Ligand‐field splitting of the energy levels of Nd3+ (4f3) in 2‐hydroxyethyl methacrylate polymer (HEMA)

Detailed absorption spectra of trivalent neodymium (Nd³⁺) in 2‐hydroxyethyl methacrylate polymer (HEMA) are reported at cryogenic temperatures and at wavelengths between 440 and 900 nm. The Nd³⁺ spectra are confirmed by comparison with spectra of HEMA samples prepared without inclusion of the rare earth ion. The structure observed on the absorption bands can be interpreted in terms of the ligand‐field (crystal‐field) splitting of the energy levels of Nd³⁺ (4f³) for multiplet manifolds, ⁴F_3/2, ⁴F_5/2, ²H_9/2, ⁴F_7/2, ⁴S_3/2, ⁴F_9/2, ²H_11/2, ⁴G_5/2, ²G_7/2, ⁴G_7/2, and ²K_13/2. The Nd³⁺ ions are complexed as species made up of nitrate anions and H₂O molecules that come from the neodymium nitrate hydrate salt used in the preparation of the doped HEMA sample. The local symmetry of the Nd³⁺ ion in HEMA is very low (possibly C_S), and a number of different environments are likely for the neodymium species in the HEMA structure. We have calculated the ligand‐field splitting of the Nd³⁺ energy levels using a point charge/dipole model for one such possible species, Nd(NO₃)₃ · xH₂O^? (x = 5–6). The results of the model give an agreement of 10 cm^?1 (rms) between 43 observed‐to‐calculated ligand‐field (Stark) levels. Comparisons between these same calculated levels and observed levels obtained for Nd³⁺ in crystalline hydrated nitrate salts and certain Nd³⁺‐doped glasses also give good agreement and support a better understanding of the environment of Nd³⁺ in a polymeric structure. Copyright © 2006 Society of Chemical Industry