Laser-Induced 16-μ Fluorescence in SF6: Acoustic Effects

Abstract

Damped oscillations in the laser‐excited 16‐ and 10‐μ infrared fluorescence decay from SF₆ and SF₆–rare‐gas mixtures have been observed. This fluorescence modulation is shown to be due to a laser‐induced sound wave propagating in the sample cell. The generation and propagation of this sound wave is analyzed by solving the coupled mass, momentum, and energy transport equation for the gas following laser excitation. Similar effects have been observed in BCl₃, and the phenomenon is expected to occur in general for molecular systems which strongly absorb pulsed laser radiation and also have very rapid vibration–vibration and vibration–translation energy transfer rates. The risetime of the 16‐μ fluorescence from SF₆ following excitation by a Q‐switch CO₂ laser oscillating at 10.6 μ gives a lower limit of 5 × 10⁴ sec⁻¹ at 1 torr for the vibration–vibration energy transfer rate in SF₆ between states fluorescing at 10 μ and states fluorescing at 16 μ.