Attempts on the selective promotion of diamond growth were investigated by deploying laser-assisted vibrational excitation of reactant molecules, which deposits energy selectively into specific molecules and activate the molecules towards the selected reaction pathways. Laser-assisted combustion chemical vapor deposition (CVD) of diamond was studied using a wavelength-tunable CO_2 laser. The CH_2-wagging mode (υ_7, at 949.3 cm~(-1)) of ethylene precursor molecules is strongly infrared active and perfectly matches the emission line of the CO_2 laser at 10.532 μm. On- and off-resonance excitations of molecules were performed via tuning the incident laser wavelengths centered at 10.532μm. With the same amount of laser power absorbed, the on-resonance vibrational excitation allowed a largest fraction of the absorbed laser energy coupled directly into C_2H_4 molecules whereas energy coupling under off-resonance excitations is less efficient in influencing the combustion process. The diamond deposition rate was enhanced by a factor of 5.7 accompanied with an improvement of diamond quality index under the onresonance excitation at 10.532 μm. The flame shape variation indicate that the resonant vibrational excitation is an efficient route coupling energy into the reactant molecules to surmount the chemical reaction barrier and steering the combustion process to favor the diamond formation. Mass spectrometry was performed to study the chemical species in the flame, which suggests that hydrocarbons-related species were selectively enhanced while etchant species were suppressed with resonant laser excitations.
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