Resonance Raman Scattering from Single Levitated Microparticles

Measurements of resonance Raman scattering from single crystalline (or amorphous) particles suspended in an electrodynamic balance have been made for inorganic and organic microparticles, potassium permanganate (KMnO₄) and p-nitrosodimethylaniline (p-NDMA), respectively. The microparticle dimensions were in the range 10–50 μm. The particles were obtained by evaporating aqueous solution droplets of KMnO₄ and p-NDMA (0.01 M concentration) and potassium nitrate or sodium nitrate (1.0 M concentration) added as a nonresonance Raman standard or reference. Resonance Raman spectra were obtained using laser illumination at wavelengths of 514.5 and 488.0 nm. For MnO⁻₄ particles at a laser wavelength of 514.5 nm, the fundamental symmetric stretch mode (ν₁) and up to four overtones were observed. In addition, the antisymmetric stretch mode (ν₃) and the combination mode (ν₁ + ν₃) could be seen. In p-NDMA particles, the phenyl-nitroso deformation mode (ν₁) and the symmetric benzene ring-stretching mode (ν₃) were observed using 488.0 nm laser radiation, but no overtones were distinguishable. As a measure of the morphological variability between particles, the ratio of the intensity of ν₁ to the intensity of the largest nitrate ion (Raman) peak (normalized by the concentrations) was determined for each spectrum. This enhancement ratio or reference ratio was of order 100 and varied by 27% for 12 KMnO₄ particles with 514.5 nm illumination, by 39% for 10 KMnO₄ particles at 488.0 nm, and by 16% for 14 p-NDMA particles at 488.0 nm. Resonance Raman spectra obtained for KMnO₄ particles before and after deliquescence indicated that photolysis occurred in the presence of water.