Soot particle disintegration and detection by two-laser excimer laser fragmentation fluorescence spectroscopy.

A two-laser technique is used to study laser-particle interactions and the disintegration of soot by high-power UV light. Two separate 20 ns laser pulses irradiate combustion-generated soot nanoparticles with 193 nm photons. The first laser pulse, from 0 to 14.7 J/cm2, photofragments the soot particles and electronically excites the liberated carbon atoms. The second laser pulse, held constant at 13 J/cm2, irradiates the remaining particle fragments and other products of the first laser pulse. The atomic carbon fluorescence at 248 nm produced by the first laser pulse increases linearly with laser fluence from 1 to 6 J/cm2. At higher fluences the signal from atomic carbon saturates. The carbon fluorescence from the second laser pulse decreases as the fluence from the first laser increases, suggesting that the particles fully disintegrate at high laser fluences. We use an energy balance parameter, called the photon/atom ratio, to aid in understanding laser-particle interactions. These results help define the regimes where photofragmentation fluorescence methods quantitatively measure total soot concentrations.