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Triplet Electronic Spin States of Crude Oils and Asphaltenes Tony Klee, Tyler Masterson, Bo Miller, Emma Barrasso, Jamal Bell, and Richard Lepkowicz Department of Physics and Optical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803
Joseph West Department of Chemistry and Physics, Indiana State University, Terre Haute, Indiana 47809
Joy E. Haley, Danielle L. Schmitt, Jonathan L. Flikkema, and Thomas M. Cooper Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States
Yosadara Ruiz-Morales Programa de Ingeniería Molecular, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas Norte 152, Mexico, D.F. 07730, Mexico
Oliver C. Mullins* Schlumberger-Doll Research, Cambridge, Massachusetts 02139, United States ABSTRACT: The resolution of asphaltene nanoscience is becoming increasingly important for a variety of purposes. One key molecular attribute of the asphaltenes is the size distribution of their polycyclic aromatic hydrocarbons (PAHs). Comparison of measured spin singletsinglet absorption and emission transitions with exhaustive molecular orbital (MO) calculations on 523 PAHs indicates that asphaltene PAHs have a population centroid of ∼7 fused rings. To further test this understanding of asphaltene PAHs, it is desirable to consider the dynamics of triplet states. Nevertheless, triplet-state spectroscopy is complex, especially on polydisperse materials such as asphaltenes. For validation, we compare simple expectations for asphaltenes against both experimental and theoretical results. Measurements were conducted on crude oil and asphaltene samples of dramatically different heavy end content to identify specific transitions being investigated. Experimental results include spectra at several wavelengths, lifetimes in the presence and absence of molecular oxygen, and temperature effects. Specifically, we use classic techniques [Horrocks and Wilkinson, Proc. R. Soc. London A 1968, 306, 257273] to measure triplettriplet spectra for crude oils and asphaltenes. These are compared with corresponding MO calculations. Again, using classic methods [Guzeman et al., J. Chem. Soc. Faraday Trans. 1973, 69, 708720], quenching effects of asphaltene triplet states by molecular oxygen are measured and compared with simple diffusion expectations. The temperature dependence provides further stringent testing. Spectral comparisons versus crude oil composition rule out significant spectral contributions from free radicals. Simple expectations regarding triplet-state spectroscopy of asphaltenes and crude oils apply and corroborate previous conclusions from singlet-state spectroscopy of crude oils and asphaltenes. The data herein are consistent with asphaltene PAHs being relatively large (e.g., 7 fused rings); this, in turn, is consistent with the predominance of a single PAH per asphaltene molecule (the “island” molecular architecture). Smaller PAHs dominate the triplet transitions for the crude oil samples and optical wavelengths used herein.
’ INTRODUCTION Asphaltene science has advanced significantly in recent years.1 A new model has been proposed to account for asphaltene nanoscience: the modified Yen model,2,3 which also is called the YenMullins model.4,5 Figure 1 shows the modified Yen model, consisting of the predominant structures for asphaltene molecular architecture, nanoaggregates, and clusters for moveable crude oils.2,3 The modified Yen model has given rise to the first predictive equation of state (EoS) for asphaltenes,6 enabling treatment of asphaltene gradients in reservoirs. FloryHuggins theory has long been used to model phase behavior of asphaltenes.7 However, r 2011 American Chemical Society
because the colloidal size of asphaltenes was unknown, there had been no way to construct the gravity term; thus, a complete EoS was precluded and no predictive asphaltene EoS existed for reservoir crude oils. The modified Yen model provides the means to obtain the gravity term.6 In addition, for reservoir crude oils, it is important to treat the effects of dissolved gases.8 Figure 2 shows an example of an asphaltene gradient from a single oil column in an oil reservoir,9 which is a gradient that can Received: November 18, 2010 Revised: April 4, 2011 Published: April 12, 2011 2065
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Energy & Fuels now be modeled. In particular, the modified Yen model and new asphaltene EoS are being combined with a new technology “downhole fluid analysis”10 to address the biggest concerns in deepwater oil productionreservoir connectivity.11,12 If asphaltenes are shown to be equilibrated across an entire oil field, then reservoir fluid-flow connectivity is indicated, because of the geologic time required for asphaltene equilibration.13 Indeed, asphaltene nanoaggregates are being used to characterize oil reservoirs that are 13 orders of magnitude larger in linear dimension. In part due to the growing applicability of asphaltene nanoscience to characterize oil reservoirs, it is required to probe the accuracy of the modified Yen model. Indeed, some molecular structural issues are difficult to address. After much debate,14,15 the asphaltene molecular weight is largely resolved, the centroid of the distribution is ∼750 Da with a full-width