PuO₂ possesses a fluorite structure group with a face-centered cubic sublattice (Sarsfield et al., 2012). Group theory analysis predicts one Raman mode, the T_2g mode located near 480 cm⁻¹, and two infrared active modes located near 270 and 580 cm⁻¹ (Gilson and Hendra, 1970; Sarsfield et al., 2012). However, the typical Raman spectrum of PuO₂ exhibits several bands which are unaccounted for by group theory. Figure 2 shows the typical Raman spectrum of PuO₂ calcined at 450, 650, and 1,000 °C (Villa-Aleman et al., 2019a; Villa-Aleman et al., 2021b). Five dominant Raman bands were present in the spectrum at 479, 581, 1,163, 2,135, and 2,635 cm⁻¹, and ten additional bands were consistently observed in the spectra at 300, 450, 625, 1,043, 1,580, 2,045, 2,425, 2,600, 2,740, and 2,940 cm^-1. Additional bands in the defect band spectral region (550–670 cm^-1) were observed to have emerged in the spectrum with age since last calcination. The T_2g mode (1LO1) located near 480 cm^-1 is the only Raman allowed transition from a fluorite structure group (Schoenes, 1980; Sarsfield et al., 2012). The defect band located at 581 cm⁻¹ has been suggested to be the 1LO2 mode which is an IR-active band (Schoenes, 1980). The 1,163 cm⁻¹ band is attributed to an overtone band of the 1LO2 mode (i.e., 2LO2) (Livneh and Sterer, 2006; Jégou et al., 2010). Naji et al. (2017) postulated that the band at ∼1,580 cm⁻¹ corresponds to C=C bonds. Unpublished laser annealing work by our group with a PuO₂ pellet calcined at 450°C and a comprehensive understanding of our work since 2015 has clearly established that the 1,580 cm⁻¹ corresponds to the graphite band of carbon often observed in graphitic materials and other carbonaceous soots (Sadezky et al., 2005; Villa-Aleman et al., 2022). In the unpublished work, PuO₂ calcined at 450 C under laser annealing shows the growth of soot emerging due a significant amount of carbon remaining in the material from the oxalate decomposition (manuscript under preparation). Meanwhile, the bands located at 2,135 and 2,635 cm⁻¹ are identified as electronic bands corresponding to the Γ₁ → Γ₅ and Γ₁ → Γ₃ transitions (Naji et al., 2017). We have identified the ∼2,940 cm^-1 band as a CH stretch from a sp³ carbon. It is shown in Section 3.2 that CH stretches from sp³ carbon have been observed with infrared spectroscopy. At high calcination temperatures, we have observed that most of the CH stretch band from sp³ carbon is converted to a CH stretch from sp² carbon geometry as the intensity of the 2,940 cm⁻¹ band decreases and a new band is observed around 3,100 cm⁻¹. The bands in the Raman spectra were consistently observed in samples prepared from different Pu-bearing precursors, such as plutonium oxalates, fluorides, and nitrates, and with different excitation wavelengths. The carbon observed in PuO₂ produced from non-carbon-containing precursors is not definitively explained at present. We believe that CO₂ absorption from the atmosphere during calcination best explains the observed carbon signatures.

