E.H. Wahl

Methyl radical measurement by cavity ring-down spectroscopy

Abstract Cavity ring-down spectroscopy has been used to measure the absorbance of methyl radicals near 216 nm. The methyl radicals are generated by a hot tungsten filament heated to 2300 K in a mixture of 0.5% CH 4 in H 2 slowly flowing through the reactor at 20 Torr total pressure. CH 3 absorbances are detected with a noise-equivalent sensitivity of two parts in 10 5 using a narrow pencil of UV light 0.5 mm in diameter, which allows measurement of spatial profiles of CH 3 for column densities of 3×10 12 radicals/cm 2 (3×10 12 radicals/cm 3 ×1 cm absorption pathlength).

Measurement of the methyl radical concentration profile in a hot‐filament reactor

The spatial profile of methyl radical concentration in a hot‐filament reactor has been measured using cavity ring‐down spectroscopy (CRDS) at a wavelength of 213.9 nm for which the CH3 absorption cross section has been shown to be nearly independent of temperature. Methyl radicals are generated with a 25 mm long tungsten filament heated to 2400 K in a slowly flowing mixture of 0.6% CH4 in H2 (20 Torr total pressure). CRDS is employed to measure CH3 absorbance as a function of a distance perpendicular to the axis of the filament. The CH3 absorbance profiles do not change when the direction of the process gas flow through the reactor is reversed, which indicates cylindrical symmetry of the CH3 distribution about the filament. Consequently, the radial CH3 concentration in the reactor is determined by Abel inversion of the CH3 absorbance profile. The CH3 concentration peaks ∼4 mm from the filament (1.04×1014 molecules/cm3). Methyl radicals decay rapidly as a function of a distance from the filament and disapp...

Measurement of absolute CH3 concentration in a hot-filament reactor using cavity ring-down spectroscopy

Abstract Methyl radicals were generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 was measured during deposition using a new line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements were carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross-section has been shown by others to be independent of the temperature over a large range. We observed that the CH 3 absolute concentration varied strongly as a function of the position between the substrate and the filament, and its value was strongly influenced by the substrate temperature.

Near-infrared cavity ringdown spectroscopy of water vapor in an atmospheric flame

We have used cavity ringdown spectroscopy CRDS to measure near-infrared overtone transitions of water in . atmospheric flames propane premixed jet and laminar methane-air flat flame burner . The strong signal output with a well-defined laser beam direction and the insensitivity to strong background emission present in hostile environments make CRDS ideal for the study of combustion environments. We have obtained spectra of water vapor from within a flame and extracted a profile of the rotational temperature and concentration of water vapor as a function of distance from the plane burner surface. q 1998 Elsevier Science B.V.

Cavity ring-down spectroscopy with Fourier-transform-limited light pulses

Abstract We have investigated the implications of using a pulsed, nearly Fourier-transform-limited, single-mode light source for cavity ring-down spectroscopy (CRDS) in the mid-infrared spectral range. We show that in the case where the coherence time and duration of the light pulse exceeds the cavity roundtrip time, mode beating generates oscillations in the ring-down waveform. When the period of the oscillations is comparable to the ring-down time, it becomes difficult to obtain meaningful decay constants. This situation can be avoided by careful choice of cavity geometry and mode matching conditions together with suitable electronic filtering.

Spatial concentration and temperature distribution of CH radicals formed in a diamond thin-film hot-filament reactor

Abstract Spatial concentration and temperature profiles of the CH radical in a hot-filament chemical vapor deposition reactor are measured by cavity ring-down spectroscopy. The CH concentration is found to be on the order of 10 11 molecules/cm 3 . The spatial distribution indicates that CH formation primarily occurs at or very near the filament. At a distance of 2 mm from the filament the [H]/[H 2 ] ratio is determined to be 0.011±0.003.

Spatially resolved measurements of absolute CH3 concentration in a hot-filament reactor

Abstract Methyl radicals are generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (20 mm long) in a slowly flowing mixture of 0.5% CH 4 in H 2 . The UV absorbance of CH 3 is measured during deposition using a line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements are carried out at 213.9 nm, a wavelength at which the CH 3 absorption cross section has been shown, by others, to be independent of temperature over a large range. We observe a strong sensitivity of the methyl radical concentration throughout the reactor to the substrate temperature. At some operating conditions, we also observe the methyl radical concentration to peak at a location several millimeters from the filament surface. This behavior of CH 3 with distance from filament is in qualitative agreement with two-dimensional models of the deposition environment, and is attributed to the effect of Soret diffusion on the balance of the primary methyl production/destruction reaction.

Near-surface reduction of cavity ring-down spectroscopy detection sensitivity

Abstract Cavity ring-down spectroscopy (CRDS) is a high-sensitivity technique used to measure the absolute concentrations of absorbing species. We have used CRDS to measure spatially dependent concentration profiles near a physical surface that obstructs the light path. A decrease in detection sensitivity resulting from interaction between the circulating cavity ring-down beam and the surface was observed. Simple calculations based on geometrical obstruction by the solid surface significantly underestimate the sensitivity loss, suggesting that diffraction is important. Calculations that include diffraction effects more closely agree with experimental data.

Spatially resolved measurements of CH concentration and temperature in a hot filament CVD reactor

We have used Cavity Ring-Down Spectroscopy (CRDS) to measure CH radicals inside a Hot Filament Chemical Vapor Deposition (HFCVD) reactor during the formation of synthetic diamond films. Although the HFCVD technique has been used for several years, the elementary chemical processes in the gas phase as well as the surface reactions are not completely understood. The high sensitivity of CRDS allows determination of rotational temperature as well as the absolute concentration of CH radicals. Furthermore, the high spatial resolution of CRDS allows two-dimensional measurement of concentration and temperature profiles between the filament and the substrate. These measurements are compared with Two-photon Absorption Laser Induced Fluorescence (TALIF) measurements of atomic hydrogen Doppler temperatures. These measurements aim to provide an accurate database for comparison with simulations of the gas phase chemistry during HFCVD of diamond. * Research Assistant, Member AIAA. † Senior Research Scientist, Member AIAA. ‡ Professor Mechanical Engineering, Member AIAA. ** Postdoctoral Fellow †† Professor Chemistry. Copyright