PHOTOCHEMISTR Y 

 Table 1. — Energy in Various Types of Radiation 



263 



Description 



Wave- 

 length, 

 A 



X-rays 



Ultra-violet 



Ultra-violet 



Ultra-violet 



Visible (violet) .... 

 Visible (blue-green) 

 Visible (orange) ... 



Visible (red) 



Visible (red) 



Near infra-red 



Infra-red 



Far infra-red 



1 



1,000 



2,000 



3,000 



4,000 



5.000 



6,000 



7,000 



8,000 



10,000 



100,000 



, 000 , 000 



Erg per 



quantum 



1.96 

 1.96 

 9.82 

 6.55 

 4.12 



93 

 27 

 81 

 42 

 96 

 96 

 96 



X 10- 

 X 10 

 X 10- 

 X 10- 

 X 10- 

 X 10- 

 X 10- 

 X 10- 



10- 

 10- 



X 10- 

 X lo- 



cal, per 



Einstein 



2.84 X 108 



284 , 500 



142,300 



94 , 840 



71,120 



57 , 000 



47 , 400 



40 , 600 



35 , 500 



28,450 



2,845 



284 



Elec- 

 tron 

 volts 



12,340 

 12 3 



6 17 



11 

 09 

 47 

 06 



1.76 



54 

 23 

 12 



0.01 



is equal to 10 A. The wave number is a unit, frequently used in spec- 

 troscopy. It is obtained by dividing the wave-length, expressed in 

 centimeters, into unity. The frequency of light is obtained by dividing 

 the wave-length of light expressed in centimeters into the velocity of 

 light, 3 X 10^° cm. It is obvious that wave numbers are converted into 

 frequencies by multiplying by the velocity of light, 3 X 10 ^*'. In the 

 last column the energy of the radiation is expressed in electron volts by 



o o 



dividing the wave-length in Angstroms into 12336 A. This relation 

 follows from equating hv to the product of the voltage and the charge 

 on the electron, expressed in the proper units. 



These relationships may be illustrated with a specific example using 

 light of 5000 A (blue-green light). The wave-length of this light in cm. 

 is 5 X 10~^ cm. The wave number is 20,000 per cm. The frequency 

 is 6 X 10^^ per sec. The energy per photon, hv, is 



(6.55 X 10-2^) X (6 X 1014) = 3 93 x lO-^^ erg. 

 Multiplying the energy of one photon by the number of photons required 

 to activate a gram mol and converting ergs to calories, it is evident that 

 one Einstein of this radiation contains 



(3.93 X 10-'-) X (6.06 X lO^') -r- (4.18 X 10^) - 57,000 cal. 

 The energy in electron volts is 2.47. 



PHOTOCHEMICAL PHENOMENA 



Absorption of Radiation. — In photochemical reactions the energy 

 necessary to start the process is supplied by the absorption of radiation. 

 This absorption of radiation in the visible or ultra-violet region of the 

 spectrum results in a displacement of the electrons. If this energy of 

 electronic excitation is transferred to give displacement of the atoms 

 within the molecule, chemical reaction may result. If this energy of 

 atomic displacement becomes sufficiently great, the atoms are driven 

 apart and the molecule dissociates. If the atoms are displaced to a 



