DIATOMIC MOLECULES 75 



can be radiative transitions to states such as II (with formation of stable 

 AB"^) and III (with formation either of AB"^ or A"^ with low kinetic 

 energy); the half life for state V for such transitions, by the usual 

 spectroscopic selection rules, will be of the order of 10~^ sec; (6) there can 

 be radiationless transition to state IV, with formation of A"*" ions with 

 some intermediate amount of kinetic energy ; the half life for this transi- 

 tion will depend greatly on the magnitude of the interaction between the 

 two states and can be anything between <10~^^ sec to >1 sec. The 

 time spent by an ion in the ionization region before collection by the ion 

 "draw-out" potential is the order of lO"*" sec. Hence, ions initially 

 formed in a state such as V will normally undergo an electronic transition, 

 with or without dissociation, before acceleration and collection. Lastly, 

 transitions to state VI will be followed immediately by dissociation to 

 give A"^ with excess kinetic energy. 



A set of potential-energy curves such as those indicated therefore gives 

 rise to AB+, A+ (low KE), and A+ (high KE). Certainly there will 

 also be states yielding B"^, both without and with kinetic energy. Also, 

 to be complete, one must include double ionization, the formation of 

 AB"'"^, Avith possible subsequent dissociation most likely to A"^ + B"*", 

 but also sometimes to A"^"^ + B and A + B"*""*". There is also the 

 possibility of forming an excited neutral molecule AB* in such a state 

 that it dissociates: AB* — > A"^ + B~". All these possibilities have been 

 observed (2). 



In H2"^, the two lowest states are located relative to the ground state 

 of H2 in similar fashion to states III and IV of Fig. 1. The observed 

 mass spectral data agree completely with those expected from the above 

 discussion in regard to appearance potentials and kinetic energy of the 

 H"*" ions. Also, a calculation based upon the simple application of the 

 Franck-Condon principle leads to a value of the rates of 'ii^/Yi.2^ to 

 D~^/D2"^ in essential agreement with experiment (13). 



A detailed discussion for diatomic molecules of the dependence of 

 peak shapes as observed with the mass spectrometer upon the relative 

 shapes of the ion and ground state curves has been given by Hagstrum 

 and Tate, with special reference to CO, NO, N2, O2 (2). They find, for 

 instance, that both C"^ and 0"^ are obtained from attractive states of 

 CO"^, the state yielding C"*" having its minimum a little further out than 

 the curve for state III, Fig. 1, so that some C"^ are formed with appreci- 

 able kinetic energ^^, and the state yielding O"^ similar but with its 

 minimum much further out, so that practically all the CO^ ions formed 

 in this state dissociate with appreciable kinetic energy. 



It is probably not necessary to point out that an appearance potential 

 for an ion, since it corresponds to a "vertical" transition on a potential- 



