asperse vs impugn

asperse

verb
  • To falsely or maliciously charge another; to slander. 

  • To sprinkle or scatter (liquid or dust). 

impugn

verb
  • To verbally assault, especially to argue against an opinion, motive, or action; to question the truth or validity of. 

How often have the words asperse and impugn occurred in a corpus of books? (source: Google Ngram Viewer )