Characterization of PuO2 With Visible and UV Raman Spectroscopy: Discrimination Between the Bulk, Surface, and an Intermediate Disordered Layer

Abstract

Raman spectroscopy is an ideal tool in the characterization of materials including PuO₂. The wavelength-dependent absorptivity of the material defines the light penetration depth and the relative Raman scattering contribution from the bulk and the surface. The surface contribution to the total Raman scattering was investigated for PuO₂ calcined at various temperatures and recorded with laser wavelengths of 355, 325, and 244 nm. These experiments provided the first glimpse of the wavelength-dependent disappearance and emergence of new phonons and electronic bands from the PuO₂ surface layers. The first indication of the wavelength transition in the Raman spectra was the loss of the 2LO2 (overtone, ~1155 cm⁻¹) band and the weakening intensity of the Г₁ → Γ₅ electronic band (~2135 cm⁻¹) with the 355-nm excitation laser. The Γ₅ electronic band was barely visible with the 244-nm excitation. The electronic band located at ~1050 cm⁻¹, corresponding to the Г₁ → Γ₄ electronic transition was observed to dramatically increase in intensity while the Г₁ → Γ₃ electronic band (2640 cm⁻¹) sharpened as the UV wavelength was increased in energy from the near- to deep-UV (355–325–244 nm). The FWHM of the T_2g band was found to vary with calcination temperature (450°C and 900°C) with the 325-nm laser and the 244-nm laser. The T_2g band attributes, the strong emergence of the Г₁ → Γ₄ electronic band, and the disappearance of the 2LO2 overtone acquired with the 244-nm excitation for the different calcination temperatures suggest a shallow penetration depth.