164 BIOPHYSICALLY ACTIVE LIGHT 



sion the band will be found to have its lines crowded closer together at 

 one end, called the band head. 



The complex organic molecules that have been considered as partici- 

 pating in absorption are said to be in their lowest molecular energy state, 

 E . A photon of energy content hv, absorbed by such a molecule, will 

 alter its energy state from E to Ex. This means that the molecule 

 absorbs energy Ex — E from the radiant energy passing over it, in 

 which these E's are definite energy states characteristic of the molecule 

 and not of the radiant energy. Thus a molecule in a liquid might 

 absorb light of frequency v in such a way that v = (Ex — E )/h, where 

 h is the usual Planck's constant. 



For simplicity's sake, let us consider a molecule of hydrochloric acid 

 in which the hydrogen and chlorine atoms are a definite distance apart. 

 This molecule, by acquiring additional energy through absorption, can 

 respond in three ways: (1) There may result a vibration of the two 

 atoms along their axis of connection. (2) There may result a rotation 

 about an axis at right angles to the above axis. (3) There may be 

 changes in radii of the orbital electrons of the atoms, i.e., quantized 

 changes in the electron energies. The spectroscopic evidence shows 

 that the vibrational energy E v = in + \)hv vi where n can take on only 

 integer values 0, 1, 2, etc. This implies that the vibrational energy is 

 quantized and that, the larger n, the farther the electron is removed from 

 the nucleus. 



The rotational energy is also quantized so that increase in its energy 

 states proceeds as 



(r + b 2 h 2 



E r = 



8x 2 / 



where r changes by integers 0, 1, 2, etc. /, the moment of inertia, is 

 included because the rotational characteristic of the molecule changes 

 with increase in orbital radii of the electrons. The electronic energy E e 

 may also change, owing to electronic rearrangements when a quantum is 

 absorbed by either atom. Thus the total energy of the molecule may 

 initially be 



E = E e + E v + E r 



Now, changes in any one, any two, or all three may take place when 

 energy is absorbed. The absorption bands under consideration are 

 found in the near ultraviolet; as a result the energy of the absorbed 

 photon is chiefly used in electronic excitation. 



We may picture the energy change involved thus: the initial energy 

 of subscript zero, E e0 + E v0 + E r0 becomes E en + E v0 + E r x- The 



