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We have used rare gas matrix isolation techniques in combination with extended x-ray absorption fine structure (EXAFS) to study the variation in interatomic distances for small Fe molecules in solid neon. A considerable contraction in the interatomic distances was observed 0 for the metal molecules. An Fe-Fe distance of 2.02+0.02 A for the lowest concentration of metal was observed. This is in good agreement with early EXAFS measurements in Fe2-Ar,We also carried out a careful study of the x-ray-absorption near-edge structure (XANES), and observed the appearance of considerable structure for a 1.5-at. % Fe sample. The XANES spectra were analyzed in terms of 1s-to-(3d, 4s) and 1s-to-4p transitions.

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The molecular structures for thermal activated and photoinduced electronic spin state of [Fe(pic)3]ClyEtOH (pic = a-picolylamine) in ethanol solution are studied using EXAFS and optical absorption spectroscopies. Structural studies of the thermally activated spin state in this compound were also conducted on the crystalline solid. The experimental results show that the Fe-N bond length change in [Fe(pic)3]Cl~*EtOH solution is similar to that observed in the crystalline solid for the thermally activated spin state where the Fe-N bond distances are 2.00 f 0.02 and 2.20 f 0.02 8, for the low spin and the high spin states respectively. However, photoexcitation of the low spin state [Fe(pic)3]Cl~*EtOH in solution at 10 K generated a metastable intermediate with a structure different from either the high spin or the low spin state. The analysis of the experimental data indicates that the Fe-N bond distances in this intermediate are divided into two sets with average bond distances of 1.94 f 0.04 and 2.1 1 f 0.04 A. This phenomenon is attributed to a Jahn-Teller distortion of the octahedral structure probably due to electron transfer between the solvent and the solute molecules or to an intermediate state with triplet character stabilized by a pseudo D 4 h field.

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A structural study based on EXAFS, FTIR, and optical absorption spectroscopies has been conducted on a photogenerated, metastable state of cyclopentadienylnitrosylnickel (CpNiNO) produced by a reversible photochemical reaction. The photogenerated, metastable state with distinctively different EXAFS, FTIR, and optical absorption spectra from those of the ground state was created by irradiating the sample at 20 K with the 365-nm line of a mercury lamp. At the same temperature, the reverse reaction was induced by irradiation with the 313-nm line from a mercury lamp. On the basis of the analysis of the EXAFS data, the photogenerated, metastable state of CpNiNO has undergone considerable nuclear rearrangements cmopared to its ground state. The nuclear movement is characterized by a 0.12-A elongation of the Ni-N bond and by a bending of the Ni-N-0.A shift of the N -0 stretching frequency from 1824 to 1387 cm-' was observed in the photoinduced reaction with 365-nm light, consistent with previous studies. This implies that a NO-like species results from intramolecular electron transfer from Ni to NO. The absorption spectrum of the long-lived, metastable, charge-transfer state exhibited reduced absorption of the 385-nm band and an additional broad band in the near-IR region, which is likely a consequence of the intramolecular electron transfer and the Ni-N-0 bending. On the basis of the structures obtained from EXAFS, ZINDO calculations for the ground state of CpNiNO reproduced the general features of the observed absorption spectrum and were qualitatively consistent with the complicated dependence of the charge-transfer photoreaction on wavelength.