Spontaneous emission from C-2(d (3)Pi g) and C-3(A (1)Pi(u)) during laser irradiation of soot particles

In order to investigate the contribution of non-thermal processes to laser-induced vaporization of soot, we have recorded temporally and spectrally resolved emission from C-2(d (3)Pi(g)-a (3)Pi(u); Swan system) and C-3(A (1)Pi(u)-X (1)Sigma(+)(g); Swings system) following laser irradiation of soot at 532 and 1064 nm over a wide range of laser fluences. We compared the measured spectra with simulated spectra from C-2-Swan and C-3-Swings emission to gain new insight into the formation mechanism of the excited species. This comparison shows that the vibrational and rotational energy distributions of the nascent C-2(d (3)Pi(g)) and C-3(A (1)Pi(u)) depend strongly on laser wavelength and are not in local thermodynamic equilibrium with the soot. These results suggest non-thermal ejection of highly excited C-2(d (3)Pi(g)) and C-3(A (1)Pi(u)) from the particle surface. Multi-photon laser-induced fluorescence from thermally sublimed C-2(a (3)Pi(u)) and C-3(X (1)Sigma(+)(g)) is unlikely to be the source of the observed emission. This work provides fluence thresholds at 532 and 1064 nm for the appearance of spontaneous emission from C-2(d (3)Pi(g)) and C-3(A (1)Pi(u)) with sufficient intensity to be observable in the presence of laser-induced incandescence from soot and emphasizes the importance of non-thermal vaporization processes during laser irradiation of soot.