Previous molecular orbital calculations have shown that polycyclic aromatic hydrocarbons (PAHs) with 4-10 fused rings account for the center electronic band positions for bulk asphaltenes. Here, this work is extended to cover low-energy electronic transitions of crude oils and asphaltenes. The primary determinants in optical absorption band location are shown to be the size and geometry of PAHs. Large PAHs are shown to exhibit optical properties exhibited by asphaltenes and crude oils. Furthermore, these results are consistent with the observed Urbach spectral profiles. The rapidly declining electronic absorption at wavelengths exceeding 600 nm is shown to be consistent with the presence of a few large ring systems. Measurements of concentration and temperature independence of crude oil and asphaltene optical spectra imply that potential contributions to their coloration from charge-transfer and potential free radicals are not significant. Nevertheless, the very small electronic absorbances for very low energy are found to fall outside of the absorption profile of large hydrocarbon PAHs. Future work is indicated to account for addressing these very low absorbances for asphaltenes.

Fluorescence quantum yield measurements are reported for visible and UV excitation for neat and dilute crude oil solutions, extending earlier work with excitation in the long wavelength visible and the NIR. Large and monotonically increasing quantum yields are found with shorter wavelength excitation (to 325 nm), and all crude oils are shown to have nearly the same relative dependence of quantum yield on excitation wavelength. These observations are explained by the energy dependence of internal conversion. Dilute solutions of light crude oils exhibit higher quantum yields than those of heavy crude oils because of their lack of large chromophores. The fraction of fluorescence emission resulting from electronic energy transfer (with subsequent fluorescence emission) for neat crude oils was previously shown to vary from ∼100% for ultraviolet excitation to ∼0% for near-infrared excitation; this large variation correlates well with and is explained by the very large variation in quantum yields with excitation wavelength. Comparison of quantum yields from neat and dilute solutions shows that 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 because of its lack of small chromophores.

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A negative capacitance effect has been observed in metal-semiconductor contacts. This phenomenon is explained by considering the loss of interface charge at occupied states below Fermi level due to impact ionization. A modified Shockley–Read treatment is proposed to interpret the experimental observations. In particular, a two-energy-level simplified model is presented to simulate the capacitance spectrum. The results are in good agreement with the experimental data.