Spontaneous emission from C2(d3Πg) and C3(A1Πu) 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 C2(d 3Πg–a 3Πu; Swan system) and C3(A 1Πu–X ; 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 C2-Swan and C3-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 C2(d 3Πg) and C3(A 1Πu) depend strongly on laser wavelength and are not in local thermodynamic equilibrium with the soot. These results suggest non-thermal ejection of highly excited C2(d 3Πg) and C3(A 1Πu) from the particle surface. Multi-photon laser-induced fluorescence from thermally sublimed C2(a 3Πu) and C3(X ) 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 C2(d 3Πg) and C3(A 1Πu) 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.