imperturbation vs serenity

imperturbation

noun
  • Freedom from agitation of mind; calmness. 

serenity

noun
  • The state of being serene; calmness; peacefulness. 

  • A lack of agitation or disturbance. 

  • A title given to a reigning prince or similar dignitary. 

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