Abstract
The recent availability of reliable and relatively low-cost GaN based semi-conductor laser diode sources emitting at 404 nm has opened many new areas for fluorescence based measurements. This article characterizes the fluorescence behavior of commercial liquid-phase gasoline samples using such excitation sources. Comparison is drawn with the emission when excited using broad-band sources at shorter wavelengths (340 nm). Here, 404 nm is shown to selectively excite the larger C<sub><i>x</i></sub>H<sub><i>y</i></sub> polycyclic aromatic hydrocarbons (PAH) commonly found as minor constituents of gasoline, mainly for (<i>x, y</i>) ≥ (14, 10). Both Stokes and anti-Stokes shifted emission was observed in all the gasoline tested and in some PAH samples. The fluorescence is usually superimposed on Raman scattered laser light, arising from vibrations within the basic benzene structures. The fluorescence features of the gasoline samples tested were found to be broadly similar, but, because of its distinctive spectroscopic features, the fluorescence arising from benzo(a)pyrene was found to be one of the main variants. More generally, principal component analysis of the spectra was able to highlight differences between both the sample provenance and the fuel variety.
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