The band position, intensity, and full width at half maximum (FWHM) of the bands observed in the Raman spectra of PuO₂ are highly dependent on the calcination temperature and the age of the material. Low calcination temperatures of a Pu oxalate precursor result in lattice defects during the formation of PuO₂. The defects in the crystal lattice are heavily influenced by the impurities of the material, such as Gd₂O₃-doped CeO₂ (Taniguchi et al., 2009; Desgranges et al., 2012; Filtschew et al., 2016; Harker and Puxley, 2016; Villa-Aleman et al., 2019b; Xu et al., 2019). During the thermal decomposition of plutonium oxalates, carbon atoms are found in substitutional and interstitial positions. The FWHM of the Raman T_2g band correlates with the disorder in the crystal lattice. The chemical transition from the oxalate to the oxide shows the formation of PuO₂ nanoparticulates at low temperatures that exhibit a broad, asymmetric FWHM T_2g band corresponding to phonon confinement. PuO₂ calcined at 450 C is known to contain significant quantities of carbon and defects as measured with the T_2g band properties (Villa-Aleman et al., 2019b; Christian et al., 2022). Calcination at higher temperatures results in the narrowing of the FWHM for the T_2g and electronic bands, corresponding to the conversion of an amorphous to crystalline lattice. Stabilization of the crystal lattice occurs in the high-fired regime (>900 C) as observed by the FWHM. Recent laser-induced annealing by our group (unpublished) of 450 C calcined PuO₂ demonstrated the presence of large quantities of carbon in the material. During the laser annealing of low-fired PuO₂, the emergence of a soot signature with bands near 1,580 (higher frequencies have been observed up to 1,610 cm⁻¹) and 1,350 cm⁻¹ with a broad luminescence is observed to grow, suggesting the coalescence of carbon atoms and reactions within the crystal lattice. The concentration of graphitic and disorder carbon structures and the luminescence are reduced at higher annealing temperatures. PuO₂ calcined at 650 °C only shows a weak soot signature and little luminescence. Throughout the manuscript, the FWHM of the T_2g band and the electronic bands as well as the defect bands can be used to characterize the crystallinity of the material.

The defect bands for the 450 °C freshly calcined ²³⁹PuO₂ are very weak, and the intensity of the defect bands relative to the T_2g significantly differ from the alpha decay aged material in equilibrium (∼10 years). The different behavior of the defect bands and the FWHM between freshly calcined and aged materials suggests that the defect bands are primarily an effect of alpha-decay or ion bombardment of the crystal lattice.

We have been investigating the time-dependent properties of the bands in the Raman spectrum since 2016 (Villa-Aleman et al., 2019b; Villa-Aleman et al., 2021a; Villa-Aleman et al., 2021b). Recent analyses of our data and comparison with published x-ray diffraction (XRD) data of alpha-irradiated UO₂, PuO₂, and other actinides have provided new insights into the time-dependent Raman data of ²⁴⁰PuO₂, the meaning of the FWHM of the T_2g band, and the source of the defect bands (Villa-Aleman et al., 2021b). The work presented in this section summarizes our previous work and new insights into the crystal lattice bombarded by alpha particles.

Raman spectroscopy is frequently used to measure how the environment affects the crystal lattice of a material. Changes in oxidation state or damage to the crystal lattice can result in new bands or changes in the band intensity, FWHM, and position. The alpha particle and the recoiling U atom have been found to be the predominant drivers of the time-dependent spectral changes observed in PuO₂ (Nellis, 1977; Weber, 1983; Matzke, 1992; Kato et al., 2009; Talip et al., 2018). The spectral changes are indicative of the effect of the alpha decay on the crystal lattice. Although the spectral changes are clearly observed in T_2g and the electronic bands at 2,135 (Γ₁ → Γ₅) and 2,635 cm⁻¹ (Γ₁ → Γ₃), most work follows the T_2g band properties since the band is much narrower than the electronic bands (Villa-Aleman et al., 2021b; Villa-Aleman et al., 2023b).

The T_2g phonon mode, located at ∼480 cm⁻¹ for PuO₂ calcined at ≥ 900 °C, corresponds to the symmetric stretch of the four oxygen atoms surrounding the Pu atom (Gilson and Hendra, 1970; Sarsfield et al., 2012). As the alpha decay dose increases with time, the bands in the Raman spectrum to shift to lower frequencies, indicative of weakening bonds in the crystal. The FWHM of the T_2g band is observed to increase and the relative intensity of the band to decrease. The increase in the FWHM and the decrease in the band intensities are correlated to the distribution of bond energies in the crystal lattice. These signatures are correlated with amorphization of the material. Once it is realized that PuO₂ is a dynamic system where the time-dependent properties of the material are driven by the alpha decay, Raman spectroscopy can be used to monitor the changes in the crystal lattice due to alpha particles and the recoil uranium atom.

