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Spectroscopic Determination of Rotational Temperatures in C2H4/C2H2/O2
Flames for Diamond Growth with and without Tunable CO2 Laser Excitation
Xiangnan He
University of Nebraska, 209N WSEC, Lincoln, 68588-0511, US
Phone: 402-472-8323, Email: ylu2@unl.edu
Z. Q. Xie
T. Gebre
Y. F. Lu
Abstract:
Optical emission spectroscopy (OES) and spectroscopic temperature determination were carried out to
study C2H4/C2H2/O2 flames used for diamond deposition with and without an excitation by a
wavelength-tunable continuous-wave (CW) CO2 laser. This diamond deposition process is a multienergy
process with both flame and laser energy. In this study, strong emissions from C2 and CH
radicals were observed in the visible range in all the acquired OES spectra. When the flames were
irradiated using the CW CO2 laser at a wavelength of 10.591-μm, the emission intensities of OH, CH
and C2 radicals in the flames increased owing to the laser excitation, resulting in higher radical
intensities which are a key factor in diamond deposition process since they affect the deposition of
diamond by affecting the concentrations of atomic H and OH radicals. High-resolution spectra of OH,
CH, and C2 after CO2 laser excitations were also taken to compare with those without CO2 laser
excitations. The CO2 laser was also tuned to a wavelength of 10.532-μm to precisely match the
resonant frequency of the CH2-wagging vibrational mode of the C2H4 molecules. OES spectroscopy of
the C2 and CH radicals were performed at different laser powers, and OES spectra taken were collected
for comparison. Comparison shows that 10.532-μm CO2 laser excitations were much significant than
10.591μm CO2 laser excitations. The rotational temperatures of CH radicals in the flames were
determined by analyzing the spectra of the R branch of the A2Δ→X2Π; (0, 0) electronic transition near
430 nm. The equation used for temperature calculation is ln(Iλ4/SJ J)= -(1/Tr)(EJ/kB)+lnC, which is
derived from I=CSJ J λ-4exp(-EJ/kBTr), in which I is the relative emission intensity of a rotational line
obtained from the experimental spectra, C is a proportionality constant which is the same for all
rotational transitions within a band, SJ J is the rotational intensity factor or Höln-London factor, λ; is
the wavelength of the emitted spectral line, EJ is the rotational energy of the initial level, kB is the
Boltzmann constant, and Tr is the rotational temperature. Afterward, Boltzmann plot, the negative and
inverse of whose slope is rotational temperature, will be generated for temperature determination. Both
intensity and temperature distributions were plotted along the flame axial direction from nozzle to
substrate. When the laser wavelength was kept at 10.591 or 10.532 μm, curve plots were generated with
laser power changed from 0 to 1000 W with an interval of 200 W. Besides, the deposited diamond thinfilms
were characterized by scanning electron microscopy, stylus profilometry, and Raman
spectroscopy. All the characterization results were collected and compared to better explain this multienergy
processing. The deposition mechanism with and without the CO2 laser excitation was then
discussed based on the OES spectral results.
Society of Engineering Science ▪ 47 th Annual Technical Meeting 416