On the absorption of light by crystals

0*58
0*49
0*065
Owing to th e larger dielectric c o n sta n t of A gB r we see th a t W has a much 
lower value for th is salt th a n for th e alkali halides; a slightly larger choice of 
n* w ould bring it dow n to th e value 
0*035
e-volt w hich we deduced above.
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On the absorption of light by crystals
391
In order th a t, for th e alkali halides, no p h o to co n d u ctiv ity should be 
observed up to a tem p eratu re T, it is necessary by (1) th a t W should be 
greater th a n
k
T \0 
0 6
. Taking T  to be 
300
° K ., th is gives a value in th e 
neighbourhood of 
0-4
e-volt. W e should th u s expect, on th e pure alkali 
halides illum inated on th e long w ave-length tail of th e absorption band, th a t 
photoconductivity would be observed near or above room tem p eratu re. 
E xperim ents above room tem p eratu re do not seem to have been m ade.
S
ummary
The m echanism of th e absorption of light by solids is discussed from th e 
theoretical po in t of view. The energy of a q u an tu m of rad iatio n can be 
converted into h eat, re-em itted as fluorescent rad iatio n , or used up in 
producing a photoelectric cu rren t w ithin th e solid; estim ates are m ade of th e 
dependence on tem p eratu re of th e relative probabilities of these processes, 
which are com pared w ith experim ent.
R
eferences
Berg and M endelssohn 1937 P roc. P h ys. Soc. 49 (extra part).
de Boer 1935 “ E lectron E m ission and A bsorption P h en om en a.” Cambridge.
Frenkel 1936 P h y s