Page 267 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 267

The ~~ission Ions                                             253
                    of
              2.  Cation  emitters: The alkali  metal  zeolites,  and other alkali metal  alumi-
                                            of
                  nosilicates,  are  efficient  emitters alkali metal  cations. The cation  emit-
                  ters  have  been  known for a much  longer time than the anion  emitters,
                  but  the  anion  emitters are better  understood from a chemical  perspec-
                  tive;  hence  they  are  discussed  here.  Both  types emitters, however,  can
                                                       of
                  be scaled  up in intensity  readily to be  used for the primary  ion  guns in
                  static SMS inst~ments. Ion  beams of 50 pA to 1  nA  focused  to a l-m
                  spot size are  routinely  produced by using  these emitters. These  emitters
                                                      to
                  are  primarily  used  in  SIMS  guns,  as  opposed being  used for isotope
                  ratio analyses.
                                                         of
              The importance of the work  function  and  temperature the surface,  the  ion-
          ization  potential for positive ion emission,  and the electron  affinity for negative  ion
          emission  are  well  established for conditions in which  the  S-L equations  are  valid.
          Expe~mentally, the IP and  EA are also  important for thermal  emitters.  For  exam-
          ple,  the alkali metals all have  low  IPS and  are  emitted  in  good  yields  from the ze-
          olites impregnated  with  the  corresponding  alkali  metal. The halide  and  perrhenate
          anions  all  have  high  EAs  and  are  emitted  in  good  yield from certain of the rare
          earth oxides, The temperature is also quite important,  but  possibly  not for the  same
          reasons as for the S-L conditions.  Under S-L conditions a higher  temperature is
          more  likely to strip an electron or  to  add  an electron to an  atom.
              Sublimation of preformed ions from the surface is a process  much  different
          from S-L ioni~ation since  these  ions  already  have  their  oxidation  and  charge states.
          It may be surmised  that  a higher  temperature  increases the migration  rates of the
          preformed  ions  and  increases  the  sublimation rate, until the temperature  gets so
          high  that  some  undesirable  process  occurs  in the matrix.  As  an  example,  the  oxi-
          dation  state of  the ion of  interest could  change  with  excessive  temperature,  de-
          stroying  the ion. emission  properties, or the  matrix  could  undergo  an  undesirable
          phase  ans sf or mat ion. The importance of the IP and  EA  may be more  related  to  the
          degree of polarization  between the preformed  ion  and  their counter ions. The WF
          of the surfaces of these  emitters  has  never  been  measured,  not  even at low  tem-
          peratures. The better  anion  emitters are in  rare  earth  oxide  matrices,  and  these
                                                                       ma-
          terials  are  known to have  relatively  low WFs at low te~peratures, but the effects
                                                                         to
          of blending  these  components  and  elevating  the  temperature  have  the  potential
          alter  these  values. This area  needs  more  study.
              Experimentally, the compound consisting  of  the preformed  ion  and its
          counter  ion  (such  as barium  perrhenate for perrhenate  emission)  does  not  produce
          ions  when  heated-instead,  only  neutral  species  sublime.  It must  be  embedded  in
          a suitable  matrix  and  then  heated. The limited  experimental  evidence  collected  to
          date indicates  that  the  ion  to be emitted  must  have  significantly  greater  mobility
          andlor  vapor  pressure  in  this  matrix  than  its counter ion,  allowing  diffusion of the
          ion of interest. When the tempera~re gets  sufficiently  hot the ion  migrates to the
   262   263   264   265   266   267   268   269   270   271   272