Sensitized onium salts

An onium salt capable of generating acid upon exposure to actinic radiation having the following structure:

• (R.sub.1).sub.a (R.sub.2).sub.b (R.sub.3).sub.c Q.sup.+ --A--M.sup.+ X.sup.- --B--X'.sup.-
• where:
• Q is S;
• R.sub.1, R.sub.2 and R.sub.3 are independently aromatic groups having 6-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• or aliphatic groups having 1-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• or aralkyl groups having 7-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• M.sup.+ is a cationic organic group having the structure:
• Q'.sup.+ (R.sub.4).sub.d (R.sub.5).sub.e (R.sub.6).sub.f
• where Q' is S;
• R.sub.4, R.sub.5, and R.sub.6 are independently aromatic groups having 6-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• or aliphatic groups having 1-12 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• or aralkyl groups having 7-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• f=0;
• d and e are 0, 1 or 2 such that d+e=2 when R.sub.4 and R.sub.5 are monovalent groups;
• and d+e=1 when R.sub.4 or R.sub.5 is a divalent group;
• A is a divalent group selected from the group of hindered alkylene groups having 4-12 carbon atoms;
• aromatic groups having 6-18 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• and aralkyl groups having 7-24 carbon atoms, which may be substituted with hydroxyl, alkoxyl, amino or alkylamino groups;
• B is a divalent aromatic sensitizer which absorbs radiation having a wavelength longer than 300 nm and is capable of transferring an electron to Q,
• x.sup.- and X'.sup.- are anionic groups independently selected from the group consisting of sulfate, sulfonate, sulfite phosphate, phosphite and carboxylate;
• and when
• c=0;
• a and b=0, 1, or 2 such that a+b=2 when R.sub.1 and R.sub.2 are monovalent groups;
• and a+b=1 when R.sub.1 or R.sub.2 is a divalent group;
• wherein A provides a spatial separation between Q and M, and B provides a spatial separation between X and X', such that the spatial separation provided by A between Q and M is substantially the same as the spatial separation provided by B between X and X'.