half-maximum (fwhm) of ∼500 1000 Da.13,5 Other molecular structural parameters are more difficult to assess, such as the size of the asphaltene PAHs and the number of PAHs per asphaltene molecule. Several studies have addressed the asphaltene PAH ring size. The combination of molecular orbital (MO) calculations with the very robust measurement of the asphaltene absorption and emission spectra probed asphaltene PAH ring size.1618 These studies utilized the Clar representation of PAHs; aromatic sextet carbon is more stable than isolated double-bond carbon.4,19 The MO studies relied on carbon X-ray Raman measurements of asphaltenes, which showed that the more-stable aromatic sextet dominates in asphaltenes.20,21 The combined MO and optical spectroscopy studies concluded that the most-probable asphaltene PAHs have 7 fused rings with roughly 410 rings fwhm.1618 The results are consistent with direct imaging of asphaltenes by scanning tunneling microscopy (STM),22 and by transmission electron microscopy (TEM).23 Infrared Raman spectroscopy studies of asphaltenes also give 7 fused rings as the most probable.24,25
Figure 1. Schematic of the Modified Yen Model,2,3 also known as the Yen-Mullins Model,4,5 showing the predominant asphaltene molecular structure and the two dominant colloidal structures in laboratory solvents and in moveable crude oils.2,3 At low asphaltene concentrations, true molecular solutions are found. As the concentration increases, asphaltene molecules self-assemble to nanoaggregates. At yet-higher concentrations, nanoaggregate clusters form.
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The first studies to find the predominance of a single PAH per asphaltene molecule relied on time-resolved fluorescence depolarization (TRFD).2629 These studies gave the correct asphaltene molecular weight and were also the first measurements to indicate that there is predominantly one PAH per molecule, the so-called island architecture. Small, blue-fluorescing asphaltene chromophores undergo rotational diffusion 10 times faster than large, red-fluorescing asphaltene chromophores; the different asphaltene chromophores are not cross-linked. A single PAH per molecule of roughly 7 fused rings is consistent with a molecular weight of 750 Da, when considering alkane carbon and heteroatom content of the asphaltenes. Recent unimolecular decomposition studies of asphaltenes support the island molecular architecture. Perhaps the best data currently known on asphaltene molecular architecture is from a relatively new technique applied to asphaltene, laser two-step ionization mass spectrometry (L2MS). These studies on asphaltenes and 23 model compounds have shown that all archipelago models studied fragment with the addition of much excess energy while asphaltenes and island models are more robust and do not fragment.5 One set of studies exploiting Fourier-transform ion cyclotron resonance mass spectroscopy (FT-ICR-MS) has shown that when individual asphaltene molecules are fragmented by collision with helium atoms, only alkane carbon fragments off the molecular core; there is no loss of aromaticity in the fragmentation process.30,31 In addition, laser-induced acoustic desorption electron impact mass spectroscopy (LIAD-EI-MS) studies show that asphaltene molecules are robust against fragmentation by electron impact32,33 while model “archipelago molecules” with multiple PAHs are rather fragile, which reinforces the validity of the island asphaltene molecule.33 The central role played by MO calculations, coupled with spectral analysis in assessing asphaltene PAH distributions, motivates further testing along these lines. The singlet electronic manifold of asphaltenes and crude oils has exhibited tremendous systematics in photophysics. For example, crude oils and asphaltenes obey the Urbach tail formalism in solid-state physics.34,35 The Urbach tail is associated with diminishing populations of absorbers with decreasing band gap. The Urbach electronic absorption edge of crude oils and asphaltenes can be accounted for by MO calculations presuming a diminishing number of PAHs with increasing numbers of fused rings beyond 7 rings.18 Crude oils have also been shown to obey the “Energy Gap Law”,36 which is well-known for photophysics, where fluorescence quantum yields decrease exponentially with decreasing band gap.37 The MO calculations could reproduce corresponding asphaltene fluorescence spectra presuming a distribution of asphaltene PAHs with 7-fused rings most probable.18 Moreover, the giant variation of the extent of electronic energy transfer with subsequent fluorescence emission (from zero to one) in fluorescence spectra
Figure 2. A series of dead oil samples (without dissolved gases) from a single oil column in an oil reservoir, deepwater Gulf of Mexico.8,9 The oil coloration is linear in asphaltene content;10 there is a gigantic asphaltene gradient in the oil reservoir that must be understood for efficient oil production. This gradient can now be modeled. 2066
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Figure 3. A blue crude oil; the blue color of the oil has been traced to fluorescence from its perylene content, a 5-fused ring PAH.40 Relating optical properties to the PAH content for crude oils is sensible.
