Measurement of collisional V→T, R energy transfer in vibrationally excited SF6 using Doppler broadening of the Hg(6 1S–6 3P1) multiplet

Abstract

A new method for measuring energy transfer between colliding molecules has been developed. It is based upon the temperature dependence of the isotopic and hyperfine line widths of the Hg multiplet in the neighborhood of 254 nm. The light source was a Hg resonance lamp whose line profiles could be precisely controlled by varying the Hg pressure. Multiphoton absorption at 944 cm−1 was used to excite SF6 vibrationally during a 250 ns CO2 laser pulse. The subsequent rate of equipartition of this excess vibrational energy with the translational and rotational degrees of freedom was determined by measuring the initial slope of the increase in the absorption of 254 nm radiation by a trace of Hg vapor. It was found that pressures above 2 Torr were required to involve all of the SF6 molecules in the absorption of the laser radiation. The V→T, R energy transferred per SF*6 –SF6 collision was found to be proportional to the 3/2 power of the excitation energy between 1500 and 5000 cm−1 and gradually changed to a first power dependence in the 300 to 1000 cm−1 range.