Phosphorescence 361 



photoconductivity was only definitely established by B. Gudden and 

 R. Pohl " in 1920. The change in electrical resistance is now recog- 

 nized as an internal or volume photoelectric effect, due to emission 

 of electrons within the substance illuminated. It has played an im- 

 portant part in theories of luminescence since the beginning of the 

 century. It was in 1899 that both P. Lenard ^^ and J. J. Thomson ^^ 

 demonstrated that the negative charges emitted from illuminated 

 metal surfaces were the same as the particles of cathode rays, that is, 

 they were electrons. 



Quite naturally these discoveries formed the basis for theories of 

 phosphorescence during the early twentieth century. The subject 

 is beyond the scope of this book and it will suffice to say that after 

 1900 a number of workers regarded the exciting light as liberating 

 electrons. The first of these was L. E. O. de Visser (1901) , while 

 P. Lenard and V. Klatt (1904) and de Kowalski '« (1907) empha- 

 sized the presence of emission centers represented by the trace of 

 heavy metal present, fiom which light separated electrons. Later 

 work of Lenard and Saeland ^^ (1909) and J. Stark (see 1911) «° 

 gave support to the photoelectric theory. Usually luminescent 

 emission was presumed to occur on return of the electron to the 

 positively charged atom or molecule. The theory is particularly 

 important in connection with thermoluminescence and cathodo- 

 luminescence and has become associated ^vith the names of Lenard 

 and Klatt. Philip Lenard won the Nobel Prize in physics in 1905 

 for work on cathode rays. Virgil Klatt was his teacher and later a 

 collaborator in their extended work on impurity phosphors in 1904, 

 some fifteen years after the Klatt and Lenard paper of 1889. Their 

 work has formed the basis for present theories of phosphorescence. 



Theories of Phosphorescence (and Fluorescence). A Review 



Although the views of various early workers on the mechanism 

 of phosphorescence have been presented in connection with their 

 discoveries, it may be helpful to review the theories as a whole, now 

 that the proper perspective has been acquired. As practically every 

 student of phosphorescence has ventured a theory, only the principal 



''^Ztschr. fur Physik 1: 365-375, 1920, and many later papers. 



■^sp. Lenard, Sitzungsber d. Akad. Wiss. Wien. 108: 1649-1666, 1899; Ann. der Phys. 

 2: 359-375, 1900. 



"J. J. Thomson, Phil. Mag. 48: 547-567, 1899. 



"J. de Kowalski, Phil. Mag. 13: 622-626, 1907, and Arch. Sci. Phys. et Nat. 21: 22-24, 

 384-386, 1907. 



^»P. Lenard and S. Saeland, Ann. der Physik 28: 476-502, 1909, and P. Lenard, Ann. 

 d. Physik 31: 641-685, 1910. 



«<•}. Stark. See Prinzipien der Atomdynamik 2:213-227, 1911. 



