Continuous-wave probe laser investigation of laser vaporization of small soot particles in a flame

Pulsed laser vaporization of small soot particles (<100-nm radius) in a flame is observed with a cw probe laser. Both light scattering and absorption along the pulsed laser beam are measured. Above a threshold of 0.2 J/cm2, both quantities decay exponentially with increased fluence for submicrosecond pulses. These two measurements demonstrate that the number of particles remains constant, but the mean size decreases: that is, small particles vaporize rather than fragment or photophorese. The vaporization process is modeled including transport across the first gas mean free path (Langmuir layer). Numerical integration of the time-dependent conservation equations demonstrates that a simple analytic treatment is adequate. The threshold and fluence dependence are predicted to within experimental uncertainty, assuming soot has the thermal properties of graphite. Laser vaporization of soot has possible application to laser beam profiling, gas velocity measurements, flow visualization, and point measurement of soot absorption.