Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Printed in Great Britain. All rights mserwd 003s105%/w s17.Oot.00 PII: 50038-1098(96)00616-3 INFRARED ABSORPTION SPECTRA OF ELECTRONICALLY EXCITED LONG-LIVED METASTABLE STATES IN Naz[Ru(CN)5NO]*2H20 J.A. Gtiida,“.’ O.E. Piro,*” P.S. Schaiquevich” and P.J. Aymonino” ‘CEQUINOR, Departamento de Quimica, Facultad de Ciencias Exactas (FCE), Universidad National de La Plata (UNLP), C.C. 962, 1900 La Plata, Argentina bDepartamento de Ciencias Basicas, Universidad National de Lujan, 6700 Lujan, Argentina ‘Departamento de Ffsica, FCE, UNLP and Programa PROFIMO, C.C. 67, 1900 La Plata, Argentina zyxwvutsrqpon (Received 1 M arch 1996 by R.C. Dynes) [Ru(CN),NO]~- can be excited selectively by two very long-lived electro- nic metastable states (MS1 and MSII) by irradiation at low temperature with blue-near UV light. Upon excitation to MS1 and MS11states, CN and NO modes suffer frequency down-shifts, a fact which implies softening of the named bonds. Shifts of vibrational modes of the RuNO group are far larger than shifts of CN modes, a fact that indicates that the metastable states involve electronic transitions mainly located at that group. The MS1 and MS11 states decay when heating at two different onset temperatures TI < T,,. A thermally driven MSI-to-MS11 excitation transfer process taking place in the temperature range Tu > T > TI shows that MS11 is energetically below MSI. Copyright 0 1997 Elsevier Science Ltd Keywords: D. electronic states (localized), D. phonons, E. light absorption and reflection. 1. INTRODUCTION Metastable electronically excited states mainly localized on a single atom or molecule (Frenkel excitons) are of great and continuous interest in solid state physics and also for the understanding of chemical and photochemi- cal reactions. Very long-lived metastable states play a key role in high capacity information storage materials where an atom or molecule can be optically switched between such electronic states and ground state. Very long-living electronic metastable states have been generated by light irradiation at low temperatures and their existence proved by several techniques in transition metal-nitrosyl coordination compounds, i.e. salts of the nitroprusside [Fe(CN),NO12- ion [3.-41, the potassium salts of [RuC15NO]*- and [Ru(NO~)~ (OH)(NO)]2- anions [5, 61 and Na2[0s(CN)5NO].2H20 [7]. As the metal(n group is a common component of all these complexes it should be the site for the electronic excitation that lead to the metastable states. Accordingly, vibrational spectroscopic [B] and neutron [9] * To whom correspondence should be addressed. and X-ray [lo] diffraction studies performed on sodium nitroprusside showed that the major changes in vibrational wavenumbers and bond lengths when populating the metastable states occur in the metal-NO group. We have already reported two sets of infrared (i.r.) absorption bands associated, respectively, with the MS1 and MS11states (from now on MS1 and MS11will refer, in general, to the metastable excited states which show the smallest and largest frequency down shift of the NO mode, respectively) generated in the nitro- prusside ion in Na2[Fe(CN)5NO]*2H20(SNP) [2] and Ba[Fe(CN)5NO]*3H20 [3]. The two sets of bands begin to decay, upon heating, respectively, at temperature T,, for MSI, and T,,, for MSII, the first temperature being higher than the second. Differential scanning calorimetry (DSC), as applied to several nitroprusside salts [4] and also to K2[RuC15NO] [5] and K2[Ru(N02)4(0H)(NO)] [6], has later confirmed such a tinding. Also, we have just reported the i.r. spectra of MS1 and MS11 metastable states in Na2[Os(CN)5N0].2H20 and found a decay temperature ordering TI < TII opposite to that reported for the other related systems [7]. As a continuation of that work, we are reporting now 471