Laser-induced opto-acoustic pulses in a flame

Another means of detection of the absorption of laser radiation by individual species in a flame environment is the production of opto-acoustic pulses in a flame at 1-atm pressure. The flame is seeded with alkali atoms and a pulsed laser is tuned to the appropriate absorption line. While some of the electronically excited atoms fluoresce, the vast majority lose their energy by collision with other gases present in the flame. This energy ultimately (though rapidly) is converted into translational kinetic energy of the flame gases, producing a pulsed pressure wave which expands from the region illuminated by the laser. This sound wave is readily audible to an observer in the vicinity of the burner. Quantitative measurements are made using a microphone, and show that the technique is very sensitive and accurate, and perturbs neither the gas dynamics nor chemical kinetics of the flame.