of crude oils, as a function of excitation wavelength,38 is a direct consequence of the Energy Gap Law.37 Fluorescence lifetimes obey the SternVolmer quenching and energy transfer formalism, which is consistent with a simple overall description of excited electronic states of crude oils and asphaltenes.39 Finally, the novel optical characteristics of a blue crude oil from the deep water, Gulf of Mexico, is traceable to its perylene content, a 5-ring PAH,40 and is consistent with moderately large PAHs dominating the electronic spectral properties of crude oils in the visible range. (See Figure 3.) To continue the investigation of the electronic properties of crude oils and asphaltenes, it is highly desirable to incorporate triplet-state measurements. Both triplet-state dynamics and spectroscopy are of interest. It is more difficult to perform direct comparisons of theory and experiment; both of which are more difficult for triplets. Nevertheless, such measurements are routinely performed on PAHs4144 and comparisons can be made to assess whether moderately large PAHs continue to dominate newly investigated electronic properties of crude oils and asphaltenes. Fluorescence quenching measurements have been performed on crude oils and PAHs and are interpretable in terms of simple quantum models.45,46 The method of pumpprobe spectroscopy (which also is called laser flash photolysis) is a very effective way to investigate the triplet state of molecules as well as long-lived radical species.47 (Flash photolysis often refers to the strong pulse coming from a flash lamp while pumpprobe often refers to the use of lasers.) In our case, as we will show, the triplet states, not free radicals, dominate our data. No evidence was found for absorption from charge transfer in crude oils or asphaltenes, especially at long wavelengths, where charge transfer is expected.17 Moreover, in previous studies of PAHs, triplet-state formation was dominant over charge transfer43 and is consistent with other studies.41,44 Moreover, as will be shown, the quenching dependence on oxygen dependence and the agreement with spectra with MO calculations also indicate that the formation of triplet states dominate. The pumpprobe process monitors the change in absorption of a sample over a broad spectral range when excited by a pulsed laser. This method is attractive for exploring uncharacterized samples, due to its sensitivity for measuring very small changes in absorption and due to the ability to tune the pump and probe
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wavelengths independently over a broad spectral range. With this sort of system, decay kinetics up to tens of milliseconds with nanosecond resolution are recorded and differentiated spectrally. The arrangement of the system makes it very easy to alter the environment of the sample and look for and characterize transient species under many conditions (for example, under variable temperature or oxygen conditions). In contrast to pumpprobe, phosphorescence normally must be conducted on chilled samples. Phosphorescence readings are typically taken between 70 K and 100 K. Here, triplettriplet (T1Tn) state transitions for crude oils and asphaltenes are explored experimentally, using pumpprobe methods. Use of crude oil and asphaltene samples with dramatically different heavy end content enabled identification of dominant transitions investigated. Results from MO calculations of PAHs compare favorably with measured results. Quenching dynamics of the lowest-lying triplet states with and without molecular oxygen are determined to validate the simple description of the quenching process. The temperature dependence of the quenching is determined and compared to expectations based on simple diffusion concepts. The results on triplet-state spectral and temporal analyses herein are consistent with the previously published singlet state experimental and MO theoretical analyses that asphaltene PAHs have, most probably, 7 fused rings and considerable width to the distribution.
