Sulfonium salts

A sulfonium salt represented by formula (1):

• wherein R1 represents a linear or branched C2 to C9 divalent hydrocarbon group;
• each of R2 to R5 represents a hydrogen atom or a linear or branched C1 to C3 hydrocarbon group;
• each of R6 and R7 represents a phenyl group or a phenyl group which has a substituent having a C1 to C30 alkyl group, or R6 and R7 together with S atom to which they are attached form a heterocyclic ring with a saturated carbon of C3 to C9 alkyl groups;
• and X− is an anion which is represented by formula (2), or represented by formula (3) which is a bis(perfluoroalkylsulfon)imide ion, or Cl−, Br−, I−, BF4−, AsF6−, SbF6−, or PF6−,
• CkHmFnSO3− (2)
• wherein each of k, m, and n is an integer of ≧0;
• when m is 0, k is an integer of 1 to 8, n is 2k+1, and formula (2) represents a perfluoroalkylsulfonate ion;
• when n is 0, k is an integer of 1 to 15, m is an integer of ≧1, and formula (2) represents an alkylsulfonate ion, a benzenesulfonate ion, or an alkylbenzenesulfonate ion;
• and when each of m and n is an integer of ≧1, k is an integer of 1 to 10, and formula (2) represents a fluorinated benzenesulfonate ion, a fluorinated alkylbenzenesulfonate ion, or a fluorinated alkylsulfonate ion,
• (CpF2p+1SO2)2N− (3)
• wherein p represents an integer of 1 to 8.