Page 268 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 268

surface  and  sublimes,  leaving the counter ion behind. This process  continues until
            something  happens  to  limit further diEusion andor sublimation. This limiting step
                                                                     The
            is thought to be  the  buildup of charge  (or  color  centers)  within  the  matrix. sec-
                                                                 for
            ond  major  requirement  seems be some  mechanism  to  compensate the  buildup
                                   to
            of these  charge  centers.
                The most  successful of the  anion  emitters  are  based  on rare earth oxide  ma-
            trices,  with  the rare earth in  the  +3 oxidation  state.  Europium  oxide, Eu,O,,  is the
            most  successful of these  anion  emitters  and also has the most  stable 4-2 oxidation
            state of all the  rare  earths.  This feature of  Eu,O,,  the stability of the  +2  oxidation
            state, is thought to be responsible for this compound’s being the best matrix.
            Whereas the matrix  in  the -1-3 oxidation  state  allows the migration of the  anions
            away  from  the  counter ion, the  stability of the  +2  oxidation  states  allows  reactions
            of the  following  type  to  take  place:

                Eu20, + Ba(ReO,),  + 2e- * 2ReO,-  (gas  phase) + 2EuO + BaO   (6.1)
                 This  reaction  involves  the  reduction of  Eu in  the +3 oxidation  state to Eu  in
            the  +2  oxidation state, freeing  an  oxygen  anion to combine  with  the  Ba  counter
            ion left after  the  perrhenate  anion  migrates. There are  a  variety of other chemical
            reactions of a  similar type that can be written,  such  as the follo~ing:

                 Eu203  + 2Ba(ReO,),  + 2e- * 2ReO,-  (gas  phase)
                                          + 2EuO + Ba2~(Re04),           (6.2)
                 This  equation  has the advantage  over the previous  one that the barium  con-
            taining  molecule  on  the  right of the equation  retains  a  partial ionic character  that
            permits it to serve  as  a  “transfer  station” additional  migration of perrhenate  an-
                                            for
            ions.
                 A model is proposed to account for the  capability of these  materials  to be ef-
            ficient  emitters of ions:
                 1.  The singly  charged  ion  needs  to be presynthesized,  paired  with  a suit-
                    able  counter  ion (all successful  systems  studied to date have  the  ion  of
                    interest singly  charged  and the counter  ion  doubly  charged),  and  em-
                    bedded  in  a  matrix that allows the singly  charged  ion to migrate  away
                                                                     the
                    from  the  doubly  charged  counter  ion  when  heated.  Otherwise, entire
                    neutral molecule can sublime, bypassing the channel for ion emission.
                 2.  The ion of interest needs to be  stable in the  matrix. This places  various
                    requirements  on  redox  properties of the matrix for each  type of ion.
                 3.  There  needs  to be  some  mechanism  available  for ~aintaining charge
                    neutrality  in  the lattice after  ion  migration.  Electron  migration  between
                    the  filament  and  the  power  supply  can  maintain  charge  neutrality,  but
                    this  alone  cannot  satisfy the bonding  requirements of the  counter  ion.
                 4.  For  a cation emitter,  an  oxidation  reaction  within the matrix is required
                    to generate  a  new  cation  to  satisfy bonding  requirements of the  anion
                                               the
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