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[0068] In formula (1), n 1 is an integer ofO to 6, with the proviso that R 1 and R 2 may bond together to form a ring structure when n$^{1 }$is 1 and R$^{1 }$is not hydrogen. Examples of the ring thus formed include cyclopropane, cyclobutane, cyclopentane and cyclohexane rings. 

[0069] Also, two R 1 may bond together to form a ring structure when n 1 is an integer of at least 2 and R 1 is not hydrogen. Examples of the ring thus formed include cyclo› propane, cyclobutane, cyclopentane, cyclohexane, norbor› nane, and adamantane rings. When two R 1 bond to a common carbon atom, two R $^{1 }$may bond together to form a spiro ring structure with Z. 

[0070] In formula (1), Q+ is an onium cation. The onium cation is preferably a sulfonium cation having the formula (2a) or an iodonium cation having the formula (2b). 

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[0071] In formulae (2a) and (2b), RH to R 15 are each independently a C$_{1 }$-C$_{20 }$ monovalent hydrocarbon group which may contain a heteroatom. The monovalent hydro› carbon group may be straight, branched or cyclic. Examples include alkyl groups such as methyl, ethyl, n-propyl, iso› propyl, n-butyl, tert-butyl, cyclopropyl, cyclopentyl, cyclo› hexyl, cyclopropylmethyl, 4-methylcyclohexyl, cyclohexyl› methyl, norbomyl, and adamantyl, alkenyl groups such as vinyl, ally!, propenyl, butenyl, hexenyl, and cyclohexenyl, aryl groups such as phenyl, naphthyl, and thienyl, and aralkyl groups such as benzyl, 1-phenylethyl and 2-pheny› lethyl. Inter alia, aryl groups are preferred. In these mon› ovalent hydrocarbon groups, some hydrogen may be sub› stituted by a moiety containing a heteroatom such as oxygen, sulfur, nitrogen or halogen, and a moiety containing a heteroatom such as oxygen, sulfur or nitrogen may intervene between carbon atoms, so that the group may contain a hydroxyl moiety, cyano moiety, carbonyl moiety, ether bond, ester bond, sulfonic acid ester bond, carbonate bond, lactone ring, sultone ring, carboxylic anhydride or haloalkyl moiety. 

[0072] Any two of R 11 , R $^{12 }$and R $^{13 }$may bond together to form a ring with the sulfur atom to which they are attached. In this case, examples of the sulfonium cation having formula (2a) are shown below. 

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[0073] Herein R 16 is a C$_{1}$-C$_{20 }$ monovalent hydrocarbon group which may contain a heteroatom. 

[0074] Examples of the sulfonium cation having formula (2a) are given below, but not limited thereto. 

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