SOME CONTEMPORARY ADVANCES IN I'l IVSICS—XI Ml 



volts. One would expect to be aljle to dissociate hydrogen by bombard- 

 ing the gas with 3.5 \-olt electrons; yet nothing of the sort happens. 

 This is an instance of the frequently-occurring observation that a par- 

 ticle or a quantum nia\- ha\e abundant energy to produce a particular 

 effect and yet be quite unable to produce it. One would expect also 

 that the minimum energy required to convert an H2 molecule into an 

 H"^ ion and an H atom and a free electron would exceed by 3.5 equi- 

 valent volts the ionizing-energy of an H atom, yet the difference ap- 

 pears to be less, which is strange. 



Detachment of Tightly-bound Electrons from Atoms 



We will now consider the most direct and striking evidence for the 

 statement that each atom (apart from those of the lightest elements) 

 possesses several distinct ionizing energies — several distinct "states of 

 ionization." This fact is taken to mean that each atom possesses 

 several or many electrons w'hich are bound, as the phrase is, with differ- 

 ent degrees of firmness or tightness; that the ionizing-energies of the 

 atom are, so to speak, the extraction-energies of these various electrons; 

 to each electron there corresponds a certain extraction-energy, the 

 amount of energy which must be imparted to the atom to extract that 

 electron, the energ^'-difference between the normal state of the atom 

 and that particular "state of ionization" which involves the absence of 

 that particular electron. I shall frequently use the language of this 

 interpretation, which is extremely convenient and likely to remain so. 

 Nevertheless it is desirable to remember that the quantities actually 

 observed are energy-difTerences between various states of the atom, or 

 energy- values of various states of the atom referred to the energy -value 

 of the normal state as zero. These energy-values are the data of ex- 

 perience; most other assertions about the states of ionization are 

 speculative.^^ 



Concei\'e a layer of atoms of an element possessing several different 

 values of ionizing-energy IT'i, W2, Ws and so forth; in other words, 

 atoms which are capable of se\'eral states of ionization of which the 

 energy-\alues exceed that of the normal state by Wi, IF2, W3 and so 

 forth. Suppose that a beam of radiation of frequency v, so chosen that 

 the product Jip exceeds all of the ionizing-energies, falls upon the layer. 

 Such a beam is absorbed as though it consisted of indi\'idual particles of 

 energ>' hv, each of which is either completely absorbed or totally ignored 

 by the layer of matter upon which it falls. Consider an atom which ab- 



'5 In some cases, although not in any which will be discussed in this section, it is 

 found necessary to suppose that several distinct states of ionization correspond to 

 the absence of a particular electron, which is somewhat of a strain upon the picture. 



