MON ATOMIC GASES 71 



for the angular deviation given above. Tliose ions having the appropriate 

 radius are refocussed on an exit sUt behind which is a collector electrode 

 connected to a sensitive electrometer circuit. The current in the 

 electron beam is usually about 10 microamp. Total ionization is the 

 order of 10~^ or 10"^ amp. A very strong peak might give an ion 

 current of 10~^^ amp. The size of the minimum peak detectable will 

 depend upon the sensitivity and the noise level of the electrometer 

 circuit; 10~^^ amp is a reasonable lower limit for most instruments. 



As is indicated by the above, in the modern mass spectrometer the 

 current density in the electron beam and the gas density in the ionization 

 region are sufficiently low to assure that all processes are the result of 

 single electron impacts upon isolated gas molecules and that all sub- 

 sequent breakups and rearrangements of the molecules are unimolecular 

 [only exceptions reported: formation of Hs"^ and HC02"^ (1)]. Proper 

 location of the filament and the pumping lead essentially prevents the 

 diffusion of products formed by thermal cracking at the filament back 

 into the ionizing region. Thus, by varying either the accelerating 

 voltage or the magnetic field, one can successively collect and measure 

 the ions of each tn/e ratio formed from a given molecule by collision with 

 electrons of known energy, without the complications that arise from 

 secondary reactions in gases at higher pressures or in condensed phases. 



One major trouble in the interpretation of data arises from the com- 

 plete lack of direct information regarding the neutral fragments formed 

 along with the ions. This matter will be discussed in some detail in 

 subsequent sections of this report. A second serious complication is that 

 the simple "single-focussing" mass spectrometer is designed to focus and 

 collect efficiently only those ions formed with essentially zero kinetic 

 energy (of the order of translational thermal energy at 200° C, 0.06 

 volt). By applying a retarding potential to the final ion collector one 

 can determine the amount or distribution of excess kinetic energy 

 possessed by the ions of any given mass number, but the geometry of 

 the spectrometer tube is such that the vast majority of ions with appreci- 

 able (over about 1 volt) translational energy will strike the sides of the 

 tube long before reaching the ion collector (2). 



MoNATOMic Gases 



The simplest substances for study in the mass spectrometer are the 

 monatomic gases. Here, all the data obtainable can be represented by a 

 set of "ioriization-efficiency" curves for the ions obtained by removing 

 one or more electrons from the atom. These curves, in which the 

 magnitude of the ion current is plotted against the energy of the electron 



