A new mechanism of Raman enhancement and its application

Strong electronic Raman bands corresponding to the transition between ⁴I_9/2 and ⁴I_11/2 manifolds of Nd³⁺, caused by a Raman-enhancement effect, are observed in the FT-Raman spectrum of Nd₂O₃. Neither resonance enhancement (RR) nor surface enhancement (SERS) accounts for the Raman enhancement observed here. We propose a new mechanism of Raman enhancement called the "feed-back" mechanism. A YAG laser excites the final state of the Raman transition (⁴I_11/2 of Nd³⁺) to the 4F_3/2 state and causes a significant decrease in the population of Nd³⁺ at the ⁴I_11/2 state. This causes the population ratio of Nd³⁺ at ⁴I_9/2 and ⁴I_11/2 to deviate from the value required by Boltzmann's law. To restore equilibrium, Raman scattering is enhanced so that more Nd³⁺ ions are brought from the ⁴I_9/2 state to the ⁴I_11/2 state. This hypothesis gets support from the temperature-variable FT-Raman spectroscopic results. Additionally, obvious differences between the Stokes and anti-Stokes Raman spectrum of Nd³⁺ provide further evidence to support the feed-back mechanism. The Raman-enhancement effect confers on the electronic Raman bands a special ability to reflect the variation of coordinated structure around metal ions. The structural variations in polymer-metal ion composites and biomineralization systems have been investigated by using the electronic Raman bands.