PnOTORYNTHESIS OF GREEN PLANTS IN ULTRAVIOLET AND INFRARED 1 1 57 



in chapter 18. page 484, as a possible consequence of the absorption of 

 blue-violet light by rhlorophyll). In other cases, the presence of excess 

 kinetic energy may help the dissociation products, formed in a tightly 

 packed medium, to escape immediate recombination (Franck and Rabino- 

 witch 1934). In still others, the vibrational energy of the excited elec- 

 tronic state may contribute directly to chemical activation; this is par- 

 ticularly likely in cases when the reaction partner forms a complex with the 

 absorbing molecule, so that no energy-dissipating collisions intervene be- 

 tween the primary and the secondary photochemical step. 



The latter explanation could be suggested for a decline in the 7 values 

 of photosynthesis at long waves, if the experimental 7 curve would indi- 

 cate that a certain minimum vibrational energy of the excited electronic 

 state is required for photosynthesis. However, according to Emerson and 

 Lewis, the decline occurs within the same band (and not upon transition 

 from one band to another, e. g., from the orange to the red band). This is 

 difficult to understand, since, within a single band, the absorption leads 

 everywhere, not only to the same electronic state, but also to the same vi- 

 brational state (at least, in respect to high-frequency band vibrations). 



One could suggest that the red band of chlorophyll is not a single band, 

 but contains vibrational bands clustering tightly under its wings. How- 

 ever, vibrational bands situated on the infrared side of the main electronic 

 band most likely originate in the vibrating states of the ground state, and 

 lead to the same excited state as the main band; they thus offer no ex- 

 planation for a diminished quantum yield. 



Another possibility is that the red absorption band of chlorophj-ll conceals a band 

 corresponding to a different electronic transition. In figure 21.20, it was suggested that a 

 comparatively weak band Xo —> Ao is hidden under the strong A'o — > Yo band; one could 

 suggest that absorption in the far red, exhibited by live cells, is caused by a shift of the 

 Xo — > Ao band, rather than by an extension of the Xo — > Yo band. One could also sug- 

 gest that, in the series of transitions Xq -^ Ao, Xo -^ Ai, Xo —* A2, . . ., the latter ones, 

 which lead to the vibrating states Ai, Ai, and give rise to chlorophyll bands in the orange, 

 yellow and green, can be followed by radiationless transitions into state Y, while the 

 first one, which leads to the non vibrating state ^0, cannot produce the same result 

 (because of insufficient energy of the excited state), and is therefore ineffective in bring- 

 ing about photosynthesis. 



Before considering any of these hj^potheses too seriously, one should 

 ascertain whether the drop in 7 above 680 m^, obsei^ved by Emerson and 

 Lewis, is not due to a more trivial cause, such as scattering (which Emer- 

 son and Lewis considered unlikely); to the presence of ferrous salts (or 

 other infrared-absorbing inorganic components) ; or the presence of a photo- 

 synthetically inactive pigment with a band greater than 680 m/i (perhaps 

 chlorophyll d; see page 1183). 



Observations of the low yield of photosynthesis in filtered extreme red 

 or infrared light (Urspnmg 1918, Gabrielsen 1940, etc.) without the meas- 