Figure 3A shows the time-dependent behavior of the T_2g band and the two main defect bands located at ∼580 and 650 cm⁻¹ (Villa-Aleman et al., 2021b). The three bands (T_2g, 580, and 650 cm⁻¹) seen in Figure 3B are termed bands A, B, and C, respectively, in this section. The observation of the defect bands in the PuO₂ system represent a complex evolution of the material which might involve group symmetry changes and the creation of new suboxides (Conradson, 2004; Taniguchi et al., 2009; Filtschew et al., 2016; Villa-Aleman et al., 2019b; Xu et al., 2019). Although we have identified four defect bands in the Raman spectrum, this work concentrates on the intense 580 and 650 cm⁻¹ bands, labeled B and C in Figure 3 (Villa-Aleman et al., 2021b). Numerous researchers have attributed the B band as the LO2 infrared band which emerges in the Raman spectrum due to changes in the symmetry rules caused by the alpha decay of the crystal lattice (Schoenes, 1980; Jégou et al., 2010; Sarsfield et al., 2012; Naji et al., 2017). Although the B band might correspond to the LO2 active in the IR, we have identified three additional bands in the defect region which cannot be assigned to other infrared bands since there are only two infrared bands (∼270 and 580 cm⁻¹) and one Raman band (480 cm⁻¹) in the fluorite structure. It is possible the three bands are the result of suboxide species in the PuO₂ system (Villa-Aleman et al., 2019a). The suboxide species hypothesis is supported by the B/A and C/A band ratios and the wavelength-dependent intensity differences among the bands. The Raman spectrum acquired with long excitation wavelengths (561, 633, and 785 nm) shows the 650 cm⁻¹ band with a greater intensity than the 580 cm⁻¹ while short excitation wavelengths (457, 488, 514, and 532 nm) show the 580 cm⁻¹ band with a greater intensity than the 650 cm⁻¹ band.

The time-dependent behavior of the Raman spectrum can be understood with knowledge of the alpha decay process. There are 2.3 × 10⁹ alpha decay events per gram per second for ²³⁹Pu (Wolfer, 2000). Each 5 MeV alpha particle travels approximately 10 μm in the crystal lattice while the 86 keV ²³⁵U recoil nucleus is known to travel ∼12 nm. The alpha particle captures two electrons during the travel, becoming a He atom. A significant number of cation and anion vacancies, known as Frenkel defects, and interstitial defects are created during the alpha particle and uranium recoil. The cationic and anionic Frenkel pairs are representative of the damage to the crystal lattice. Approximately 265 Frenkel pairs are created at the end of the alpha particle range (anions and cations), while 2,300 Frenkel pairs are created during the uranium atom recoil (Wolfer, 2000). Schwartz et al. (2005) calculated that 90% of the Frenkel pairs return to their original site, leaving approximately 10% as free interstitial defects and vacancies or clusters of both. The Frenkel pairs result in crystal lattice swelling—a phenomenon well explained by X-ray diffraction (Nellis, 1977; Weber, 1983; Matzke, 1992; Kato et al., 2009; Talip et al., 2018). Eventually, the exponential behavior of crystal lattice swelling Δa/a plateaus, where the same number of created defects equals the number of defects annealed.

The time-dependent curves of the A, B, and C bands provide information between the creation and annealing of the Frenkel pair defects and possibly the creation of other oxide species in the material. Figure 4 shows different metrics used in the analysis of the crystal lattice. The spectral properties shown in Figure 4 are FWHM band A (T_2g), B/A band area ratio (580 cm⁻¹/T_2g), C/A band area ratio (650 cm⁻¹/T_2g), and C/B band area ratio (650 cm⁻¹/580 cm⁻¹). Each curve provides a different perspective on the effect of alpha decay in the material.

For a well-ordered crystal lattice of PuO₂ (high-fired), the greatest rate of change in the crystal lattice damage occurs early in the aging process. Eventually, the rate of change becomes zero and the curve plateaus indicate an equilibrium state between the creation and annihilation of Frenkel pairs. The time-dependent damage is proportional to the specific activity of the radionuclide composition. Therefore, since ²⁴⁰PuO₂ decays 3.67 times faster than ²³⁹PuO₂, the curve for ²⁴⁰PuO₂ should reach the plateau 3.67 times faster than that of ²³⁹PuO₂. For ²³⁸Pu, ²³⁹Pu, ²⁴⁰Pu, and ²⁴²Pu, the alpha decay has approximately the same energy; thus, the graphs in Figure 4 can be normalized (Nellis, 1977; Browne and Tuli, 2007; Browne and Tuli, 2015).

