A system of physical chemistry - Index of

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A system of physical chemistry - Index of

134

A SYSTEM OF PHYSICAL CHEMISTRY

IV.). Baly has likewise shown the connection between absorption bands

and fluorescence and phosphorescence bands. A summary of the latter

work is given by Baly in the Astrophys./otirn., July, 19 15. Fluorescence

is touched on briefly at the close of Chapter VII. For further informa-

tion regarding the photo-electric effect, the reader is referred to Allen's

Photo-electricity or to Hughes' Photo-electricity.

Photochemical Reactions. Einstein's Law of the Photochemical

Equivalent.

In Chap. XIV. of Vol. II. we have already discussed photochemical

reactions at some length.

the mode of application

Our present purpose is

of the quantum theory

to indicate briefly

to photochemical

change.

At the present time there appears to be a somewhat general tendency

to regard photochemical reactions as primarily due to a photo-electric

effect. 1

As the photo-electric effect is itself independent of temperature, the

whole process is only slightly affected by temperature, i.e. photochemical

reaction velocity constants have very small temperature coefficients.

The most important application of the quantum theory to photochemical

processes is that of Einstein as expressed in his law of the

photochemical equivalent.

(A. Einstein, Ann. d. Physik, [4] 37? 832, 191 2.)

According to Einstein, a photochemical reaction takes place owing

to the absorption of radiation in terms of quanta, each single molecule oj

a photo-sefisitive substance requiringjust one quantum hv(^ {of the requisite

frequency vo which the substance itself can absorb) in order that it may be

decomposed. So far as experiment has gone, the law has been approximately

verified. Bodenstein {Zeitsch. Physik. Chem., 85, 329, 19 13)

gives a table of several reactions from which it will be seen that if not

a single quantum, at least a very small number, 2 to 5, are required per

molecule of the substance photochemically decomposed. Such dis-

crepancy as does exist is probably to be attributed to the fact that the

experimental conditions did not correspond closely enough to those

postulated by Einstein, the principal one of which is that there shall be

the same intensity or density of radiation throughout the system. If the

system absorbs light very strongly, and is only illuminated in one direction,

this condition will not be fulfilled, and instead of reaching a true

equilibrium governed, as Einstein supposes, by an expression analogous

' In this connection, compare Landsberg {yourn. Russ. P/iys. Chem. Soc, 47, 908

(1915)) ; Nazarov, j6jrf., p. 943 ; {Chem. Soc. Abstracts, 108, ii., 718, 754 (1915)).

Volmer (Zeitschr. Elektrochem., 21, 113 (1915)) concludes that the close relation

between photo-electric conductivity and photochemical sensitiveness " confirms the

Luther theory that a loosening of electrons is the immediate cause of photochemical

processes ".

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