Ryo Fukaya

Photoinduced charge-transfer process in rubidium manganese hexacyanoferrate probed by Raman spectroscopy

The photoinduced charge-transfer process in Rb(0.94)Mn[Fe(CN)(6)](0.98).0.2H(2)O is investigated by observing the valence states of the metal ions by Raman spectroscopy. The sample in the high-temperature phase is irradiated at the ligand to metal, CN(-)-->Fe(III) and charge-transfer band (lambda=395 nm). The Fe(III)-CN-Mn(II) pair valence state corresponding to the high-temperature configuration is totally depleted after prolonged irradiation, and the Fe(II)-CN-Mn(III) pair valence state corresponding to the low-temperature configuration appears. In addition, two kinds of CN stretching modes, ascribed to Fe(II)-CN-Mn(II) and Fe(III)-CN-Mn(III) pair valence states, are found. The photoproduction process of each pair valence states is well reproduced by a kinetic model assuming a charge transfer from Mn(II) to Fe(III). During irradiation, continuous shifts of the Raman peaks are found and ascribed to a release of the strain due to the lattice mismatching between the high-temperature and the photoinduced phases. This behavior indicates that the photoinduced phase created locally in the high-temperature-phase lattice grows up to a photoinduced phase domain. The conversion efficiency is lowered with decreasing temperature, indicating the existence of an energy barrier. We propose a model, which can explain the existence of an energy barrier in the electronic excited state.

Probing of local structures of thermal and photoinduced phases in rubidium manganese hexacyanoferrate by resonant Raman spectroscopy.

Resonant couplings of the electronic states and the stretching vibrations of CN(-) ligands, which bridges metal ions, is investigated by resonance Raman spectroscopy for Rb(0.94)Mn[Fe(CN)6](0.98)·0.2H2O. Large excitation wavelength dependences over one order of magnitude were found for Raman peaks corresponding to different valence pairs of metal ions in the excitation wavelength range between 350 and 632 nm. In the thermal low-temperature phase, the CN(-) stretching modes due to the low-temperature-phase configuration (Fe(2+)-Mn(3+)) and the phase-boundary configuration (Fe(3+)-Mn(3+)) are coupled to the Fe(2+)-to-Mn(3+) intervalence transfer band and Jahn-Teller distorted Mn(3+) d-d transition band, respectively. In the photoinduced low-temperature phase, the Fe(3+)-Mn(3+) mode shows strong resonant enhancement with the CN(-)-to-Fe(3+) charge-transfer band, which exists in the high-temperature phase with a cubic structure. From these resonance behaviors, we conclude that the local lattice symmetry of the photoinduced phase is cubic in contrast with the tetragonal symmetry in the thermal low-temperature phase.

Temperature dependence of photoinduced valence changes in rubidium manganese hexacyanoferrate probed by Raman spectroscopy

Temperature dependence of the photoinduced charge transfer process in Rb0.94Mn[Fe(CN)6]0.980.2H2O was investigated by observing the pair valence states of adjacent metal ions by Raman spectroscopy. After irradiation by 395 nm light in resonance with ligand-to-metal charge transfer band (CN- → Fe(III)), the photoinduced phase containing low-temperature-like phase was generated as a result of the charge transfer from Mn(II) to Fe(III). Since this process was suppressed upon cooling, it was suggested that the charge transfer process was assisted by a thermal-activated process.