The lifetimes calculated from the FWHM of A, the ratio of B/A, and the ratio of C/A curves are 344, 238, and 287 days, respectively. In contrast, the age-dating curve corresponding to the C/B ratio has a much shorter lifespan of 113 days. An overview of the graphs in Figure 4 indicates that the FWHM of band A (Figure 4A) and the C/A band area ratio (Figure 4B) are very similar (a 344- and 287-day lifespan, respectively). Unlike the FWHM of band A and the C/A band area ratio, the B/A curve’s (Figure 4B) lifespan (238 days) does not follow the same pattern, suggesting that the intensity of the B band (580 cm⁻¹, probably 1LO2) is caused by a different effect than crystal damage. Meanwhile, the C/B band ratio (Figure 4D) shows a 113-day lifespan, much faster than the other three curves. This indicates that the rate of production of an oxide (band C) rises at a much faster rate than the symmetry change of the LO2 or the formation of a different oxide described by band B (580 cm⁻¹). Our long excitation wavelength Raman data (785, 633, and 561 nm), in contrast to the short excitation wavelength Raman data, clearly show that the intensity of band C is at least twice as intense as band B (Villa-Aleman et al., 2019b). Any differences in the shape of the curves should be indicative of different mechanisms governing the damage caused by the alpha decay to the material. Therefore, the data from Figures 4A and C suggest that the FWHM of the T_2g and the area of band C describe the same phenomena, while the B band (580 cm⁻¹) most likely corresponds to a different phenomenon such as symmetry change.

Nellis developed a simple theory for the swelling and annealing of crystal lattice where the interatomic distances can be expressed as: Δ a a 0 = A 1 − e − B τ t , where a₀ is the lattice parameter after preparation, Δa is the change in lattice parameter via self-radiation, A and B are constants, τ is the decay constant of an actinide isotope, and t is storage time (Nellis, 1977). The established relationship is dependent on three different constants: τ (the decay constant associated with the alpha-emitting isotope(s) present), A (the maximum change in lattice parameter), and B (a measure of the rate of change of the lattice parameter, kinetic constant). A and B are intrinsic to the PuO₂ lattice and will be the same for each isotope of plutonium. Because τ is different for each plutonium isotope, using the same τ to generate the exponential regressions for each isotope transforms all the regressions onto the same timescale, allowing for direct comparisons. Like the XRD measurements, where the distance between atoms in the crystal lattice and the swelling can be calculated, the Raman spectra suggest that it is possible to estimate the crystal lattice expansion by comparison with the XRD data. This hypothesis was tested with a comparison of the data in Figure 4 to XRD swelling plots obtained by Weber (1983).

Although several studies have been conducted to measure the Δa/a of the crystal lattice for PuO₂, mixed oxide fuel, and irradiated UO₂ (Rand et al., 1962; Nellis, 1977; Weber, 1983; Matzke, 1992; Kato et al., 2009; Talip et al., 2018), the crystal lattice swelling of UO₂ versus alpha particle dose is the most significant data for comparison with our ²⁴⁰PuO₂ aging data curves in Figure 4 (Weber, 1983). Weber (1983) irradiated UO₂ using ²³⁸PuO₂ as an alpha particle source and measured the Δa/a for a wide range of doses. The data from Weber were re-created and compared to our curve from Figure 4A and are shown in Figure 5. The curve generated by Weber for the irradiation of UO₂ with ²³⁸PuO₂ is nearly identical to the time-dependent curve of the FWHM of the T_2g band (band A) and the C/A band ratio (FWHM—344 days, C/A—287-day lifespan) observed with Raman spectroscopy (Weber, 1983). It is evident that the T_2g (symmetric stretch of oxygen atoms surrounding a plutonium atom) can be correlated to the Δa/a of the material and, therefore, is also correlated to Frenkel pairs’ generation and destruction. Similarly, since the C/A band ratio also follows the Δa/a curve, we postulate that the creation of the C band is correlated to crystal lattice swelling and the formation of a suboxide during the alpha decay. On their own, the intensity of the emerging defect bands is meaningless unless the intensities are anchored to another parameter (band ratios) and the C/A ratio curve is different from the B/A and B/C band ratios; we suggest that the B band originates from a different source. The most logical explanation for B band behavior is that the emergence of this band relies on symmetry selection rules and not on the formation of suboxide species.

