Resonant excitation of ethylene molecules in the combustion flame CVD of diamond using a wavelength tunable CO₂ laser

CO₂ laser resonant excitations of precursor molecules were applied in combustion flame synthesis of diamond films. The combustion flame was produced from a mixture of ethylene (C₂H₄), acetylene (C₂H₂) and oxygen (O₂). A wavelength-tunable CO₂ laser with wavelength range from 9.2 to 10.9 μm was used for wavelength-matched excitation of the ethylene molecules. By irradiating the flame using CO₂ laser at 10.532 μm, the ethylene molecules were resonantly excited through the CH₂ wagging vibrational mode (ν₇, 949.3 cm^-1). Irradiation of the flame using the common CO₂ laser wavelength at 10.591 μm was also carried out for comparison. It was found that diamond synthesis was more obviously enhanced by the CO₂ laser resonant excitation at 10.532 μm as compared to that at 10.591 μm. Firstly, the flame was shortened by 50%, indicating a promoted reaction in the process. Secondly, the diamond grain sizes as well as the diamond film thicknesses were increased by 200~300% and 160% respectively, indicating a higher growth rate of diamond films. Finally, Raman spectra of the diamond sample showed a sharp diamond peak at 1334 cm^-1 and a suppressed G-band, indicating higher diamond quality.