 °C are consistent with theoretical predictions.

Laser Raman Gas Diagnostics

Abstract : Examining the use of Raman scattering as a gaseous systems probe, this volume emphasizes the application of laser Raman gas diagnostics to fluid mechanics and combustion measurements. Topics ranging from the basic physics and chemistry involved in scattering measurements to present and potential applications in flame systems, industrial process control, and the atmosphere are discussed. This book investigates important fluid mechanics problems for which Raman scattering can contribute new information and surveys efforts in the development of diagnostic capabilities designed to correspond to the needs of fluid mechanics. In addition, this work reports on significant nonthermal and nonchemical equilibrium problems and explores types of laser sources, spectral discrimination systems, and signal processing.

Raman-scattering cross sections for water vapor

We present cross sections for Raman scattering from water vapor excited at the four argon-laser wavelengths 476.5, 488.0, 496.5, and 514.5 nm. These cross sections, for the strong vibrational band centered at a shift near 3654 cm−1, are approximately 2.5 times larger than the corresponding N2Q-branch cross sections, and follow closely the expected (1/λR)4 dependence, where λR is the Raman scattering wavelength. We also report observations of the room-temperature depolarization and spectral profile of the band, and the overall spectral distribution of the scattering from 470 to 630 nm, for incident light at 488 nm. The latter observation indicates the absence of any other strong vibrational Raman-active modes.

Light Scattering in Terms of Oscillator Strengths and Refractive Indices

Quantum-mechanical expressions for nonresonance-dipole light-scattering cross sections are related to corresponding expressions for oscillator strengths and refractive indices. In particular, Rayleigh and Raman cross sections for atoms are expressed in terms of oscillator strengths and vector-coupling coefficients. Several of our results, including the relationship between Rayleigh scattering and refractive index, differ in general from the corresponding results of classical dispersion theory. We show that these differences arise from antisymmetric components Ckja = (Ckj − Cjk)/2 of the polarizability tensor, which have been neglected in the classical analyses. Calculated Rayleigh cross sections for cesium and aluminum atoms, and Raman cross sections for aluminum atoms are presented to illustrate our results. The antisymmetric contribution is found to be substantial in all of these cross sections; its most obvious effect is to cause the depolarization (for linearly polarized incident light) to exceed 34 over extended wavelength ranges away from resonance. On the other hand, for atoms initially in states of zero angular momentum, and molecules under conditions which allow the use of Placzek’s polarizability approximation, our results agree with those of classical dispersion theory.

Laser generation of ultrasonic waves for nondestructive testing

An object is tested for thickness or for presence and location of flaws by impacting a pulsed laser beam on the surface of the object so as to generate ultrasonic waves by rapid heating of a thin surface layer thereon. The reflected ultrasonic echo in the object is detected by monitoring surface motion with a rough surface interferometer.

Corneal refractive surgery using an ultraviolet (213 nm) solid state laser

The authors demonstrate the use of a frequency multiplied Q-switched Nd:YAG laser to generate the fifth (213 nm) and the fourth (266 nm) harmonic frequencies to ablate porcine corneas and synthetic collagen materials. This new strategy is discussed for corneal refractive surgery using a solid state laser. The potential for spatially resolved refractive correction based on a 213 nm solid state laser is reviewed in light of new corneal imaging technology that can detect localized refractive errors of the eye. Finally, the authors discuss remaining problems that need to be addressed before this technology can be applied to clinical investigation.

High-speed, high-resolution 3D range camera

There is a large class of machine vision inspection problems which requires detecting and characterizing voids in mostly diffuse materials. Typical examples include surface chips or gouges in ceramic parts, voids in powered metal components prior to sintering, and surface distress in construction and paving materials. There are two interrelated difficulties: achieving lighting which provides enough image contrast for reliable detection; developing image processing algorithms which can distinguish reliably between voids and image artifacts. The problems are particularly acute when inspecting composite materials containing dark inclusions which can be confused for voids, such as the aggregates in cementitious materials. We have examined this problem theoretically to understand the contrast-forming mechanisms. In this paper we present theoretical methods for modeling image contrast in images of small voids. We show how to use these methods to design appropriate illuminating systems and image processing algorithms. We validate the approach by comparing the theoretical results with experimental measurements. The prototype system we will discuss uses machine vision methods to measure air voids in cured and sectioned samples of portland cement concrete. These measurements allow estimation of air entrainment--a material property which, when properly controlled, can enhance the concretes ability to resist microcracking and structural deterioration during repeated cycles or freezing and thawing.

High-average-power solid state deep-UV lasers for medical applications

We present the results of an experiment to demonstrate high average power uv radiation at 266 and 213 nm obtained from the fourth and fifth harmonics of a Nd:YAG laser. The second harmonic of the infrared laser was generated in an LBO (lithium triborate) crystal, the fourth harmonic by doubling the output of the LBO in a BBO (beta barium borate) crystal, and the fifth harmonic was obtained by sum frequency mixing of the fourth harmonic and the fundamental beam in another BBO crystal. In this paper we report high average power uv radiation at 266 nm (1.3 W) and at 213 nm (0.54 W). To our knowledge, these are the highest powers ever achieved at these wavelengths in an AO Q-switched, Gaussian single-mode Nd:YAG system.

Measurement of fluorescence spectra and quantum yields of 193 nm ArF laser photoablation of the cornea and synthetic lenticules

Photorefractive keratomileusis procedures are currently being evaluated for both direct ablation of the human cornea and for the shaping of synthetic lenticules, with the ArF excimer laser offering the promise of reduced photochemical damage of underlying tissues due to the non-penetrative nature of 193 nm radiation. In order to better consider the possible effects of any secondary luminescence produced by the interaction of 193 nm radiation with these materials, an optical system based on a gated diode array spectrometer was constructed and used to record their fluorescence spectra at known laser fluences. A broadband fluorescence peak centered around 290-330 nm was observed upon irradiation of corneal buttons and collagen-based biopolymers. The optical efficiency of the fluorescence cell was calibrated by the use of a scattering material of known reflectivity allowing estimates of the fluorescence quantum yield of this process to be made.

Acoustically modulated optical transmission for low level gaseous species measurement

A new optical technique has been developed for low level optical extinction measurements in gaseous media. It is a variation on direct light transmission where the sample gas density in a cell is modulated acoustically by a loudspeaker. The amplitude variation of the light transmission is proportional to the gas density and is insensitive to window contamination and detector instabilities. Preliminary analysis and experiments indicate tat the instrument promises to measure light absorption to < 1%/m. The technique has been demonstrated through the use of an absorbing gas, namely, 200 ppm of NO(2) in N(2) which produces 2.5% absorption over the observed path length of 10 cm.