Cavity ring-down spectroscopy of CH and CD radicals in a diamond thin film chemical vapor deposition reactor

A mixture of H2 and CH4 is passed over a hot-wire tungsten filament in a diamond thin film chemical vapor deposition reactor. The resulting CH radicals are measured in absorption using cavity ring-down spectroscopy (CRDS). The concentration of the CH radicals increases as the filament is approached. The rotational temperature measurements indicate a large temperature discontinuity between the filament and the CH in the gas phase. The pathways for CH production were investigated by replacing H2 by D2 in the feed gas mixture, which resulted in the exclusive production of CD. From this observation it is concluded that rapid H/D isotope exchange dominates in the gas phase. Nonperiodic temporal oscillations in the CH concentration are observed when a rhenium filament is used in place of a tungsten filament. The oscillations are attributed to the nonperiodic changes in the amount of carbon at the filament surface. PACS: 81.15.Gh; 82.80.Ch; 82.40.Bj Cavity ring-down spectroscopy (CRDS) is a novel laserbased method for absorption measurements of species that either weakly absorb or exist at low concentrations [1, 2]. In this study CRDS is used as a gas-phase diagnostic in a diamond thin film chemical vapor deposition (CVD) reactor. The high sensitivity (which can approach the shot-noise limit [3]), the simple quantification without the need for a calibration procedure, and the capability of spatial resolution make this technique especially well suited for this purpose. Since its invention, CRDS has been used to study a variety of molecules in CVD reactors [4–6]. Competitive diagnostic methods that have been used in diamond CVD reactors are laser-induced fluorescence (LIF) [7–9], optical emission spectroscopy (OES) [10], coherent anti-Stokes Raman spectroscopy (CARS) [11, 12], resonance-enhanced multiphoton ionization (REMPI) [13–17], third-harmonic generation [18, 19], degenerate four-wave mixing (DFWM) [20], ∗Corresponding author. (Fax: +1-650/723-9262, E-mail: [email protected]) mass spectrometry [21–24], vacuum UV[25–27], IRabsorption [28], and gas chromatography (GC) [29, 30]. One of the main disadvantages of these other methods is the need for a calibration procedure to derive (absolute) concentrations. In the simplest implementation of CRDS, a short lightpulse from a laser is injected into a high-finesse linear resonator built from two highly reflective mirrors. The absorbance of a sample is measured by observing the decay of light intensity within the optical resonator that encloses the sample [2]. Detecting the light exiting through one of the mirrors is a way of monitoring the intensity decay within the cavity. For a sufficiently short laser pulse, the light inside the cavity can be visualized as a light pulse bouncing back and forth between both mirrors. On each round-trip a constant fraction of the light inside the resonator is transmitted through the mirrors. With a fast photodetector a train of pulses whose successive intensities decay exponentially would be observed. A slow detector shows only the envelope of the decaying train of pulses. Thus the intensity I(t) at time t is related to the initial intensity I0 by I(t)= I0 exp(−t/τ) (1) where τ is the ring-down lifetime. The value of τ is inversely proportional to the losses of the empty resonator (e.g., mirror and scattering losses) and to losses from absorption of gaseous species present within the resonator. By measuring the ring-down lifetimes τ and τ0 for a cavity with and without a sample present, respectively, the sample absorbance A is determined according to