The production of alpha particles is accompanied by a high energy uranium recoil atom. It is known that the energy of the recoil atom is high enough to produce a significant number of Frenkel pairs while tunneling through the matrix but also to induce annealing. Since the date-aging curve from the FWHM of the T_2g band matches the Δa/a data from irradiated UO₂ with an alpha source, the recoil uranium atoms do not seem to contribute to the FWHM of A in our ²⁴⁰PuO₂ decay experiments. Therefore, the data suggest that either the Frenkel pairs from the recoil atom are not important in the equality due to the high energy and melting of the material or are just another multiplier in the intensity of the curve without affecting the curve’s shape.

The new meaning associating the FWHM of the T_2g band with Δa/a (crystal swelling) and the age-dating behavior of band B (580 cm⁻¹) provides a method for differentiating crystal swelling from symmetry group changes. Consequently, the FWHM of the T_2g band describes the crystallinity of the material and can provide an estimation of the Δa/a from a calcination experiment or from an aged material affected by the alpha decay. An illustration of this be found in the calcination of Pu₂(C₂O₄)₃ and Pu(C₂O₄)₂ at different temperatures to form the oxide from temperatures in the 350 °C–900 °C range (Villa-Aleman et al., 2023a). These experiments showed that the FWHM of the T_2g band can be used to describe the crystallinity of PuO₂ at different temperatures and can depend on the precursor and only reach an equilibrium at high calcination temperatures.

For high-fired PuO₂, the curves in Figure 4 provide the necessary information to estimate the age of the material since the last calcination. The high-fired PuO₂ exhibited the highest crystalline state, and the T_2g exhibited the narrowest FWHM, with the defect bands showing the lowest intensity. For high-fired PuO₂, a single point in time is sufficient to estimate the age since last calcination, with the narrowest FWHM of the T_2g attaining approximately 9 cm⁻¹ (Villa-Aleman et al., 2021b; Villa-Aleman et al., 2023a). Low-fired calcination PuO₂ results in bands with much wider FWHM, as demonstrated by the FWHM acquired at temperatures as low as 450 C. PuO₂ calcined at that temperature shows a FWHM between 18 and 21 cm⁻¹ (Villa-Aleman et al., 2023a); meanwhile, unpublished data show a similar FWHM of the T_2g for high-fired ²⁴⁰PuO₂ aged for 7 years. Unlike the FWHM, the intensity of the defect bands is highly dependent on the accumulated alpha decay dose and not on the original calcination temperature of the material. The B/A band ratio for PuO₂ freshly calcined at 450 C is approximately 0.03 compared to a B/A band ratio of 0.01 for high-fired PuO₂. Our age-dating data show a maximum band ratio of 0.20–0.22 for an aged material (Villa-Aleman et al., 2021b). This information can be used to estimate the age of the material since last calcination for any calcination temperature. Two-point measurements, two temporal datasets, can be used to make more accurate age estimates (Villa-Aleman et al., 2021b). Our work has shown that the FWHM of the T_2g band is related to radiolytic aging and calcination temperature, and the metric can only be used for age determination from medium (650 C) to high-fired material. Low-fired materials have bands with a FWHM which are a composite of age and calcination temperature which describe the crystallinity of the material; thus, the effect of calcination temperature on the FWHM can obfuscate the increase observed with age. Conversely, we know that the growth of the B and C bands are dictated by the alpha decay process with little contribution from disorder in the crystal. In the worst-case scenario, for PuO₂ calcined at 450 C, the contribution from the material crystallinity could be 15% of the total intensity of the B band. In these cases, the error could be reduced significantly by taking two measurements at different times separated by at least 1 month. The B and C bands and their ratios to the FWHM of the T_2g band can be used to make accurate age determinations up to ∼10 years for ²³⁹PuO₂, where an equilibrium between defect creation and annealing reaches steady state.

