Fig. 1 Schematic of the soot flow tube.

Fig. 2 SMPS electric-mobility size distributions of nonirradiated soot (solid curve) and particles irradiated at $532\text{\hspace{0.17em} nm}$ and $0.7\text{J}/{\text{cm}}^{\text{2}}$ (dotted curve). Burner flow rates were 0.24 SLM for ethylene, 27.5 SLM for the air coflow, and 65 SLM for the air cross flow (see text for further details). Symbols represent the average geometric diameters determined from 325 measurements on each of two TEM samples, one collected with the DMA set to a mobility size of $100\text{\hspace{0.17em} nm}$ and the other collected at a mobility size of 110 nm.

Fig. 3 TEM images of nonirradiated soot particles. (a) Soot aggregate from a distribution with a mean geometric diameter of $110\left(\pm 15\right)\text{\hspace{0.17em} nm}$ and a mean electric-mobility diameter of $112\left(\pm 1\right)$ $\mathrm{n}\mathrm{m}$ . (b) Higher-magnification TEM image of the same particle.

Fig. 4 SMPS electric-mobility size distributions of irradiated and nonirradiated particles represented as particle volume distributions. The measured number concentrations (Fig. 2) were converted to volume assuming the particles to be spherical with diameters equivalent to the electric-mobility diameters. Particles were irradiated at $532\text{\hspace{0.17em} nm}$ with a fluence of $0.7\text{J}/{\text{cm}}^{\text{2}}$ .

Fig. 5 TEM images of a soot particle irradiated at $1064\text{\hspace{0.17em} nm}$ with a fluence of $1\text{J}/{\text{cm}}^{\text{2}}$ . The particle electric-mobility diameter was $100\text{\hspace{0.17em} nm}$ . (a) Aggregate in this TEM image has a branched-chain morphology similar to that of the nonirradiated particles, despite changes in the particle fine structure (b). (b) Higher-magnification TEM image of a part of the same aggregate.

Fig. 6 TEM images of soot nanoparticles produced by laser irradiation. Particles were produced at (a) and (b) $1064\text{\hspace{0.17em} nm}$ with a laser fluence of $1\text{J}/{\text{cm}}^{\text{2}}$ and (c) $532\text{\hspace{0.17em} nm}$ with a laser fluence of $0.87\text{J}/{\text{cm}}^{\text{2}}$ and were collected at an electric-mobility diameter of $30\text{\hspace{0.17em} nm}$ . (b) and (c) Higher-magnification TEM images of nanoparticles that appear to be predominantly composed of carbon with no apparent long-range order and that have isolated regions with some long-range order (arrows).

Fig. 7 Single-energy images of representative particles imaged by STXM at the carbon edge. (a) Nonirradiated soot imaged with a step size of $60\text{\hspace{0.17em} nm}$ . The image size is $15\times 15\mu \text{m}$ . (b) Soot particle irradiated at $532\text{\hspace{0.17em} nm}$ with a fluence of $0.8\text{J}/{\text{cm}}^{\text{2}}$ and imaged with a step size of $30\text{\hspace{0.17em} nm}$ . The image size is $2\times 2\mu \text{m}$ .

Fig. 8 Representative carbon K-edge NEXAFS spectra of soot particles before (dotted curve) and after (solid curve) laser irradiation with a single laser pulse at $532\text{\hspace{0.17em} nm}$ and a laser fluence of $0.8\text{J}/{\text{cm}}^{\text{2}}$ . The spectrum of HOPG (dashed curve) from Lenardi et al. (Ref. [11]) is provided for comparison.

Fig. 9 Number concentration of electric mobility size distributions of nonirradiated soot particles (filled circles) and soot irradiated at $532\text{\hspace{0.17em} nm}$ with fluences of $0.21\text{J}/{\text{cm}}^{\text{2}}$ (open circles), $0.33\text{J}/{\text{cm}}^{\text{2}}$ (squares), and $0.76\text{J}/{\text{cm}}^{\text{2}}$ (triangles). The distributions are plotted as n(ln D). Solid curves are the best fits to the data with a sum of two lognormal distributions, i.e., Eq. (3). The values of fitting parameters are listed in Table 1.

Fig. 10 Electric-mobility size distributions plotted as a function of laser fluence. Soot was irradiated at $532\text{\hspace{0.17em} nm}$ and sampled with the SMPS. The contour line spacing is $2\times {10}^6{\text{cm}}^{-\text{3}}$ .

Fig. 11 Fluence dependence of median size and number concentration of small- and large-mode distributions. Results are shown for $532\text{\hspace{0.17em} nm}$ (circles) and $1064\text{\hspace{0.17em} nm}$ (squares) irradiation. Fits of Eq. (3) to the distributions were used to derive (a) the median particle size (D) for each mode and (b) the total number concentration (N) for each mode. Large-mode results (open symbols) correspond to the right axis, and small-mode results (closed symbols) correspond to the left axis. (c) Equations (3) and (4) were used to calculate the average number of small particles produced per soot aggregate. The curve represents the calculated mass fraction of carbon volatilized into small molecular clusters during $532\text{\hspace{0.17em} nm}$ laser irradiation. Symbols correspond to the left axis; curves correspond to the right axis.

Fig. 12 TEM images of a soot particle irradiated at $1064\text{\hspace{0.17em} nm}$ with a fluence of $0.24\text{J}/{\text{cm}}^{\text{2}}$ . The particle electric-mobility diameter was $100\text{\hspace{0.17em} nm}$ . (a) TEM image of an aggregate with a branched-chain morphology and fine structure similar to that of the nonirradiated particles. (b) Higher-magnification TEM image of a part of the same aggregate.

Fig. 13 TEM images of a soot particle irradiated at $532\text{\hspace{0.17em} nm}$ with a fluence of $0.3\text{J}/{\text{cm}}^{\text{2}}$ . The particle electric-mobility diameter was $100\text{\hspace{0.17em} nm}$ . (a) TEM image of an aggregate with a branched-chain morphology similar to that of the nonirradiated particles. (b) Higher-magnification TEM image of a part of the same aggregate.

Fig. 14 TEM images of small particles produced by laser irradiation of soot. (a) Higher-magnification TEM image of particles produced at $1064\text{\hspace{0.17em} nm}$ with a fluence of $0.24\text{J}/{\text{cm}}^{\text{2}}$ and collected at a mobility diameter of $10\text{\hspace{0.17em} nm}$ . The circle highlights an isolated region of turbostratic graphitic structure. (b) and (c) Higher-magnification TEM images of particles produced at $532\text{\hspace{0.17em} nm}$ with a fluence of $0.3\text{J}/{\text{cm}}^{\text{2}}$ and collected at a mobility diameter of $28\text{\hspace{0.17em} nm}$ .

Fig. 15 Average carbon K-edge NEXAFS spectra of soot particles (a) before (top curve) and after laser irradiation with a single laser pulse at $532\text{\hspace{0.17em} nm}$ and laser fluences of (a) $0.01\text{J}/{\text{cm}}^{\text{2}}$ , (a) $0.05\text{J}/{\text{cm}}^{\text{2}}$ , (b) $0.2\text{J}/{\text{cm}}^{\text{2}}$ , and (b) and (c) $0.80\text{J}/{\text{cm}}^{\text{2}}$ , as indicated. The curves have been normalized and offset from one another for clarity. Vertical dotted lines indicate energy levels for the 1s–π* transition of aromatic carbon (285.5 eV), 1s–σ* carbon transition $\left(\sim 292.8\text{\hspace{0.17em} eV}\right)$ , and the exciton peak (291.7 eV).