’ EXPERIMENTAL SECTION Table 1 gives some properties of the crude oils used in the triplettriplet spectral measurements. Figure 4 shows the ground-state absorption spectra of five oils (1, 2, 6, 20, and 21) as well as the spectra of the UG8 asphaltene in benzene. For oils with more asphaltenes, the absorption spectra extend further to the red.34,35 All samples have been diluted in benzene and referenced against benzene. As expected,34,35 they all show broad monotonically increasing absorption at shorter wavelength, with a long tail beyond 1000 nm. The absorption spectra of the crude oils extend to longer wavelength in concert with their asphaltene and resin content. Dramatically different C36þ fractions are noted for these crude oils; this is very useful in ruling out significant impact of certain chemical species on our data. The fluorescence spectra of these crude oil and asphaltene samples are shown in Figure 5 (excitation at 375 nm). As expected, the asphaltenes exhibit a broadened emission and a red-shift in fluorescence, with respect to the crude oils, which are all relatively light. The absolute fluorescence signal for a given sample is a complex function of its absorption and its quantum yield, along with the optical design of the spectrometer. Laser PumpProbe Spectral Measurements. Nanosecond transient absorption measurements herein use the technique of laser flash photolysis (also called pumpprobe measurements) at the U.S. Air Force Research Laboratory and shown schematically in Figure 6. The third harmonic (355-nm) of a Q-switched Nd:YAG laser (Quantel Brilliant, pulse width of ∼5 ns) is used for laser flash excitation. Pulse fluences of up to 8 mJ/cm2 are typically used at the excitation wavelength. Laser-induced transmittance changes are monitored using white light from a 75-W xenon source (Photon Technology International) focused through the sample, and reimaged on the entrance slit of a Digikrom 240 monochromator. The resolved or dispersed light is detected with a Hamamatsu R-928 photomultiplier tube, and the current is routed through a back-off circuit that measures and compensates for Io, the background transmitted intensity. The real time current is recorded across 50 ohms on a Tektronix TDS 3054 digital oscilloscope. Data were collected and analyzed using routines written in National 2067
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Table 1. Crude Oil Properties for Samples Used for TripletTriplet Spectra
a
viscosity (cP)
cutoff λa (nm)
0.84
9.4
720
0.882
24.1
540
60.8
0.905
34.2
805
38.7
51.2
0.919
31.3
655
13.0
86.1
0.80
0.9
455
crude oil
C36þ (mol %)
asphaltene (wt %)
resin (wt %)
aromatics (wt %)
saturates (wt %)
density (g/cm3)
1
14.9
0.4
9.8
28.9
60.8
2
1.7
0
4.2
34.5
61.3
6
17.4
0
10.5
28.6
20
4.8
0.1
10
21
0.04
0
0.9
Wavelength (nm) for Abs. = 1 for a path length of 2 mm.
Figure 4. Crude oil and asphaltene visible near-infrared (groundstate) absorption spectra for samples used for triplet-state spectral properties. Samples were diluted in benzene.
Figure 5. Singlet-state fluorescence emission spectra with excitation at 375 nm of the crude oils and asphaltene used in this triplet excited-state study. The asphaltene spectrum exhibits broadening and a red shift, with respect to these fairly light crude oils. Instruments Lab View 5.1. All samples were deoxygenated three times using the freeze pump thaw method.
