PHOTOCHEMISTRY 



273 



Frequently a spectrum shows a region of discontinuous lines coming 

 closer and closer together and merging into a region of continuous absorp- 

 tion, as shown diagrammatically for iodine in Fig. 6. Under these condi- 

 tions the fine structure in part of the spectrum shows that the resolving 

 power of the spectrograph is adequate and dissociation is indicated by 

 the region of continuous absorption. The long-wave-length edge of the 

 band of continuous absorption marks the least amount of energy which is 

 necessary to dissociate the molecule. Important work has been done in 

 correlating this long-wave-length edge of the absorption band with 



5500 A 

 -1' 



5000A 



Convergence frequency'^ 



Fig. 6. — Absorption spectrum of jodine vapor, showing continuous absorption below 



4995 A caused by dissociation. 



the heat of dissociation as determined experimentally. In some cases 

 there is complete agreement. In other cases the energy of dissociation, 

 determined chemically or thermally, is less than the energy of dissociation 

 as calculated from the edge of the region of continuous absorption. In 

 such cases the discrepancy is usually traced to the fact that the fragments 

 into which the molecules are dissociated are excited. Several examples 

 of this relationship between calculated and observed dissociation are given 

 in Table 2. 



Table 2. — Heats of Dissociation Calculated from Spectroscopy 



Absorbing 

 molecule 



Hi. 

 I2.. 

 Br2 



Maximum 



wave- 

 length of 

 continuous 

 absorption, 



o 



A 



850 

 4995 

 5107 



4785 



Heat of 

 dissoci- 

 ation, cal. 

 per mole 



334,000 

 56,800 

 55 , 600 

 59 , 400 



Probable 



energy of 



excitation 



of products 



234.000 



21,600 



10 , 400 



2,500 



Estimated 

 heat dis- 

 sociation 



into normal 

 atoms 



100,000 

 35 , 200 

 45 , 200 

 56 . 900 



Heat of 

 dissoci- 

 ation de- 

 termined 



calori- 

 metrically 



101,000 

 34 , 500 

 46.200 

 57,000 



In a similar manner it is possible sometimes to calculate the heat of 

 dissociation from the wave-length at which sharp lines in an absorption 

 spectrum change into hazy or indistinct lines. Such a spectrum of indis- 

 tinct lines, which cannot be further resolved, is known as a predissociation 

 spectrum. When such a spectrum is produced, it is supposed that the 

 electronic and vibrational energies are subjected to the regular quantum 

 number restrictions, but that the rotational energy is not quantized, 