Figure 6 shows the evolution of the Raman spectrum of PuO₂ with wavelengths from 785, 633, 561, 514, 488, 457, 405, and 355 nm (Villa-Aleman et al., 2019a). Additional excitation wavelengths (325 and 244 nm), although not present in Figure 6, were used to probe PuO₂. For clarity, the spectra taken with 785, 633, 514, 488, and 457 nm excitations were collected from the same sample with a similar age since the last calcination, while the spectra collected with 405 and 355 nm excitations were taken from PuO₂ samples with different aging which exhibited differences in the defect band region and band shifts relative to other laser excitation wavelength spectra. The Raman spectrum observed at 785 nm showed the T_2g band, very weak defect bands, and electronic bands. The overtone 2LO2 and the electronic bands were barely visible in the Raman spectrum with the 785 nm excitation wavelength. The Raman spectrum acquired with the 633 nm showed the first clear indication of the 2LO2 and electronic bands. As the excitation wavelength was changed from 561 to 457 nm (561, 514, 488, and 457 nm), the 2LO2 overtone and the electronic bands at 2,135 and 2,635 cm⁻¹ were observed to increase relative to the T_2g band. The intensity of these bands from 785 nm to the 457 nm was observed to increase by several orders of magnitude. The evolution of the band intensities relative to the T_2g band illustrates the effect of resonance Raman versus excitation energy. The most important observations from the spectra are the growth in intensity of the overtone, electronic bands, and the band associated with soot located at 1,580 cm⁻¹ from the 785 nm to the 457 nm excitation line and the decrease in band intensity for 405 nm and 355 nm. In contrast to laser excitation wavelengths at 405 and beyond, Raman spectra with 355, 325, and 244 nm show the disappearance of the 2LO2 band, while the band at 1,045 cm⁻¹ is significantly stronger with the UV wavelengths than in the visible excitation wavelengths. The disappearance of the overtone band is most likely related to differences between the laser excitation wavelength and the electronic state precluding resonance enhancement. Another possibility for the disappearance of the 2LO2 overtone might be related to the interaction of the UV laser excitation wavelength primarily with the surface, where the structure of PuO₂ has been modified to a different molecular structure. It has been shown in the uranium system that the overtone is very sensitive to the molecular structure (Jégou et al., 2010; Elorrieta et al., 2018). Laser-heated UO₂ results in the formation of U₃O₈, while laser-heated U_xPu_xO₂ results in a different molecular structure with primary bands at 465 and 630 cm⁻¹ where the overtone band has disappeared.

Resonance Raman has been explained in terms of the Kramer–Heisenberg–Dirac (KHD) theory, which explains the wavelength-dependent behavior of the PuO₂ Raman spectrum (Smith and Dent, 2019). The KHD theory predicts the presence of overtones in the Raman spectrum when the energy of an excited state is similar to the laser excitation energy and the possibility of observing numerous overtones in contrast to normal Raman scattering. The optical band gap for PuO₂ is located at approximately 442.8 nm (McCleskey et al., 2013; Scott et al., 2014). The proximity of the energy between the conduction band and the excitation wavelength at 457 nm agrees with the enhancement of the bands observed in Figure 6.

