54 LIGHT ABSORPTION EFFECTS 



(b) Fluorescence is the simplest kind of result of light absorption: 

 the re-emission of most of the light. If the light is emitted very soon 

 after absorption (in no more than one microsecond), the re-emission is 

 termed fluorescence. Because the system cannot emit more energy than 

 it absorbed, the fluorescence is quite generally of a longer wavelength 

 (more toward the red end of the spectrum) than the incident absorbed 

 light. Because of rapid rearrangement of internal electron orbits [these 

 are the changes shown going from (b) to (c) in Fig. 22], fluorescence is 

 normally due to a transition from a particular excited electron orbit to a 

 particular lower orbit very near the normal electron orbit; thus the light 

 is usually monochromatic. If a substance absorbs in several regions of 

 the spectrum (e.g., chlorophyll absorbs strongly both in the blue and in 

 the red) the internal rearrangements referred to yield the result that the 

 fluorescence is as though only the longer wavelength is absorbed (chloro- 

 phyll fluoresces only in the red). 



(c) Phosphorescence is the emission of light considerably later than 

 fluorescence emission — phosphorescences lasting a few seconds are not at 

 all uncommon. The light emitted is primarily the same as in fluorescence. 

 The reason for the delayed light emission in phosphorescence is that the 

 electron is trapped in an orbit (perhaps E 2 ) from which, according to 

 the rules of quantum physics, it cannot readily jump down to the normal 

 orbit. After a time, which may be as long as a few seconds, the electron 

 manages to get to the particular excited orbit from which it can jump 

 with the emission of the normal fluorescent light. Because the molecules 

 exhibiting appreciable phosphorescence are, de facto, in high energy states 

 for considerable periods, they tend to be very reactive chemically. 



(d) Energy transfer refers to the transfer of the absorbed light energy 

 from the receiving molecule to another molecule. It will occur if the 

 molecules are sufficiently close to each other and if the energy of the 

 excited electron in the absorbing molecule chances to be matched to a 

 possible excited energy state of the other molecule. 



(e) Internal conversion covers a number of experimental situations. 

 When electrons are excited by absorption of photons, they usually do 

 not reach the next highest orbit but an orbit somewhat higher, so that 

 they execute a vibration around the new orbit (the vibration, naturally, 

 must be one allowed by the rules of quantum physics). This extra vibra- 

 tional energy is dissipated by being communicated to the surrounding 

 medium or to vibrations of the lattice if the atom is part of a larger 

 structure. The electron thus reaches the lowest state of vibration of the 

 new orbit, and this is the particular excited electron orbit referred to 

 above in the discussion of fluorescence. The extra vibrational energy 

 may be the source of a number of other effects ranging from rearrange- 

 ments of the atoms composing a molecule to the dissociation of molecules. 



