. . . "Preferably, these are mono- or dicarbonyl compounds, such as benzoin and its derivatives, in particular benzoin methyl ether, benzil and benzil derivatives, for example 4,4-oxydibenzil and other dicarbonyl compounds such as diacetyl, 2,3-pentanedione and a-diketo derivatives of norbornane and substituted norbornanes, metal carbonyls such as magnanese pentacarbonyl or quinones such as 9,10-phenanth" . . .