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of rare earth ions in individual bonding environments. It is based on the use of a narrow-band excitation source, such as a tunable laser, to selectively excite subsets of ions from a large distribution of bonding environments. The fluorescence resulting from this selective excitation is said to be "line-narrowed," because it contains the fluorescence contribution from only those ions which are resonant with the exciting wavelength. By tuning the narrow-band excitation source across an inhomogeneously broadened absorption band, a series of FLN spectra can be obtained which can be used in the context of crystal field theory to obtain site-specific structural information. The Eu3 + ion is particularly amenable to FLN studies because the 7 F 0-) 5 D0 absorption band is non-degenerate. Any broadening observed in this transition results solely from the presence of dissimilar Eu 3 + bonding environments. It is important to note that the term "energy-selective" is a more correct description of FLtN than "site-selective." It is possible that ions with different bonding environments may have absorption bands that are resonant with the exciting laser. Consequently, resultant fluoreseenc(

will be a superposition of contributions from more than one type of site. This is called accidental degeneracy and it compromises the site-selectivity of the experiment. Another situation in which site-selectivity is compromised is when excited-state energy transfer populates ions that are non-resonant with the excitation source. Resultant emission is again a superposition of contributions. These topics are discussed in more detail in the text. EXPERIMENT Eu 3 +-doped silica samples were prepared by dissolving a europiumn salt (either EuCl 3 0 6H 2 0 or Eu(N0 3 )3 -6H 2 0) in a mixture of tetraethoxysilane (TEOS), water, and ethanol (mola ratio 1:4:4). The europium salt concentration was adjusted to give a densified Eu 2 O3 -Si0 2 product concentration of 1 to 10 wt.% Eu 2O3 . The addition of the europiumn salt lowers the so] pH, so all reactions are considered acid-catalyzed (pH = 0 to 2). In some samples, HCl was added to adjust the pH. Reactions were carried out in sealed test tubes in a 40*C bath. Two methods were used to introduce A13 + into the matrix. The first method involved dissolving Al(N0 3 )3 .9H 2 0 along with the europium salt in the silica sol described above. The second method involved prehydrolysis of the above Eu: TEOS:H 2 0: ethanol mixture for approximately two hours. After prehydrolysis, an aluminum-silicon double metal alkoxide ((BuO) 2 A1-O-Si(OEt) 3 ) was added to the silica sol. Eu 3 +-doped silica gels with and without added alumina were also prepared with the method developed by Thomas et.al. [3] to prepare Nd 3 +-doped silica gels. Our preparation utilized the aluminum-silicon alkoxide rather than aluminum-sec-butoxide used by Thomas, and europiumn chloride was used in place of neodymium chloride. Other procedures were as described in reference [3]. All samples formed transparent, monolithic gels. Gel times var