Refractive-Index Matching Avoids Local Field Corrections and Scattering Bias in Solid-State Na 2 SO 4 Ultraviolet Raman Cross-Section Measurements

We report a refractive-index matching method to measure nonabsorbing solid ultraviolet (UV) Raman cross-sections that avoids the local field correction and interface scattering of incident light. We used refractiveindex-matched chloroform as an internal standard to determine the solid-state 995 cm⁻¹ Na₂SO₄ 244 nm Raman cross-sections. The pure liquid chloroform 668 cm⁻¹ 244 nm Raman cross-section was determined by using acetonitrile as an internal standard and by calculating the local field corrections for the observed Raman intensities. Our measured 244 nm UV Raman cross-section of the solid-state 995 cm⁻¹ SO₄^2– band of 1.97 ± 0.07 × 10⁻²⁸ cm²/(molc·sr) is about half of its aqueous solution Raman cross-section, indicating interactions between the sulfate species in the solid that decrease the Raman polarizability.