QUANTUM YIELDS OF CRUDE OILS

Fluorescence quantum yield measurements are reported for visible and U V excitation for neat and dilute crude oil solutions, extending earlie r work with excitation in the long wavelength visible and the NIR. Lar ge and monotonically increasing quantum yields are found with shorter wavelength excitation (to 325 nm), and all crude oils are shown to hav e nearly the same relative dependence of quantum yield on excitation w avelength. These observations are explained by the energy dependence o f internal conversion. Dilute solutions of light crude oils exhibit hi gher quantum yields than those of heavy crude oils because of their la ck of large chromophores. The fraction of fluorescence emission result ing from electronic energy transfer (with subsequent fluorescence emis sion) for neat crude oils was previously shown to vary from similar to 100% for ultraviolet excitation to similar to 0% for near-infrared ex citation; this large variation correlates well with and is explained b y the very large variation in quantum yields with excitation wavelengt h. Comparison of quantum yields from neat and dilute solutions shows t hat quenching is the other major process which occurs with chromophore interactions. The quantum yields of a maltene and resin are large and similar, while the asphaltene exhibits much smaller quantum yields be cause of its lack of small chromophores.