In Raman scattering, the intensity of the Raman bands depends on the laser power, the frequency of the incident light radiation, and the polarizability of the electrons in the molecular vibration. To understand the resonance Raman effect observed in the CeO₂, UO₂, and PuO₂ systems, we find it important to review the KHD expression (Smith and Dent, 2019). For our simplistic description of polarizability, we start with the KHD expression and describe the polarizability of the electrons under laser irradiation. The KHD expression is as follows: α ρ σ G F = k ∑ F r ρ I I r σ G ω G I − ω L − i Γ I + I r ρ G F r σ I ω I F + ω L − i Γ I , where k is a constant, ρ and σ are incident and scattered polarization directions, respectively, and G, I, and F are the ground, intermediate, and final vibronic states, respectively, containing integral mixing waveforms related to the ground, intermediate, and excited vibrational states through dipole operators in the numerator, while energies related to excited states and laser excitation frequency are in the denominator. Bra-ket notation is used to represent integrals where operators modify the wavefunctions of the intermediate, ground, and final states. The integrals describe the mixing of the ground and excited states. The term iΓ _I describes the lifetime of the excited state and is related to the natural FWHM of the Raman band. It is important to realize that, when the excited state energy is similar to the excitation energy, as shown in the denominator of the first term in the equation, ( ω G I − ω L − i Γ I ), the term becomes i Γ I . The first term in the equation increases in value by several orders of magnitude, resulting in a similar enhancement to the observed bands in the Raman spectrum. Enhancements of 10² to 10⁶ are often observed in Raman resonance (Morris and Wallan, 1979). Since the first term in the equation is much larger than the second term due to the denominator ( ω G I − ω L − i Γ I ), an approximation can be used to retain only the first term.

The selection rules for Raman spectroscopy can be obtained by separating the electronic, vibrational, and rotational contributions during ground, excited, and final-state transitions. Electronic transitions are known to occur within a picosecond or less. On this timescale, where electrons are excited, the nucleus remains virtually stationary (vibrations occur at ∼10⁻⁹s). Therefore, it is feasible to separate the electronic, vibrational, and rotational terms in the KHD expression using the Born–Oppenheimer approach with the wavefunction Ψ= Θ·Φ·r, where Θ, Φ, and r represent the electronic, vibrational, and rotational wavefunctions, respectively. Therefore, the integral in the first term I r σ G can now be described by electronic and vibrational products θ I r σ θ G Φ I | Φ G . This separation, in conjunction with a Taylor series expansion, describes the electronic transition while the nucleus is at equilibrium and the motion of the nucleus along a particular coordinate. After few mathematical manipulations, an equation can be derived which depends on two terms related to electronic transitions and to the motion of the nucleus. The first term is identified as the A-term, and the second is identified as the B-term. When the excitation energy is significantly different than the electronic excited state energy, the B-term dominates the polarizability, and only vibrations that contain one quantum of energy (no overtones) are observed in the Raman spectrum. Therefore, only symmetric vibrations are allowed in the Raman spectrum. Conversely, when the excitation energy is close to the energy of the electronic state, the A-term dominates the polarizability factor, and it is quite common to observe overtones in the Raman spectrum.

The two independent effects, wavelength-dependent Raman scattering and wavelength-dependent defect band scattering, present two aspects of the material (the interactions of the laser wavelength with the electronic states near the conduction band and the presence of multiple oxides characterized by the defect bands). The wavelength-dependent Raman scattering was monitored by the relative band intensity of the numerous bands (electronic bands) to the T_2g band. The Raman spectrum acquired with the 785 nm excitation line consisted of the T_2g band with weakly observed defect bands and electronic bands. Thereafter, the overtone and electronic bands were observed to increase in intensity toward shorter excitation wavelengths.

The wavelength-dependent studies have shown the presence of resonantly enhanced Raman bands and the 2LO2 overtone in the spectrum. Similarly, the bands in the defect spectral region, consisting of four bands (in this study, we only discuss the two most dominant bands in the spectrum, B (580 cm⁻¹) and C (650 cm⁻¹)) show a different excitation wavelength dependence which suggests the presence of distinct oxides.