Laser Flash Photolysis Measurements To Determine Environmental Effects. Flash photolysis experiments were used to acquire the triplet-state lifetimes, as a function of oxygen content and of temperature, at Rose-Hulman Institute of Technology. These measurements were
Figure 6. Nanosecond laser flash photolysis (pumpprobe) used for measurement of T1Tn spectra for crude oils and asphaltene in the Air Force Research Laboratory. performed using the output of an optical parametric oscillator (OPO) pumped by a repetition rate of 10 Hz, a nanosecond pulse width (∼5 ns fwhm), and Q-switched Nd:YAG laser. The OPO is tunable from 420 nm to 2300 nm and possesses energy of several millijoules through this range. The OPO pumped a flash photolysis is shown in Fig. 7; this is the same layout as shown in Figure 6, but with different components (Rose-Hulman used a commercially available PROTEUS system from Ultrafast Systems). The samples of UG8 asphaltene were dissolved in toluene with concentrations ranging from 100 mg/L to 1.0 g/L, and the linear absorption spectra were recorded over a range of 1901100 nm on a Thermo Scientific Evolution spectrophotometer to confirm concentration. Measurements were performed on these samples in a 1 cm cuvette using pump wavelengths between 700 nm and 1400 nm from the OPO and excitation energies between 1 mJ and 3 mJ, and 256 shots were recorded for each spectral measurement. In addition to the measurements that were obtained under standard room conditions, temperature-dependent and oxygen-dependent measurements were made with the pumpprobe system. For the temperaturedependent experiments, a temperature-controlled cuvette holder replaced the standard holder (with aperture entrance and exit ports to match that of the standard holder). The sample temperature was controlled with a TLC 50 Fluorescence Cuvette Holder with a computercontrolled temperature monitor, using a water flow and dry air purged Peltier compartment with optical window feeds. Temperatures of 20 °C through 105 °C, are possible, with 0.02 °C precision. The experiments reported here were limited to a precision of ∼1 °C, because of the need to get the nitrogen and oxygen air lines into the top of the cuvette. The temperature-dependent data was used to create an Arrhenius plot and calculate activation energies. When taking the oxygen-dependent measurements, it was essential to have full control over the oxygen concentration. To achieve ample control of the system, a two-gas flow mixer and meter by Aalborg (S/N 260800-1) was added to the setup. The flow meter allowed for the concentration of oxygen and nitrogen to be precisely set and varied individually. After taking a baseline scan, the nitrogen was set to have a 2068
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Figure 7. Pumpprobe system used for measurement of triplet-state lifetimes of asphaltene at Rose-Hulman Institute of Technology.
Figure 9. Typical lifetime data for (top) excited singlet states from the femtosecond pumpprobe measurement at an excitation wavelength of 465 nm, and (bottom) the ground triplet-state transition recorded with the nanosecond flash photolysis measurement at an excitation wavelength of 420 nm. Note the enormous difference in time scales for the two plots. Figure 8. Duration of gas saturation, as determined by measuring lifetimes. Here, the substantial increase of lifetime with bubbling nitrogen is observed. The excitation wavelength was 420 nm with an excitation energy of ∼2 mJ. reading of 100 on the flow meter; once the sample had saturated, the level of oxygen was slowly increased, keeping the nitrogen constant at 100. In order to ensure that the readings were accurate, the flow meter was given enough time to situate to get an accurate reading for the amount of oxygen being bubbled through the sample. The readings could be varied from no oxygen flowing into the sample all the way to there being more oxygen than nitrogen being bubbled through the sample. In Figure 8, data are shown for the pure oxygen (red line) to pure nitrogen (blue line) bubbling through the sample. Femtosecond PumpProbe Measurements. Femtosecond white light continuum pumpprobe measurements were conducted in order to compare the kinetics and spectral response of asphaltenes, using very different excitation sources. Femtosecond transient absorption spectra were acquired using a commercially available pumpprobe spectroscopy system (Newport, Helios). The pump wavelength was tunable from 465 nm to 2900 nm and the pulses (∼120 fs, fwhm) were generated from an optical parametric amplifier (Newport, TOPAS), which, in turn, was pumped by a Ti:Sapphire regenerative amplifier (Newport, Spitfire) operating at a repetition rate of 1 kHz. In order to generate pump wavelengths below 465 nm, the TOPAS output was frequency doubled using a BBO crystal. The pump beam was sent through a depolarizer, prior to overlapping the probe beam at the sample, in order to ensure observation of pure depopulation dynamics. The probe pulse was obtained using ∼5% of the fundamental 800-nm pump (Spitfire) to generate a white-light continuum (420950 nm) in a sapphire plate. The temporal step resolution of the system was 7 fs, and the maximum extent of the temporal delay
