高真空条件下的激光诱导白炽光研究

Laser-Induced Incandescence (LII) occurs when a high-energy pulsed laser beam
encounters graphitic particulate matter particles like soot or carbon black. The
particles absorb laser energy from the beam and see an increase in their internal
energy, resulting in an increase of temperature. At the same time, the particles loose
energy through heat transfer mechanisms. If the energy absorption rate is sufficiently
high, particle temperature will rise to levels where significant incandescence
(blackbody emission) can occur .Typically, Laser-Induced Incandescence produces
50ns to 1s long light pulses at atmospheric pressure.
So far, LII measurements had been restrained to conduction-dominated conditions,
whereby signals are short-lived (less than one microsecond) and require sensitive
nanosecond resolution instrumentation. This thesis introduces a novel LII based
measurement method performed under high vacuum conditions. The novelty of LII
under vacuum resided in the fact that heat conduction away from the soot particle
becomes negligible below 10-2 mbar and this constituted a step away from the typical
situation, whereby laser absorption is followed by heat conduction from the particles
to the surrounding medium. Instead, sublimation and radiative heat transfer would
follow laser absorption. The consequence was the obtention of long-lived LII signals
(up to 100 microseconds) and a large gain of photons (ranging between 50 to 300)
emitted per primary soot particle during LII temperature decays. Furthermore, the
refractive index function E(m) value could be determined directly from measured
radiative temperature decays, with potentially an uncertainty of circa 7%, which
outperformed current soot extinction measurements. In addition, for laser fluences
below 0.06 J/cm2, a regime where only laser energy absorption and radiative heat
transfer apply would be reached and LII signals became independent of particle size. 激光诱导白炽光烟雾粒子成像分析仪(LII) Imager SX PIV相机