The Electronic Absorption Edge of Petroleum OLIVER C. MULLINS,* SUDIPA MITRA-KIRTLEY, and YIFU ZHU Schlumberger-Doll Research, Old Quarry Road, Ridgefield, Connecticut 06877 The electronic absorption spectra of more than 20 crude oils and as- phaltenes are examined. The spectral location of the electronic absorp- tion edge varies over a wide range, from the near-infrared for heavy oils and asphaltenes to the near-UV for gas condensates. The functional form of the electronic absorption edge for all crude oils (measured) is characteristic of the "Urbach tail," a phenomenology which describes electronic absorption edges in wide-ranging materials. The crude oils all show similar Urbach widths, which are significantly larger than those generally found for various materials but are similar to those previously reported for asphaltenes. Monotonically increasing absorption at higher photon energy continues for all crude oils until the spectral region is reached where single-ring aromatics dominate absorption. However, the rate of increasing absorption at higher energies moderates, thereby de- viating from the Urbach behavior. Fluorescence emission spectra exhibit small red shifts from the excitation wavelength and small fluorescence peak widths in the Urbach regions of different crude oils, but show large red shifts and large peak widths in spectral regions which deviate from the Urbach behavior. This observation implies that the Urbach spectral region is dominated by lowest-energy electronic absorption of corre- sponding chromophores. Thus, the Urbach tail gives a direct measure of the population distribution of chromophores in crude oils. Implied population distributions are consistent with thermally activated growth of large chromophores from small ones. Index Headings: Fluorescence; UV-visible spectroscopy. INTRODUCTION The optical properties of crude oils are useful in un- raveling their complex composition and are used in the petroleum industry for a variety of monitoring and anal- ysis purposes. Here, we are interested in the electronic absorption of crude oils at the low-energy absorption edge. The coloration of crude oils varies over a wide range; the low-energy electronic absorption tail can occur in the UV for gas condensates, the visible for medium crude oils, and the near-infrared for heavy oils, tars, and asphaltenes. The origin of the coloration of crude oils is generally associated with aromatic molecules of various sizes, 1yet electronic absorption of crude oils is not simply associated with 7r-~r* and n-~r* transitions of these mol- ecules but also with transitions involving charge transfer complexes and free radicals. 2 Recently, we have explored the electronic absorption edge of asphaltenes, the solid and most aromatic component of crude oils2 For the asphaltenes, the electronic absorption edge occurs in the near-infrared spectral range. The asphaltenes exhibit ex- ponential tails of nearly the same decay width in their electronic absorption spectra. This observation relates to a phenomenology observed in the electronic absorp- tion spectra of many materials. The electronic absorption edges of increasingly broad classes of materials have been treated within the frame- Received 17 April 1992. * Author to whom correspondence should be sent. work of the Urbach phenomenology 4 in which the ab- sorption coefficient a depends exponentially on the pho- ton energy he0: = a0exp • (1) Eo is the Urbach decay width, and for many materials the width is simply thermal (i.e., Eo = kT). 4 This decep- tively simple concept, which says that a thermal distri- bution of absorber sites produces an exponential ab- sorption tail with a thermal width, is, in fact, difficult to justify theoretically, although substantial progress has recently been made) ,s The Urbach absorption tail with thermal widths has been observed in many semiconduc- tors. 4 In addition, in glassy materials, the thermal dis- order is effectively frozen in at the glass transition tem- perature, and the glass transition temperature gives the width of the Urbach absorption tail. 4 Structural disorder can also increase the Urbach width, producing a tem- perature-independent component to the Urbach width, v In addition to solid-state materials, organic molecules dissolved in solution exhibit thermal widths in the elec- tronic absorption edge as well as their fluorescence emis- sion edge. s,9 Asphaltenes exhibit exponential tails of nearly the same decay width, but the decay width is much larger (× 10) than thermal widths2 Furthermore, the decay width of the asphaltenes is far larger than would be expected if the maturation temperature of the crude oil resulted in the decay width of the asphaltenes, analogously to glass transition temperatures of glassy materials. Initially, it may seem strange that complex mixtures such as as- phaltenes have any relation in their electronic absorption profile to amorphous semiconductors. However, upon further reflection, similarities become apparent. Single- component systems such as amorphous semiconductors have absorber sites that differ in their thermal activation, each site having its own absorption characteristics. The asphaltenes have a distribution of chromophores (ab- sorbers) produced in a thermally activated (maturation) process; each chromophore has its own absorption char- acteristics. The asphaltene absorbers are not themselves thermally activated. Thus, the decay widths are not given by kT; rather, the decay widths are characteristic of the chromophore population distribution, which results from a thermally activated process. Here, we investigate the electronic absorption and flu- orescence emission properties of crude oils and two as- phaltenes. The electronic absorption edges of the crude oils are exponential, and, surprisingly, all crude oil spec- tra exhibit similar decay widths, independent of the spec- tral location of the absorption edge. This observation is consistent with and extends the concept of the Urbach phenomenology, which has been observed in diverse ma- Volume 46, Number 9, 1992 0003-7028/92/4609-140552.00/0 APPLIED SPECTROSCOPY 1405 © 1992 Society for Applied Spectroscopy