Laser Induced Incandescence and Particle Size

The cooling rate of soot particles after laser induced incandescence (LII) is thought to provide an estimate of the size of nanoparticles (NPs) in the engine exhaust. It is shown that cooling rate is not applicable to NPs, and instead is only a measure of the size of micron-sized particles. The problem is that the conservation of absorbed laser photon energy in NPs is subject to quantum electrodynamics (QED) constraints. Although QED allows the temperature of the NPs to increase during the absorption of the laser photons, QED limits the amount of photon energy that may be absorbed by the NPs to insignificant fractions of the laser photon energy, and therefore the cooling rate of the absorbed laser energy following LII would not be detectable. In effect, the absorption of laser photons in a NP cannot be conserved by an increase in temperature. Instead, the absorbed photon energy is conserved by emission of visible (VIS) radiation in a broadband electromagnetic (EM) spectrum produced during confinement within the NP geometry. The broadband EM emission spectrum is known in LII terminology as the emission signal. Indeed, this paper recommends that the LII emission signal be used as measure of the NPs leaving the cooling rate as a measure of micron sized particles. Key-Words: Nanoparticle, heat transfer, suppressed IR, QED, conservation principle