ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 121 



(3) arc, or (4) discharge. The modern tungsten lamp is an excellent 

 example of a source of the first category. The radiation takes place as a 

 result of heating of the surface of the radiator by some means; in this case 

 it is by the passage of an electric current. For fundamental reasons 

 sources of this category have limited utility, and that only in the near 

 ultraviolet. In most cases the intensity of the emitted radiation falls 

 rapidly between 4000 and 3000 A, approaching zero at the latter figure. 



Spark sources emit radiation excited by the passage of a high-voltage 

 discharge between electrodes. The material of the electrodes enters the 

 spark stream, contributing the major fraction of the radiation through its 

 excitation by the electrical energy. In the arc also the electrode material 

 evaporates into the arc stream to produce a large portion of the emitting 

 ions in this stream. Arcs are generall}^ low-voltage, high current dis- 

 charges. Radiation is produced in the discharge tube by excitation and 

 ionization of the gas contained at reduced pressure. The radiation is 

 excited by a relatively high potential between electrodes which, them- 

 selves, do not contribute significantly to the ion stream. The distinction 

 between these methods of excitation is not sharp and the reader will find 

 that the foregoing system of classification is not rigidly adhered to in the 

 literature. This is understandable when it is noted that discharge tubes 

 operated at very high current densities may show evidence of evaporation 

 of the electrode material into the ion stream by the appearance of radia- 

 tion characteristic of the electrode material. The heating of the elec- 

 trodes of an arc by ion bombardment ma^^ be sufficient to make the ther- 

 mal radiation from the electrode a significant contribution to the total 

 radiation from the source. A spark operated in air produces radiation 

 which is characteristic of the electrode material, but if operated under 

 water, the radiation produced bears no relation to the electrode material. 



Spectral Distribution. Of somewhat more practical importance is the 

 classification of light sources with respect to the distribution of the spectral 

 energy emitted. Sources are classified as continuous, line, or band. 

 Continuous spectra generally arise from thermal emittors or from non- 

 quantized energy transitions; line spectra arise from quantized atomic- 

 energy transitions; and band spectra arise from molecular-energy transi- 

 tions or from atomic-energy transitions occurring at high temperatures 

 and pressures. On the basis of the method of production of these various 

 types of spectra it would be expected that many sources would exhibit 

 other than the nominal type of spectrum. Thus, when the hydrogen 

 discharge tube is operated at extremely high current densities in an effort 

 to achieve high brilliance, line spectra are frequently found superimposed 

 upon the typical continuous ultraviolet spectrum of hydrogen. These 

 lines arise from the evaporation and subsequent excitation of electrode 

 material in the ion stream. Mercury discharge tubes, which at low pres- 

 sures and current densities show well-defined line spectra, show increasing 



