200 RADIATION BIOLOGY 



electrons is involved, any such energy is included in the electronic 

 energy. 



Transitions between states may take place by emission or absorption of 

 electromagnetic radiation or in impact with other systems. In radiative 

 transitions a single atom is raised to a "higher" state (i.e., one of greater 

 energy), or reverts to a lower one, by absorbing or emitting a single 

 photon of Hght. The energy hi> of this photon is exactly equal to the 

 difference in energies of the two states of the atom. A few of the more 

 important radiative transitions of H, Na, and Hg are labeled in Fig. 3-1. 

 (It should be noted that hydrogen is diatomic, and the transitions from 

 the ground state shown in the diagram cannot, therefore, be observed in 

 absorption under ordinary conditions.) Because excitation of valence 

 electrons involves energy differences of the order of magnitude of a few ev, 

 the corresponding photons lie in the visible, or ultraviolet or infrared 

 regions; this is the optical region of the electromagnetic spectrum. 



The various lines of an atomic spectrum have different intrinsic intensi- 

 ties. Thus, the absorption coefficient will vary considerably from line 

 to hne, some possible hues (i.e., excitation to some upper levels) being 

 very weak or not present at all; such lines are referred to as "forbidden." 

 Similarly variable is the emission probabihty, which can be defined as 

 the reciprocal of the mean or probable lifetime (usually called simply life- 

 time) of an at^om in a particular excited state for emission of a particular 

 hne. In the event that all emission lines from a given excited state are 

 forbidden, an atom in the state will have a long lifetime and is referred to 

 as being in a metastable state. The intrinsic probability of absorption 

 of a given atomic spectral hne is proportional to that of its emission ; thus 

 metastable atoms are not ordinarily produced directly by absorption of 

 hght. 



Transitions between levels may also take place in an impact with an 

 electron or other charged particle, atom, molecule, or ion. A de-excita- 

 tion by impact, i.e., transition of an atom already excited to a lower state, 

 is called a "collision of the second kind" (q.v.). An impact in which 

 translational energy is converted into excitation energy of an atom (or 

 molecule) is called a "colhsion of the first kind." Thus, colhsion of a 

 moving electron with an atom may excite the latter, the excitation energy 

 being provided by and subtracted from the kinetic energy of the electron. 

 A curious feature of this type of excitation, if the electron is fairly slow, is 

 that the relative probabihties for excitation of different levels may be 

 markedly different from those for optical excitation; for example, some 

 metastable states may be produced with high probability. With very 

 swift electrons (energies greater than, say, a few hundred ev) this behavior 

 does not obtain: the relative excitation probabilities in impact are about 

 the same as the optical ones. Swiftly moving heavy, charged particles 

 (a particles, protons, etc.) behave in this respect like electrons of equal 