Although Raman spectroscopy provides unique information on the decomposition of the oxalate en route to the formation of PuO₂, very little information can be extracted from organic impurities contained in the oxide. In contrast, infrared spectroscopy shows the frequencies of the oxalate ligands and the carbon products from the oxalate decomposition still available at temperatures as high as 900 C. Infrared spectroscopy depends on the change of dipole moment for a vibrational mode to be active. In general, symmetric motions are easily observed in the Raman scattering spectrum while asymmetric motions are predominantly observed in the infrared spectrum. Pu (III) and Pu (IV) oxalates are the primary precursors used in the production of PuO₂ via the calcination process (Wick, 1980; Patterson and Parkes, 1996; Bridges and Shehee, 2015). The TO (270 cm⁻¹) and 1LO2 (580 cm⁻¹) corresponding to the PuO₂ frequencies are not observable in our data since the BaF₂ windows of the DWCs prohibit transmission below 800 cm⁻¹ (Schoenes, 1980; Sarsfield et al., 2012). Figure 7 shows the IR spectrum for Pu(C₂O₄)₂ and several of its thermal decomposition products up to 900 °C (Christian et al., 2022). Plutonium(IV) oxalate can be described by the water band (∼3,600 cm⁻¹) and several bands related to the oxalate group (Myser, 1956; Kartushova et al., 1958; Glasner, 1964; Nissen, 1980; Karelin et al., 1990; Vigier et al., 2007; Orr et al., 2015; South and Roy, 2021; Christian et al., 2022). At 100 °C, a significant amount of water has been removed from the material and the presence of sp3 C-H bonds are observable at ∼2,960 cm⁻¹. By 220 °C, additional water is removed from the oxalate structure and the destruction of the oxalate groups has started; it is noted by the formation of a single band corresponding to CO₂ and the baseline rise under the oxalate group region (∼1,250–1,750 cm⁻¹). At 350 °C, the oxalate group has been converted to a carbonate moiety. CO₂ trapped within in the material or absorbed on the surface coexists with plutonium carbonate. At 450 °C, even though the Raman spectrum shows the formation of PuO₂, the infrared spectrum shows significant quantities of organics remaining after conversion to PuO₂. The shape of the bands in the oxalate region suggests a combination of carbonate remnants and additional organics. Thereafter, in the 600 °C–900 °C temperature range, the bands in the organic region are very similar but decrease in intensity with increasing temperature, suggesting the destruction of the organics. Additionally, in the 600 °C–900 °C temperature region, a new set of bands are observed to emerge and sharpen with temperature in the 2,500–2,750 cm⁻¹ spectral region. We believe these bands to be of electronic origin, similar to the electronic bands observed in the Raman spectrum at the 2,135 and 2,635 cm⁻¹ spectral regions (Villa-Aleman et al., 2023b).

The thermal decomposition of Pu(III) oxalate to PuO₂ (Figure 8) is similar to the thermal decomposition of the Pu(IV) oxalate, although the decomposition of Pu(III) has been observed to occur at lower temperatures (Kartushova et al., 1958; Rao et al., 1963; Karelin et al., 1990; De Almeida et al., 2012; Christian et al., 2023). The Pu₂(C₂O₄)₃ infrared spectrum is characterized by the OH stretch in the 3,600 cm⁻¹ region and vibrational modes from the oxalate groups in the 1,250–1,750 cm⁻¹ region. Heating the material at 160 °C results in decomposition of the oxalate group, indicated by the CO₂ band, and a rise in the baseline under the oxalate bands. It is known that, at 160 °C, a significant number of water molecules are removed from the plutonium(III) oxalate (De Almeida et al., 2012; Orr et al., 2015; Christian et al., 2023). While Pu(IV) oxalate remains stable until higher temperatures, it is possible that water molecules are required for the stabilization of the Pu(III) oxalate compound. At 200 °C, the oxalate continues to decompose and the transformation to the oxalate–carbonate complex begins to occur. The same process continues at 250 °C, with full conversion to the plutonium carbonate at 300 °C (IR measurements). Significant decomposition of the carbonate is observed at 350 °C (possibly plutonium oxy-carbonate as measured with Raman scattering measurements) with further decomposition of residual carbon species at 450 °C. Thereafter, the fingerprint signatures in the 800–1,750 cm⁻¹ are similar, although the material undergoes further degradation as calcination temperature increases.

It is important to realize that the carbon signatures observed from the decomposition of Pu(III) and Pu (IV) are different. Unfortunately, the carbon signatures are not necessarily reproduced from batch to batch, and significant work will be required to understand the differences. For instance, the peak observed at 1,263 cm⁻¹ in Figure 8 for thermal decomposition products ≥450 °C has not been reproduced. Batch size, calcination time, location of the sampled material during calcination (core versus crust of the material), and impurities may play a role.