raison d'être vs rationale

raison d'être

noun
  • Reason for being. The claimed reason for the existence of something or someone; the sole or ultimate purpose of something or someone. 

rationale

noun
  • An explanation of the basis or fundamental reasons for something. 

  • A liturgical vestment worn by some Christian bishops of various denominations. 

  • A justification or rationalization for something. 

How often have the words raison d'être and rationale occurred in a corpus of books? (source: Google Ngram Viewer )