Investigation of the metastable state of Na2[Fe(CN)5NO] · 2H2O by optical spectroscopy I. Comparison of the Raman spectra of the ground and metastable state

When excited by the 514.5 nm radiation of an argon ion laser at temperatures below 150 K crystalline sodium nitroprusside Na₂Fe(CN)₅NO] · 2H₂O can be transformed into an extremely long-living metastable state. The Raman spectra of the crystal in the metastable state and ground state are compared in the range of 50–2350 cm^-1. In the metastable state the ?(NO)-stretching vibration is shifted by more than 100 cm^-1 to smaller frequencies depending on the orientation of the sample with respect to the crystallographic axis. This indicates the population of the antibonding π¹(NO)-orbital. In addition two new vibrations appear at 566 and 422 cm^-1. The other vibrational frequencies of the molecular ion Fe(CN)₅NO]^2- are scarcely shifted, but relative intensities change.