preferably contain 1 to 6 carbon atoms most preferably 1 to 4 carbon atoms the preferred aryloxycarbonyl groups are phenyloxycarbonyl groups which may optionally be substituted by halogen atoms or c 1 c 3 alkyl groups chlorine and bromine atoms are the preferred halogen atoms of r 1 to r 4 in the dialkylamino compounds of formulae ii and iii r 5 r 6 r 9 and r 10 are alkyl groups which preferably have 1 to 8 most preferably 1 to 4 carbon atoms r 7 preferably is an alkyl group having 1 to 6 most preferably 1 to 3 carbon atoms or an alkoxyalkyl group preferably containing 2 to 8 most preferably 3 to 6 carbon atoms r 8 preferably has 2 to 6 carbon atoms preferred are compounds of the formula i in which r 1 is alkyl and r 2 to r 4 are hydrogen the preferred coinitiators are compounds of the formula ii as component c any known n heterocyclic compound capable of acting as a photoinitiator may be used examples of preferred n heterocyclic compounds are described in de a 20 27 467 these are compounds of the acridine or phenazine type which may contain an additional nitrogen atom as a heteroatom.