Absorption spectroscopy in BCl3 inductively coupled plasmas: determination of density, rotational, translational and vibrational temperatures of BCl molecule

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Broadband absorption spectroscopy experiments have been carried out in low pressure (<120 mTorr) inductive plasmas of pure BCl3 to study the A 1Π–X 1Σ+ system of BCl radicals formed in these discharges. We have detailed the simulation code of the absorption spectrum of the transition, which is essential for the correct analysis of the recorded absorption profiles when the absorbing medium is optically thick. A comparison between recorded and simulated spectra provides the absolute line-averaged density of BCl molecules, together with population distributions in rotational and vibrational levels of the BCl (X 1Σ+) ground state. In pure BCl3 plasmas, the BCl density increases with the discharge power and pressure from 4 × 1017 m−3 to 2 × 1019 m−3 at the maximum, its mole fractions staying always lower than 5%. Rotational and vibrational distributions are found to be close to Boltzmann distributions, with vibrational temperatures ranging from 700 to 2000 K, while rotational temperatures vary between 550 and 1800 K. Using the diode laser absorption technique, the translational gas temperature was also measured from the Doppler width of the 811.5 nm line absorbed by Ar*(3P2) metastable atoms, with argon added as a trace gas to BCl3. Our results show that in low pressure conditions (<50 mTorr), vibrational and rotational temperatures of BCl in both excited A 1Π and ground X 1Σ+ states are not in equilibrium with the gas temperature. It is concluded that rotationally and vibrationally hot BCl radicals are produced mainly by dissociative excitation in the gas phase of heavier BCl2 and BCl3 radicals and that they are rapidly lost by diffusion and sticking, with a high probability, on the reactor walls. All these characteristics suggest a very high reactivity of BCl radicals on the surfaces exposed to the plasma.