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Fluorescence at high temperatures (fwd)

Sender: "Yuri E.Gorbaty" <hunch@iem2.sherna.msk.su>
Subject: WSN: Fluorescence at high temperatures


  Dear Netters,

  Could anybody help us to understand the reason for strong
  fluorescence arising in aqueous fluids at high temperatures and
  pressures? We encountered this phenomenon a few years ago when we had
  started Raman studies of aqueous solutions. It looks like a broad
  band that spreads over a large spectral region, from the exciting Ar
  laser line (514.5 nm) up to 4000-5000 cm^-1 with the maximum at about
  2500 cm^-1. The effect becomes quite noticeable at 400 K and grows in
  an increased pace with increasing temperature. At 700 K even the
  strong valence band of water "drowns" in the sea of fluorescence. We
  tried, but unsuccessfully, to purify water with multiple
  distillation, treatment with ion exchange resins, degassing,
  oxidation with ozone, and so forth. It is hard to say, what method has
  not been tried.

  At first, we thought that the reason for this undesirable effect
  might be interaction of a hot fluid with the metallic walls of the
  high-pressure cell. That for we designed a cell, in which the fluid
  under study could contact only with leucosapphire and gold.  And
  again - the result was bad!  We have found later that not only water
  but other liquids, e.g. alcohols, dioxane, shows the same behaviour
  at high temperatures and pressures. Just two exceptions have been
  found from the great number of systems studied -  aqueous solution of
  zinc nitrate and NaNO_3 solution with a small quantity of perchloric
  acid added. The Raman spectra of the substances were free of
  fluorescence up to 750 K and the background was very low. However,
  the same NaNO_3 solution in absence of perchlorate shows strong
  fluorescence. On the other hand, addition of the perchlorate to other
  fluids (water, KCl solution ,for example) had no effect - fluorescence
  was all the same very strong.

  The instrumental techniques allowing one to avoid fluorescence in
  Raman studies are well-known and widely used nowadays. Unfortunately,
  because of the present situation in Russia we cannot even
  dream to purchase so an expensive equipment. But we have enough money
  to write in gold all over the walls of our laboratory the name of the
  scientist who will explain the source of that torturing phenomenon or
  advice us as how to wriggle out from the troubles.

    Thanking everybody in advance,
                                      Yu. Gorbaty
                                      (hunch@iem2.sherna.msk.su)
                                      Institute of Experimental
                                      Mineralogy,
                                      Chernogolovka, Moscow district