was 3200 ps. The instrument response function was determined to be ∼200 fs. At each temporal delay, data were averaged for 1.5 s. The pump beam was chopped at 500 Hz to obtain, sequentially, both pumped (signal) and nonpumped (reference) absorption spectra of the sample. A correction factor to account for the chirp of the white-light continuum probe was generated using the ultrafast response of CS2 and was applied to all datasets. The data were stored as 3-D Wavelength-Time-Absorbance matrices that were exported for use with the fitting software. The solutions were studied in 2-mm path-length cuvettes, had optical densities in the range of 0.070.45 at the associated pump wavelength, and were stirred continuously throughout the data acquisition. Figure 9 shows the decay of the UG8 asphaltene after excitation by femtosecond and nanosecond excitation, which demonstrated the complex nature of the decay kinetics. In addition to the measurements that were obtained under standard room conditions, temperature-dependent and oxygen-dependent measurements were made with the pumpprobe system. For the temperature dependent experiments, a temperature controlled cuvette holder replaced the standard holder. The temperature-dependent data was used to create an Arrhenius plot and calculate activation energies. When taking the oxygen-dependent measurements, it was essential to have full control over the oxygen concentration. To have ample control over the system, a two-gas flow mixer and meter was added to the setup. The flow meter allowed for the concentration of oxygen and nitrogen to be precisely set in the asphaltene in toluene sample.
’ THEORETICAL METHODS PAH model compounds were selected here by the same criteria used for calculations of singlet manifold spectroscopy.18 All the calculations, including geometry optimizations, were 2069
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Figure 10. The singlet (S) and triplet (T) manifold. All population starts out in the ground singlet state. Photoexcitation is allowed only between states of the same spin. The excessive lifetimes of the triplet T1 state occur because of a required electron spin flip of the T1 state to return back to the S0 state. Pumpprobe (or photolysis) experiments use this long T1 lifetime to probe electronic transitions.
carried using the high level quantum DFT (density functional theory) approach with the self-consistent generalized-gradient GGA and the PerdewWang 91 (PW91) exchange-correlation potential (DFT GGA-PW91)48,49 with a DNP basis set (triple numerical basis set)50 with a radial cut off of 3.0 Å, as implemented in the DMol3 code.5052 We have calculated the lowest T1T2 transitions for systems with 4FAR to 11FAR (number of rings in the fused aromatic ring). The optimization of the PAHs in triplet basal state is carried out considering an unrestricted calculation, with the GGA(PW91) functional and DNP basis set, to calculate the energy levels. The lowest triplet state for all the molecules considered in this study is that state obtained by a promotion of a π-electron from the highest-filled MO to the lowest-empty MO. Therefore, we consider only, and all of, those electronic excitations in which the highest-filled and/or lowest-empty MOs are involved in order to construct the wave functions for the lowest triplet configuration 3 Ψ0 and for the higher triplet configurations. There are some difficulties in the T1Tn transition calculation. In triplet-to-triplet absorption spectra, one is dealing with an open-shell-to-open-shell transition—where the intershell correlation energy changes—whereas, in (ground-state) singlet-totriplet spectra or even in (ground-state) singlet-to- (excited state) singlet spectra, one is confronted with a closed-shell-toopen-shell transition—where only the intrashell-correlation energy changes. First-order configuration interaction (CI) is included in the calculations; however, additional second-order CI worsens the agreement with experimental data.48 In CI calculations, the MOs for the ground state are calculated and then used unchanged to construct a series of further electronic configurations (microstates) that are mixed to form new electronic states. The microstates are used for the calculation of excited states. There are no reasons for expecting similarities between the S0S1 transitions and T1Tn spectra for molecules with 1Lb and 3 La lowest excited states; however, similarities are observed, as discussed in Figure 10 and in ref 48.
