KEYWORDS: Diamond, Carbon dioxide lasers, Molecules, Combustion, Emission spectroscopy, Molecular lasers, Gas lasers, Temperature metrology, Chemical vapor deposition, Thin films
Optical emission spectroscopy (OES) measurements were carried out to study premixed C2H4/O2 and C2H4/C2H2/O2
combustion flame for diamond deposition with and without a CO2 laser excitation. Strong emissions from radicals C2
and CH were observed in the visible range in all the OES spectra acquired. By adding a continuous-wave CO2 laser to
irradiate the flame at a wavelength of 10.591 μm, the common CO2 laser wavelength, it was discovered that the emission
intensities of the C2 and CH radicals were increased due to the laser beam induced excitation. OES measurements of the
C2 and CH radicals were performed using different gas combinations and laser powers. The rotational temperatures in
the flame were determined by analyzing the spectra of the R-branch of the A2Δ→X2Π (0, 0) electronic transition near
430 nm (CH band head). Information obtained from the OES spectra, including the emission intensities of the C2 and CH
radicals, the intensity ratios, and the rotational temperatures, was integrated into the study of diamond deposition on
tungsten carbide substrates for mechanism analysis of the laser induced vibrational excitation and laser-assisted diamond
deposition.
Excited C2 and CH species occur abundantly in diamond growth using C2H2/O2, C2H2/C2H4/O2 and C2H4/O2 flames. The
irradiation of some flames by a continuous-wave (CW) CO2 laser beam has resulted in increased optical emission
intensity from the excited species and a change in the physical appearance of the flames due to resonant absorption of
laser energy. Gas temperature in the flames is one of the most important parameters in the application of diamond
growth. In atmospheric plasmas, the gas kinetic temperature is closely related to the rotational temperature of radical
species in the plasmas. Optical emission spectroscopy (OES) was used to obtain molecular spectra of the excited C2 and
CH species in the flames for a fixed gas of C2H2/C2H4/O2 flame at several laser energies. The rotational temperatures of
CH were calculated using the Boltzmann plot method. In addition, synthetic C2 molecular spectra were compared with
the experimental spectra to obtain temperature by the intensity ratio of selected spectrum components. For each
condition, the temperatures obtained using these methods were correlated with the quality, grain size, and growth speed
of diamond films on cemented tungsten carbide (WC-Co) substrates.
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