Marshall Lapp

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.

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.

Absorption cross sections of alkali-vapor molecules: I. Cs2 in the visible II. K2 in the red

Abstract Detailed measurements have been made of the absorption cross section of Cs 2 in the visible ( C « X 1 ∑ + g , and other higher systems) and of K 2 in the red ( B 1 ∏ u « X 1 ∑ + g ). The C s 2 values, extending from 4700 to 6700 A, vary from about 0·01 to 1·2 A 2 . A representative value in the more prominent blue and red regions of the smeared-out spectrum is several tenths A 2 . The K 2 cross section varies from 5 to 25 A 2 for averaged data from 6200 to 6750 A, with the peak of the clearly-visible band structure reaching 35 A 2 at the (2, 0) band head. The cross sections were measured from absorption spectra taken as a function of optical depth for varying amounts of several inert broadening gases. Maximum effective cell-length and temperature uncertainties lead to maximum estimated errors of roughly +25 to -35 per cent.

Raman Scattering from Flames

Laser Raman scattering data for nitrogen, oxygen, and water vapor have been obtained from hydrogen-air and hydrogen-oxygen flames. The resulting ground-state and upper-state vibrational bands exhibit strong asymmetrical broadening. Experimental spectral profiles have been fitted theoretically to give a new measurement technique for the determination of rotational and vibrational excitation temperatures.

Measurements of temperature and concentration fluctuations in turbulent diffusion flames using pulsed raman spectroscopy

Instantaneous values of temperature and major component concentrations have been measured simultaneously using pulsed Raman spectroscopy in hydrogen-air diffusion flames of low turbulence produced in a well-controlled co-flowing fan-induced combustion tunnel. Results are presented in the form of probability density functions for each of these scalar variables as a function of flame position. Simultaneously measured values of temperature and concentration (nitrogen or hydrogen) are compared with adiabatic equilibrium flame calculations of these variables. Close agreement with this simplified escription is obtained in the lean regions of the flame; in the fuel-rich regions, the concentrations of N2 are higher than theoretically predicted and those of H2 are lower. Analysis strongly suggests that these systematic deviations arise from differential diffusion of H2. Initial conditioned sampling measurements are reported for hydrogen-propane-air turbulent diffusion flames and for premixed laminar propane-air flames.

Visualization of Turbulent Flame Fronts with Planar Laser-Induced Fluorescence

This report concerns the quantitative time-resolved visualization of reaction zones in laminar, transitional, and turbulent nonpremixed flames. Two-dimensional OH molecular concentrations were measured with planar laser-induced fluorescence excited by a sheet of light (formed from a single tunable ultraviolet laser pulse) and detected with a two-dimensional, image-intensified photodiode array camera. From the resulting data details of instantaneous flame front structures (including positions, shapes, and widths) were obtained.

Single-pulse, laser-saturated fluorescence measurements of OH in turbulent nonpremixed flames

A single-pulse, laser-saturated fluorescence technique has been developed for absolute OH concentration measurements with a temporal resolution of 2 nsec, a spatial resolution of <0.1 mm(3), and an estimated accuracy of +/-30%. It has been applied in laminar, transitional, and turbulent hydrogen-air diffusion flames, providing the first reported quantitative measurements of average values, rms fluctuations, and probability density functions of OH radical concentration in nonpremixed flames.

Vibrational Raman scattering temperature measurements

Abstract Experimental data and analyses are presented for the determination of gas temperature by measurements of vibrational Raman scattering intensity ratios of Stokes Q -branch fundamental bands. The method is demonstrated for two thermal equilibrium experiments: (1) CO 2 (a gas well-suited for use in multi-component mixtures near ambient temperatures) in a test cell, and (2) N 2 (a gas well-suited for use at elevated temperatures) in a flame. This method of temperature measurement is of particular value for non-thermal equilibrium conditions, for which vibrational excitation temperatures can be assigned to each pair of vibrational level corresponding to observable Raman bands.

Raman scattering studies of combustion

Abstract Abstract–The need for spatially and temporally well-resolved non-perturbing measurement techniques for modern combustion systems has stimulated development of various new methods for sensing the physical and chemical conditions. Strong requirements are imposed upon new potential techniques by the specific needs of combustion science and technology, such as the necessity to probe rapidly fluctuating, luminous hot flows which can contain more than one phase. Furthermore, the proven capabilities for some of these applicat ions of presentday probes, such as thermocouples, hot wires, gas sampling apparatus, etc., lead to the conclusion that new combustion sensors should be developed for experimental conditions for which present probes are clearly not adequate or for which they cannot survive. Raman scattering is described here as a candidate measurement technique which can satisfy many of the needs of combustion experiments, but which must be applied with care and discretion in order to avoid disadvan...

Flame Temperatures from Vibrational Raman Scattering

Raman scattering signatures are functionally dependent upon temperature, and are therefore useful as diagnostic probes. Various Raman scattering techniques for the measurement of temperature are outlined here. Those methods based upon rotational molecular structure are then briefly discussed in order to compare and contrast them with the ones based upon vibrational structure. For flame gases, the elevated temperatures and the multicomponent, variable composition make the vibrational scattering techniques appear to be more useful than those based upon pure rotational scattering. Temperature measurements based upon vibrational Raman scattering are described next, with an emphasis on the vibrational techniques developed in this laboratory. These techniques are based upon the spectral structure of the fundamental Stokes vibrational band series, which consists of the ground state band (initial → final molecular vibrational levels: v = 0 → v = 1) and the upper state or “hot” bands (1 → 2, 2 → 3, etc.).