’ RESULTS AND DISCUSSIONS Figure 10 shows a schematic of the relevant electronic states. Initially, all of the population of the asphaltene PAHs is in the ground singlet state (S0). Normal absorption spectroscopy probes excitation from S0 to S1 or S2 (or higher) electronic states. The triplet states are populated by forming S1 in photoexcitation, with some fraction of that population undergoing intersystem crossing to the triplet manifold.37 This process
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corresponds to PAHs whose allowed transition energy matches the 355-nm pump laser.18 Triplet-State Spectra. Pumpprobe experiments were employed to measure the T1Tn excited-state absorption spectrum. Samples were excited at 355 nm. This only selects a population of a range of PAHs that absorb at 355 nm.16 Some of the selected population within the singlet manifold then undergoes intersystem crossing, forming the T1 states. Comparison of the excited state spectra with the composition of different crude oil samples used here rules out much contribution of free radicals (see below), which are present in crude oils in very low concentration.53 Identification of the triplet excited state, as expected from PAH studies,43 was confirmed by molecular oxygen quenching and agreement with MO calculations. After formation of the T1 state, a flash lamp then further excites the T1 states to higher Tn levels, giving the corresponding difference absorption spectra shown in Figure 11. All the spectra are rather similar in defining characteristics in Figure 11. For the crude oils, the “colored” heavy end concentrations are, from smallest to largest, samples 21, 2, 20, 1, 6, then the asphaltenes sample. This is registered in the ground-state absorption spectra in Figure 4. In the triplet excited-state absorption spectra, the lighter oils (Figure 11, left three panels) exhibit a subtle blue shift, compared to the somewhat heavier oils (Figure 11, left, lower two panels), and the asphaltenes exhibit the greatest red shift. This is in accordance with the PAH ring size.1618 In particular, the asphaltenes are deficient in smaller PAHs2,3,54 and exhibit the largest red-shifted triplet spectra. The difference spectrum is defined as the excited-state absorption minus the ground-state absorption: ΔA ¼ εES bCES εGS bCGS where ε defines the molar absorption coefficient of either the ground state or the excited state, b is the cell path length, and C is the concentration of the ground state or the excited state. The negative absorption in Figure 11 corresponds to hole burning in the spectra. That is, there is a depletion of the S0 state, because of the initial pump excitation. This depleted population gives rise to a decreased probe-laser absorption with the pump source, compared to without the pump source. This comparison of absorption with and without the pump laser produces the negative absorbance in Figure 11. Consequently, there is a negative absorption at the pump wavelength which is clearly evident in the spectra of Figure 11. The positive absorption in Figure 11 is due to the absorption of the newly created triplet states. Minimal Free Radical Contribution. There is a question whether, in addition to triplet-state transitions, there is significant signal in our excited-state spectroscopy, in Figure 11, from crudeoil free radicals. This appears very unlikely. It is well-known that the free radicals in crude oil are, by far, concentrated in the asphaltene fraction and, to a lesser extent, in the resin fractions.53,5557 An exponential decrease in free-radical concentration has been observed versus aromatic content of petroleum fractions.56 The saturate and aromatic fractions of crude oil have almost no free radicals. For example, the free-radical concentrations of 9 1018/g-asphaltene and 2 1017/g-resin have been reported, showing the dramatic decline in free-radical content as the PAHs become smaller.57 Consequently, if free radicals dominated our signal, it is very unlikely that a very light crude oil with 10% resin and 2070
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Figure 11. Nanosecond pumpprobe T1Tn absorption spectra of several crude oils and UG8 asphaltene diluted in benzene. The excitation wavelength is 355 nm, thereby selecting a population of a range of PAHs. Samples were deoxygenated using a freezepumpthaw method. All spectra show similar features. For lighter crude oils (right three panels), the selected PAHs exhibit a subtle blue shift. For somewhat heavier crude oils (left, lower two panels), the spectra are red-shifted. Asphaltenes (upper left panel) are deficient in small PAHs2,3,54 and exhibit the largest red shift. The negative absorption is due to population depletion from the pump laser (or “hole burning”).
asphaltene fractions. Yet, examination of Figure 11 shows just this occurrence when comparing oils 1 and 6 with >10% asphaltene and resins with oil 21, which is a very light crude oil with
