PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 193 



is the book "Electronic and Ionic Impact Phenomena" by Massey and 

 Burhop (1952). 1 Also very useful are the little volume by Willey, " Colli- 

 sions of the Second Kind" (1937); the review by Laidler and Shuler 

 (1951); and standard books on photochemistry (Noyes and Leighton, 

 1941; Steacie, 1946; RoUefson and Burton, 1939). 



1-2. PHOTOCHEMICAL REACTIONS 



Photochemical reactions differ most strikingly from' thermal reactions 

 in that they involve in intermediate stages electronically excited atoms, 

 radicals, or molecules which are virtually never excited in a system in 

 thermodynamical equilibrium at room temperature. The development 

 of modern photochemistry began, indeed, when physics made possible - 

 an understanding of these excited states in terms of the quantum 

 theory. 



A great simplification of photochemistry arises from the possibility, by 

 suitably controlling or filtering the external light source, of Hmiting excita- 

 tion to a single excited state of one of the reactants. If the absorbing 

 system is sufficiently simple, the nature of this excited state will often 

 be known from standard spectroscopic studies. Moreover, in nearly 

 all circumstances, the concentration of absorbing entities is sufficiently 

 small, and the intensity of exciting light suflaciently low, that the primary 

 processes are independent of one another and an excited atom or molecule 

 reacts with normal molecules only.- The initial density of excited atoms 

 or molecules is almost uniform; for a beam of exciting light the average 

 density decreases exponentially with the thickness of absorber which the 

 beam has traversed (the change in density over a distance equal to the 

 molecular separation always being small), and fluctuations from the aver- 

 age are ordinarily unimportant. Thus a photochemical reaction presents 

 in its initial stages great elements of simplicity. If, however, the possible 

 primary and subsequent elementary processes which the excited atoms or 

 molecules undergo are numerous, or are very complex, analysis of the 

 over-all mechanism may still be extremely diflncult. This is more often 

 the case than not; and, indeed, reactions which appear to be simple often 

 conceal highly intricate mechanisms. Of course, such often-present, fun- 

 damentally irrelevant, but practically very trying compUcations such as 

 reactions with impurities or in wall surfaces enhance further the difficul- 

 ties of investigation. 



1 One of us (R. P.) must record his appreciation of the generosity of the Delegates 

 of the Clarendon Press in providing him with proof of this volume in advance of its 

 publication. 



2 There are, however, exceptions to this conclusion. A few cases in which meta- 

 stable excited atoms react with one another have been known for many years. Impor- 

 tant progress was made recently in the studies by Norrish and Porter (1949), using 

 ultra high light intensity, of photochemical reactions in which the density of excita- 

 tions is great. 



