Single pulse laser induced reactions of hexafluorobenzene/silane mixtures at 1027 and 944 cm-1

Article abstract of DOI:10.1021/j100286a013

C₆F₆/SiH₄ mixtures were irradiated with a single pulse of a megawatt CO₂ infrared laser at 1027 and 944 cm^-1, using fluences which ranged from 0.26 to 2.0 J/cm². Neat C₆F₆ (7.5 Torr, 1027 cm^-1) underwent decomposition to C₂F₄ at a fluence of 0.7 J/cm² with a conversion per flash (CPF) of 4.5%. At 0.3 J/cm² no reaction was observed, setting a fluence threshold for the laser-induced decomposition of C₆F₆ between 0.3 and 0.7 J/cm². In the presence of SiH₄ explosive reactions occurred with conversion of C₆F₆ as high as 70%! Different decomposition products were observed depending upon the amount of SiH₄ present. At constant C₆F₆ pressure (7.3 Torr) and at high C₆F₆ mole fraction (R = P_C6F6/(P_C6F6 + P_SiH4) ≥ 0.55), fluorinated carbonaceous products were observed (C₂F₄, C₂F₆). At low C₆F₆ mole fraction (R ≤ 0.55), non-fluorinated carbonaceous products were observed (C₂H₂, C₄H₂). SiF₄ was the major gaseous product in both regions, while SiF₃H was observed when R values were lower than 0.55. A polymeric black material was also deposited in the cell in both R zones. The highest CPF of C₆F₆ was obtained when the mole fraction of C₆F₆ was 0.55 and under these conditions only SiF₄, SiF₃H, and polymeric material were observed. Irradiation of C₆F₆/SiH₄ mixtures (constant C₆F₆ pressure, 7.5 Torr) at 944 cm^-1 using fluences below 1 J/cm² did not induce reaction. At 1.6 J/cm² no reaction was observed at C₆F₆ mole fractions above 0.3. However, between 0.3 and 0.1 mole fraction values identical products as those obtained in this zone under 1027-cm^-1 irradiation were observed. Identical products were also obtained when the C₆F₆ mole fraction was varied by adding C₆F₆ to SiH₄ (constant SiH₄ pressure, 30 Torr). However, the threshold for reaction was observed at a C₆F₆ mole fraction of R = 0.5 and no reaction was observed when R > 0.5. At higher overall pressure (P_C6F6 = 72 Torr; P_SiH4 = 37 Torr, with R = 0.66) irradiation at 944 cm^-1 gave the same products as those observed in the high R zone of the 1027-cm^-1 irradiation. A higher fluence (2.05/cm²) was necessary, however, to induce the reaction at 944 cm^-1. These results are discussed in terms of (a) the low fluence threshold observed for the laser-induced decomposition of C₆F₆, (b) the effects that added gases have on the decomposition of C₆F₆, (c) the use of C₆F₆ as a sensitizer for laser-induced reactions, and (d) the potential for using SiH₄ for the laser-induced reduction of C-F bonds (C-F + Si-H → C-H + Si-F).